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Isotopic composition of gadolinium, samarium and europium in the Abee meteorite Loveless, Arthur John 1970

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ISOTOPIC COMPOSITION OF GADOLINIUM, SAMARIUM AND EUROPIUM IN THE ABEE METEORITE  by  ARTHUR JOHN LOVELESS B.Sc, M.Sc,  The U n i v e r s i t y of. Toronto, 1966 The U n i v e r s i t y o f Toronto, 1967  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE  s  REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of GEOPHYSICS  We accept t h i s t h e s i s as conforming to the r e q u i r e d standard  THE UNIVERSITY OF BRITISH COLUMBIA October, 1970  In p r e s e n t i n g t h i s t h e s i s  in p a r t i a l  f u l f i l m e n t of the requirements  for  an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and Study. I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g of t h i s  thesis  f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s .  It  i s understood t h a t c o p y i n g or p u b l i c a t i o n  of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my written permission.  Department of The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada Date  ABSTRACT  The w r i t e r has m e a s u r e d the i s o t o p i c c o m p o s i t i o n of g a d o l i n i u m , e u r o p i u m and s a m a r i u m i n the Abee m e t e o r i t e i n two t e r r e s t r i a l o r e s . Gd, E u and Sm  and  have l a r g e t h e r m a l  neutron capture c r o s s s e c t i o n s ; they may  t h e r e f o r e be used to  r e v e a l d i f f e r e n c e s i n the i r r a d i a t i o n h i s t o r i e s of s a m p l e s c o n t a i n ing t r a c e amounts of these elements.  It i s w i d e l y b e l i e v e d that  the c o n t r a c t i n g protosun p a s s e d through a phase of h i g h e n e r g y p a r t i c l e r a d i a t i o n d u r i n g the e a r l y h i s t o r y of the s o l a r s y s t e m . The i n t e r a c t i o n of these p a r t i c l e s with the m a t e r i a l of the s o l a r nebula may  have p r o d u c e d a l a r g e t h e r m a l neutron flux. The Abee enstatite chondrite is r e p r e s e n t a t i v e of the  m o s t h i g h l y r e d u c e d c l a s s of c h o n d r i t i c m e t e o r i t e s .  The  chondrites  are .stones w h i c h a r e thought to be v e r y p r i m i t i v e m a t e r i a l of the solar system.  M a s o n , M i y a s h i r o and others have p r o p o s e d that  v a r i a t i o n s i n the o x i d a t i o n state of chondrites r e s u l t f r o m t h e i r f o r m a t i o n i n d i f f e r e n t regions of the nebula: c h o n d r i t e s r e p r e s e n t m a t e r i a l w h i c h was  the enstatite  d e r i v e d f r o m the hot  i n n e r r e g i o n of the s o l a r nebula, w h i l e the h i g h l y o x i d i z e d c a r b o n a ceous c h o n d r i t e s had t h e i r o r i g i n i n the c o o l e r , outer r e g i o n .  On  the b a s i s of this t h e o r y , the e a r t h was p r o b a b l y d e r i v e d f r o m an i n t e r m e d i a t e r e g i o n of the nebula.  Isotopic a n a l y s e s w e r e p e r f o r m e d on m i c r o g r a m quantities of Gd, E u and Sm.  A p r e c i s i o n of 0.02-0.2% (at the 9 5 % confidence  l e v e l ) was a c h i e v e d f o r a l l i s o t o p i c r a t i o s of i n t e r e s t .  P r i o r to  1970 the b e s t p u b l i s h e d i s o t o p i c a n a l y s e s of these elements had a p r e c i s i o n of 1-2%.  The higher p r e c i s i o n r e p o r t e d here was made  p o s s i b l e through i m p r o v e m e n t s i n the m a s s s p e c t r o m e t e r i o n optics and the use of d i g i t a l r e c o r d i n g of m a s s s p e c t r a . T h i s w o r k i s c o m p a r a b l e to r e c e n t m e t e o r i t e a n a l y s e s on G d by E u g s t e r , T e r a , B u r n e t t and W a s s e r b u r g at the C a l i f o r n i a Institute of Technology, although t h e i r l u n a r a n a l y s e s w e r e of s u p e r i o r quality and had a p r e c i s i o n of 0.01%. No s i g n i f i c a n t Gd, E u o r Sm i s o t o p i c a n o m a l i e s were 157 o b s e r v e d f o r the Abee and t e r r e s t r i a l samples.  The G d  15 8 /Gd  r a t i o , w h i c h i s the m o s t s e n s i t i v e to neutron i r r a d i a t i o n , was i d e n t i c a l f o r the three s a m p l e s studied w i t h i n 0.1%. T h i s p l a c e s an upper l i m i t of 3 x 1015n e u t r o n s / c m 2 f o r the d i f f e r e n t i a l i r r a d i a t i o n of the source m a t e r i a l f r o m w h i c h the e a r t h and the Abee m e t e o r i t e w e r e d e r i v e d . A s i m i l a r c o n c l u s i o n was r e p o r t e d by the C a l t e c h group.  The absence of any a n o m a l i e s m a y be  a t t r i b u t e d to u n i f o r m i r r a d i a t i o n and d i l u t i o n of the s o u r c e m a t e r i a l , e f f i c i e n t s h i e l d i n g inside l a r g e p l a n e t a r y bodies d u r i n g the i r r a d i a t i o n phase, the absence of an intense i r r a d i a t i o n phase, or a c o m m o n s p a t i a l o r i g i n w i t h i n the s o l a r nebula f o r c h o n d r i t i c and t e r r e s t r i a l matter.  T A B L E OF  CONTENTS  ABSTRACT TABLE OF  CONTENTS  LIST O F F I G U R E S LIST O F T A B L E S ACKNOWLEDGEMENTS C H A P T E R 1. ., -  INTRODUCTION  1.1 C h o n d r i t e s - R e m n a n t s of P r i m i t i v e Planetary Material 1. 2 Q u a n t i z a t i o n of the O x i d a t i o n State of Chondrites 1 . 3 H i g h E n e r g y P a r t i c l e I r r a d i a t i o n and its Consequences 1.4 D e l i n e a t i o n of the P r e s e n t Study  C H A P T E R 2.  A R E V I E W O F T H E E V I D E N C E CONCERNING IRRADIATION ANOMALIES  2 . 1 Synthesis of the E l e m e n t s and H i g h E n e r g y Particle Irradiation  ..  2 . 2 A R e v i e w of the S e a r c h f o r I r r a d i a t i o n -Anomalies 2 . 3 The E f f e c t of Recent C o s m i c I r r a d i a t i o n .  C H A P T E R 3.  INSTRUMENTATION  3. 1 O p e r a t i o n a l P r i n c i p l e s of a M a s s Spectrometer 3. 2 Ion O p t i c s 3 . 3 The Ionization  Process  V  3.4 D i g i t a l R e c o r d i n g of M a s s S p e c t r a  42  3.5 N o i s e R e j e c t i o n U s i n g a L o w - P a s s F i l t e r  45  C H A P T E R 4.  P R E P A R A T I O N OF S A M P L E S  47  4.1 S e p a r a t i o n of the R a r e E a r t h E l e m e n t s  47  4. 2 P r e p a r a t i o n of F i l a m e n t s  55  4. 3 P r o d u c t i o n of Ion S p e c t r a with M i n i m u m Interference  57  C H A P T E R 5. C O M P U T E R R E D U C T I O N O F  COMPLEX  SPECTRA  59  5. 1 E v a l u a t i o n of P e a k Heights  59  5. 2.Least Squares R e d u c t i o n of a Set of Scans  61  5. 3 C a l c u l a t i o n of Means and Standard D e v i a t i o n s of Isotopic R a t i o s f o r a Set of Scans 5.4 C a l c u l a t i o n of F r a c t i o n a t i o n - C o r r e c t e d Parameters 5.5 C o r r e c t i o n f o r the Isotopic C o m p o s i t i o n of O x y g e n C H A P T E R 6.  I N T E R P R E T A T I O N OF  68 71  76  METEORITIC  & T E R R E S T R I A L ISOTOPIC RATIOS  78  6.1 T e r r e s t r i a l G a d o l i n i u m  78  6. 2 G a d o l i n i u m i n the Abee M e t e o r i t e  84  6. 3 Do Abee and T e r r e s t r i a l G a d o l i n i u m have Similar Composition?  '  90  6.4 I n t e r p r e t a t i o n of E u r o p i u m Data  93  i6. 5 I n t e r p r e t a t i o n of S a m a r i u m Data 6.6 C o n c l u s i o n s  95 99  vi  A P P E N D I X I.  E F F E C T OF NEUTRON C A P T U R E  ON  GD, S M A N D E U  102  A P P E N D I X II.  S E C O N D O R D E R F O C U S S I N G SHIMS  109  A P P E N D I X III.  M E A S U R E D ISOTOPIC RATIOS FOR A L L GADOLINIUM ANALYSES 119 S L O P E OF T H E O R E T I C A L C O R R E L A T I O N LINES FOR NEUTRON C A P T U R E 129  A P P E N D I X IV.  BIBLIOGRAPHY  131  \  \  LIST OF FIGURES  F I G U R E 1-1  ABUNDANCES OF THE ELEMENTS  METALLIC  F I G U R E 1-2 O X I D I Z E D A N D R E D U C E D I R O N I N CHONDRITES F I G U R E 1-3  F R E Q U E N C Y O F O C C U R R E N C E VS OXIDATION STATE  F I G U R E 1-4  T H E R M A L N E U T R O N C A P T U R E IN LUNAR GADOLINIUM  F I G U R E 2-1  M O D E L F O R W H I C H 5% O F P R I M I T I V E M A T E R I A L WAS IRRADIATED  F I G U R E 2-2  UNIFORM IRRADIATION M O D E L  F I G U R E 3-1 C O N T I N U O U S S C A N N I N G O V E R THE GADOLINIUM S P E C T R U M F I G U R E 3-2 I O N B E A M P R O D U C E D B Y ION S O U R C E  NEW  F I G U R E 3-3 D I S C O N T I N U O U S S C A N N I N G O V E R THE GADOLINIUM S P E C T R U M F I G U R E 3-4 F R E Q U E N C Y R E S P O N S E O F DIGITAL FILTER F I G U R E 4-1 E L U T I O N C U R V E S F O R E U AND SM  I B , GD,  F I G U R E 5-1 S L O W D I S C O N T I N U O U S S C A N N I N G F I G U R E 5-2 P O L Y N O M I A L R E P R E S E N T A T I O N ION B E A M I N T E N S I T Y F I G U R E 5-3 M A S S S P E C T R O M E T E R F I G U R E 6-1  OF  FRACTIONATION  ISOTOPIC A N A L Y S E S ON T E R R E S T R I A L GADOLINIUM  F I G U R E 6-2  CORRELATION B E T W E E N B AND A FOR TERRESTRIAL GD  F I G U R E 6-3  C O R R E L A T I O N O F GD ISOTOPES IN M E T E O R I T E S  F I G U R E 6-4  CORRELATION B E T W E E N A AND B IN M E T E O R I T E S  F I G U R E II-l  M O D I F I C A T I O N O F MAIN E L E C T R O M A G N E T T O A C C O M M O D A T E FOCUSSING SHIMS  F I G U R E II-2  FRINGING F I E L D O F MAIN M A G N E T  F I G U R E II-3  C U R V A T U R E O F SECOND ORDER F O C U S S I N G SHIMS  F I G U R E II-4  O P T I M U M C U R V A T U R E O F SHIMS AS A FUNCTION OF B E A M DIVERGENCE  F I G U R E II-5  P E A K SHAPE FOR TWO V A L U E S O F THE ANGULAR DIVERGENCE  F I G U R E II -6 B E A M S P R E A D I N G A T T H E C O L L E C T O R F O R FIRST A N D S E C O N D O R D E R ;FO CUSSING  ix  Page LIST OF  T A B L E 1-1  T A B L E 2-1  TABLES  THERMAL NEUTRON CROSS SECTIONS  CAPTURE 8  M A X I M U M D I F F E R E N T I A L IRRADIATION OF P R E - T E R R E S T R I A L AND PREMETEORITIC MATERIAL  28  T A B L E 4-1  DESCRIPTION OF S A M P L E S STUDIED  48  T A B L E 4-2  I N T E R F E R I N G IONS N E A R GD, AND SM SPECTRA .  50  T A B L E 4-3  T A B L E 4-4  T A B L E 5-1  T A B L E 6-1  T A B L E 6-2  T A B L E 6-3  T A B L E 6-4  T A B L E 6-5  T A B L E 6-6  EU  B E A M INTENSITIES FOR S A M P L E S CONTAINING A L L R A R E EARTHS  51  E X T R A C T I O N O F GD, E U SM F R O M A B E E  53  FRACTIONATION PARAMETERS  AND  CORRECTED 75  I S O T O P I C R A T I O S O F GD I N TERRESTRIAL SAMPLES  79  ADDITIONAL RESULTS FOR TERRESTRIAL GADOLINIUM  83  I S O T O P I C C O M P O S I T I O N .OF.GD IN T H E A B E E M E T E O R I T E  87  ISOTOPIC COMPOSITION OF T E R R E S T R I A L AND ABEE EU  94  ISOTOPIC COMPOSITION OF SMIN TERRESTRIAL SAMPLES '  96  ISOTOPIC COMPOSITION OF S A M A R I U M IN T H E A B E E M E T E O R I T E  97  T A B L E S III -1 to III-8 G D I S O T O P I C R A T I O S F O R A L L T E R R E S T R I A L AND A B E E ANALYSES  121  X  ACKNOWLEDGEMENTS  The w o r k r e p o r t e d i n this t h e s i s r e c e i v e d c o n s i d e r a b l e support f r o m s e v e r a l p e r s o n s i n two different i n s t i t u t i o n s . The study of the Abee enstatite chondrite was f i r s t p r o p o s e d by Dr. H. M a b u c h i of the D e p a r t m e n t of C h e m i s t r y , U n i v e r s i t y of Tokyo.  He and M r . S. Yanagita a s s u m e d r e s p o n s i b i l i t y f o r  s e p a r a t i n g gadolinium, e u r o p i u m and s a m a r i u m f r o m a s a m p l e of the Abee m e t e o r i t e .  T h e i r m a j o r c o n t r i b u t i o n to this  study is g r a t e f u l l y acknowledged. The w r i t e r i s indebted to Dr. M. O z i m a of the  Geophysi-  c a l Institute, U n i v e r s i t y of Tokyo, f o r h i s a s s i s t a n c e with the m a s s s p e c t r o m e t r y d u r i n g the i n i t i a l phase of this r e s e a r c h . Dr. O z i m a a l s o c o o r d i n a t e d the w o r k of the two l a b o r a t o r i e s and was a v i s i t i n g p r o f e s s o r at U. B.C. 1969.  f r o m M a y to O c t o b e r ,  D u r i n g this p e r i o d the w r i t e r benefited f r o m m a n y  p r o f i t a b l e d i s c u s s i o n s with  hiirK  Dr. R u s s e l l s h a r e d m u c h of h i s e x p e r i e n c e i n m a s s spect r o m e t r y w i t h the w r i t e r and p r o v i d e d able s u p e r v i s i o n throughout this r e s e a r c h .  The w r i t e r i s p a r t i c u l a r l y g r a t e f u l f o r  Dr. R u s s e l l ' s a s s i s t a n c e i n s o l v i n g many of the p r o b l e m s r e l a t e d to m a s s s p e c t r o m e t e r  i n s t r u m e n t a t i o n and data p r o c e s s i n g .  Without the generous c o o p e r a t i o n of other graduate  students and f a c u l t y m e m b e r s this w o r k would not have been possible.  In p a r t i c u l a r , the close c o o p e r a t i o n of John O z a r d ,  John B l e n k i n s o p and L i l y L e e was a r e a l asset. The e x c e l l e n t s e r v i c e s of the Computing C e n t e r at U. B. C. d e s e r v e a s p e c i a l w o r d of thanks.  T h e i r facilities were  u s e d e x t e n s i v e l y and w e r e always r e l i a b l e . F i n a n c i a l support came p r i m a r i l y f r o m the N a t i o n a l R e s e a r c h C o u n c i l of Canada i n the f o r m of f a c u l t y grants and a studentship to the w r i t e r . A 40 g r a m s a m p l e of the Abee m e t e o r i t e was p r o v i d e d by the G e o l o g i c a l S u r v e y of Canada.  1  CHAPTER 1  INTRODUCTION  1. 1 C h o n d r i t e s - R e m n a n t s of P r i m i t i v e P l a n e t a r y M a t e r i a l The study of m e t e o r i t e s has l e d to a vast amount of e x p e r i m e n t a l data without m u c h u n a n i m i t y of i n t e r p r e t a t i o n . Yet, i n m a n y r e s p e c t s , the m e t e o r i t e s contain the best r e c o r d a v a i l a b l e to us f o r events w h i c h o c c u r r e d d u r i n g the h i s t o r y of the s o l a r s y s t e m . M e t e o r i t e s c o n s i s t p r i m a r i l y of s i l i c a t e , m e t a l l i c and sulphide phases; the three phases often o c c u r together, i n undifferentiated form.  T h e r e are three m a i n c l a s s e s of m e t e o r -  ites w h i c h a r e b a s e d on the p r o p o r t i o n s of the s i l i c a t e and m e t a l l i c phases: stones, i r o n s and s t o n y - i r o n s . s u b d i v i d e d into chondrites  The  stones are f u r t h e r  ( m e t e o r i t e s containing round s i l i c a t e  g r a i n s c a l l e d chondrules i n a m a t r i x of s i l i c a t e s and m e t a l s ) and a c h o n d r i t e s ( m e t e o r i t e s l a c k i n g c h o n d r u l e s ) .  The  achon-  d r i t e s r e s e m b l e t e r r e s t r i a l igneous r o c k s , w h i l e the i r o n s are thought to be s i m i l a r to the m a t e r i a l of the earth's c o r e .  It i s  w i d e l y b e l i e v e d that the a c h o n d r i t e s and.irons are b r o k e n f r a g ments of one (or m o r e than one) parent body having a d i a m e t e r in e x c e s s of 10 km.  Since the extent of d i f f e r e n t i a t i o n into m e t a l l i c  and s i l i c a t e f r a c t i o n s has not been f i r m l y e s t a b l i s h e d , the stonyi r o n s may  r e p r e s e n t either m a t e r i a l f r o m the v i c i n i t y of a  2  c o r e - m a n t l e b o u n d a r y or f r a g m e n t s d e r i v e d f r o m the contact s u r f a c e s between m e t a l l i c i n c l u s i o n s of m e t e o r i t i c s i z e and a stony matrix. The r e m a i n i n g c l a s s of m e t e o r i t e s , the c h o n d r i t e s , c o m p r i s e s 8 5 % of a l l m e t e o r i t e s , b a s e d on the n u m b e r of r e c o v e r e d m e t e o r i t e s w h i c h were o b s e r v e d to f a l l .  D e t a i l e d studies of t h e i r  m i n e r a l o g y and c h e m i s t r y have l e d m o s t r e s e a r c h e r s to b e l i e v e that the c h o n d r i t e s r e p r e s e n t the m o s t p r i m i t i v e m a j o r c l a s s of m e t e o r i t e s ( A n d e r s , 1964;  Wood, 1968).  It i s t h e r e f o r e this  c l a s s w h i c h p r o v i d e s the c r u c i a l test of any t h e o r i e s f o r the o r i g i n of m e t e o r i t e s .  The b u l k c h e m i c a l c o m p o s i t i o n of the c h o n d r i t e s  c l o s e l y r e s e m b l e s the s o l a r c h e m i c a l c o m p o s i t i o n , n e g l e c t i n g v o l a t i l e e l e m e n t s . ( F i g u r e 1-1).  T h i s fact, i n a d d i t i o n to the  unique c h o n d r i t i c t e x t u r e , s t r o n g l y supports the b e l i e f that these a r e v e r y p r i m i t i v e objects. 1. 2  Q u a n t i z a t i o n of the O x i d a t i o n State of C h o n d r i t e s Although the b u l k c h e m i c a l c o m p o s i t i o n of c h o n d r i t e s i s  s u r p r i s i n g l y u n i f o r m , there e x i s t m a r k e d oxidation states.  differences in their  In fact, the o x i d a t i o n states b a s e d on e l e m e n t a l  and o x i d i z e d i r o n abundances appear to be quantized.  Mason  (1962) has plotted weight per cent i r o n (as m e t a l or as FeS) against weight per cent o x i d i z e d i r o n (as s i l i c a t e s ) for s e v e r a l p r e c i s e analyses of c h o n d r i t e s ( F i g u r e 1-2).  S i x d i s t i n c t groups are  3  FIGURE 1-1.  ABUNDANCES OF THE METALLIC  ELEMENTS  Atmosphere o f the sun  10~  2  IO  - 1  1  10 Ordinary  10  2  10  3  10^  IO  5  10  chondrites  E l e m e n t a l abundances i n t h e atmosphere o f t h e sun ( r e l a t i v e t o 1 0 " S i a t o m s ) a r e compared w i t h t h o s e i n o r d i n a r y ( h y p e r s t h e n e and b r o n z i t e ) c h o n d r i t e s . Elements w i t h l o w b o i l i n g p o i n t s have b e e n e x c l u d e d . Note t h e h i g h a b u n d a n c e o f L i i n c h o n d r i t e s when compared w i t h t h e sun. The d i a g r a m was r e p r o d u c e d f r o m M e t e o r i t e s and t h e O r i g i n o f P l a n e t s by J o h n A. Wood, 196W.  4  evident, ranging f r o m f u l l y r e d u c e d enstatite c h o n d r i t e s to f u l l y o x i d i z e d carbonaceous c h o n d r i t e s . M a s o n (1963) p r o p o s e d that d i f f e r e n c e s i n o x i d a t i o n . state a r o s e i n the p l a n e t a r y nebula.  The enstatite c h o n d r i t e s  r e p r e s e n t h i g h l y r e d u c e d m a t e r i a l f r o m the hot i n n e r r e g i o n s , w h i l e the carbonaceous chondrites r e p r e s e n t m o r e p r i m i t i v e , h i g h l y o x i d i z e d m a t e r i a l w h i c h a c c r e t e d i n the c o l d outer regions of the nebula. The q u a n t i z a t i o n of the o x i d a t i o n states may  result from  the a c c r e t i o n of m a t e r i a l i n the nebula to f o r m p l a n e t a r y b o d i e s , e a c h of w h i c h p o s s e s s e d p h y s i c a l and c h e m i c a l p r o p e r t i e s w h i c h w e r e r e p r e s e n t a t i v e of the r e g i o n f r o m w h i c h i t was  derived.  The c h o n d r i t i c m e t e o r i t e s may then r e p r e s e n t a b i a s e d s a m p l e of c o l l i s i o n f r a g m e n t s f r o m only four or five p l a n e t a r y bodies. The  r e d u c t i o n of iron-magnesium.; s i l i c a t e s p r o c e e d s by  a d e c r e a s e i n the F e O  content of the s i l i c a t e s and a  correspond-  ing i n c r e a s e i n the amount of F e i n the m e t a l l i c phase ( A n d e r s , 1964;  Ringwood, 1966;  M i y a s h i r o , 1968).  The MgO  i n the  silicates remains essentially unaltered until extreme reduction has o c c u r r e d .  T h e r e f o r e , the m o l e f r a c t i o n f — F e O / ( M g O + F e O )  i n s i l i c a t e s i s a u s e f u l index of the degree of o x i d a t i o n . plotting frequency of o c c u r r e n c e  By  against f for c h o n d r i t e s , M i y a s h i r o  (1968) d r e w attention to the l a r g e hiatus between enstatite chondr i t e s and the other groups ( F i g u r e 1-3).  He o f f e r e d two p o s s i b l e  5  FIGURE 1-2.  Weight per  cent  iron i n metal  OXIDIZED AND REDUCED IRON IN CHONDRITES  E - Enstatite  35 i « 30 25  « 1 •  | .i  B - Bronzite  E  and FeS  A - Amphoteric  I" w  V  20  X  V  chondrites  - Hypersthene  H  4  chondrites  *  P - Pigeonite  chondrites chondrites  chondrites  C - Carbonaceous 15  chondrites  • ••  H  10  a  X  P  °p  A  5  X  B -  ° C 0  •  c)  5  •  10  i  15  20  25  30  Weight p e r cent o x i d i z e d i r o n R e l a t i o n s h i p between reduced i r o n (metal + FeS) and o x i d i z e d i r o n (FeO) i n c h o n d r i t e s . The diagram was adapted from Mason (1962) and Anders (196^).  FIGURE 1-3.  FREQUENCY OF OCCURRENCE VS OXIDATION STATE  4  B  H  FeO FeO + MgO Oxidation state  of s i l i c a t e s  The frequency of occurrence i s p l o t t e d as a f u n c t i o n o f the f - v a l u e ( o x i d a t i o n s t a t e ) f o r c h o n d r i t e s ( a f t e r M i y a s h i r o , 1967). See F i g u r e 1-2 f o r i d e n t i f i c a t i o n o f the c h o n d r i t e c l a s s e s .  6  explanations f o r this gap: (a) Since c a r b o n and h y d r o g e n are the p r i n c i p l e r e d u c i n g agents, the l a r g e gap between the enstatite chondrites and the o r d i n a r y c h o n d r i t e s r e p r e s e n t s a m a r k e d d i f f e r e n c e i n the r e d u c i n g power of these two elements.  C a r b o n was  the  p r i n c i p l e r e d u c i n g agent at h i g h e r t e m p e r a t u r e s c l o s e to the p r o t o s u n w h i l e h y d r o g e n r e d u c t i o n dominated i n the c o o l e r , outer r e g i o n s . (b) A n a l t e r n a t i v e explanation i s that the gap i n the r e g i o n f =0.01-0.14 c o r r e s p o n d s to the oxidation state of the p r i m i t i v e m a t e r i a l f r o m w h i c h the e a r t h a c c r e t e d . an F e O  B i r c h (1964) r e p o r t e d  content for the mantle of 10-2 weight per cent.  This  i m p l i e s that the amounts of o x i d i z e d and r e d u c e d i r o n for the e a r t h as a whole ( a s s u m i n g the core contains 9 0 % m e t a l l i c i r o n ) are a p p r o x i m a t e l y 4% and 2 3 % r e s p e c t i v e l y .  On this b a s i s ,  the o x i d a t i o n state of the e a r t h i s i n t e r m e d i a t e between groups E and B i n F i g u r e 1-2.  The f-value of the e a r t h i s l e s s c e r t a i n  because of the unknown MgO  content of the mantle.  If e i t h e r of M i y a s h i r o ' s explanations i s v a l i d , then v a r i a tions i n the o x i d a t i o n state of chondrites r e s u l t f r o m d i f f e r e n c e s i n the  .amount of s o l a r i r r a d i a t i o n incident on the p r i m i t i v e m a t e r i a l  f r o m w h i c h they w e r e f o r m e d .  It i s g e n e r a l l y accepted that the con-  t r a c t i n g p r o t o s u n p a s s e d through a phase when its l u m i n o s i t y was  7  s e v e r a l hundred time.s i t s p r e s e n t - d a y value ( H a y a s h i , 1966; H o y l e et a l , 1968).  The t e m p e r a t u r e  d i s t r i b u t i o n i n .the s o l a r  nebula has not been e s t a b l i s h e d , however. On the b a s i s of M i y a s h i r o ' s h y p o t h e s i s , one would expect the enstatite c h o n d r i t e s to show other evidence of t h e i r c l o s e p r o x i m i t y to the sun i n t h e i r e a r l y h i s t o r y .  Such evidence m a y be  r e v e a l e d through a c o m p a r i s o n of the i r r a d i a t i o n h i s t o r i e s of m e t e o r i t i c and t e r r e s t r i a l m a t e r i a l .  The e a r t h p r o v i d e s a convenient  s t a n d a r d of r e f e r e n c e against w h i c h the o r b i t a l p o s i t i o n s of other p l a n e t a r y bodies m i g h t be c o m p a r e d . S e v e r a l t h e o r i e s f o r the f o r m a t i o n of the s o l a r s y s t e m postulate the e x i s t e n c e of a p e r i o d of high e n e r g y ( ^ v 500 M e V ) p a r t i c l e i r r a d i a t i o n , of s o l a r o r i g i n , d u r i n g the c o l l a p s e of the p r o t o s u n and the t r a n s f e r of angular m o m e n t u m to the s u r r o u n d i n g nebula v i a a magnetic coupling.  The high e n e r g y p a r t i c l e f l u x  would r e s e m b l e the p r e s e n t - d a y p r i m a r y c o s m i c r a y f l u x , although its i n t e n s i t y was p r o b a b l y s e v e r a l o r d e r s of magnitude g r e a t e r . When these p a r t i c l e s ( m a i n l y protons and other light n u c l e i s t r i p p e d of t h e i r e l e c t r o n s ) b o m b a r d other n u c l e i they c h i p , o r spall, nucleons f r o m t h e m , generating a l a r g e n u m b e r of s e c o n d a r y particles.  In p a r t i c u l a r , the n u m b e r of neutrons p r o d u c e d b y  these s p a l l a t i o n r e a c t i o n s w o u l d be c o m p a r a b l e to the n u m b e r of incident p r i m a r y p a r t i c l e s ( F o w l e r et a l , 19 62).  B o t h the p r i m a r y  8  s p a l l a t i o n r e a c t i o n s and s e c o n d a r y neutron capture p r o c e s s e s a r e capable of a l t e r i n g the isotopic c o m p o s i t i o n of theelements  ;  i n the i r r a d i a t e d m a t e r i a l .  1.3  H i g h E n e r g y P a r t i c l e I r r a d i a t i o n and its Consequences G a d o l i n i u m , s a m a r i u m and e u r o p i u m a r e known to have  e n o r m o u s l y h i g h t h e r m a l neutron capture c r o s s s e c t i o n s w h i c h m a y b e a t t r i b u t e d to a few s p e c i f i c isotopes (Table 1-1). Hence  Isotope  Cross Section  Gd"  242,000 t 4000 b a r n s  G d  7  1  "  Sm ? 14  Eu Eu  1 5 1  1 5 3  _ 58,000 ± 3000 :  40,800 t 900 7, 800 t 200 450.± 20  T A B L E 1-1 T h e r m a l (20° C) n e u t r o n capture c r o s s s e c t i o n s f o r (n,V ) r e a c t i o n s ( r e p o r t e d i n Cook et a l , 1968).  these m a y s e r v e as  h i g h l y s e n s i t i v e neutron f l u x i n d i c a t o r s .  An  e x a m i n a t i o n of the i s o t o p i c c o m p o s i t i o n s of Gd, Sm and E u i n m e t e o r i t e s and i n t e r r e s t r i a l s a m p l e s would r e v e a l any s i g n i f i c a n t differences in their irradiation histories.  In p a r t i c u l a r , a study  9  of these elements i n enstatite c h o n d r i t e s m i g h t p r o v i d e a c r i t i c a l test of the h y p o t h e s i s of M i y a s h i r o . P r i o r to 1969, no n a t u r a l l y - o c c u r r i n g i s o t o p i c a n o m a l i e s had been detected f o r these e l e m e n t s , e i t h e r i n t e r r e s t r i a l or i n meteoritic samples. however.  No enstatite c h o n d r i t e s had been i n v e s t i g a t e d ,  R e c e n t advances i n i n s t r u m e n t a t i o n have made it p o s s i b l e  to m e a s u r e i s o t o p i c r a t i o s f o r m i c r o g r a m s a m p l e s of Gd, Sm and E u w i t h i n 0.1% s t a n d a r d deviation.  Using very precise tech-  niques E u g s t e r et a l (1970a, 1970b) w e r e able to detect a d e f i c i e n c y 1 ^ 7  of Gd  1 i n l u n a r r o c k and s o i l s a m p l e s , as w e l l as i n the N o r t o n  County a c h o n d r i t e .  The d e f i c i e n c y of Gdl57 r e l a t i v e to t e r r e s t r i a l  Gdl57, and the c o r r e s p o n d i n g enhancement i n G d  1 5 8  ( F i g u r e 1-4)  m a y c l e a r l y be a t t r i b u t e d to the Gdl57 (n,X )Gdl58 r e a c t i o n .  The  neutrons r e q u i r e d f o r this p r o c e s s w e r e p r o d u c e d by s p a l l a t i o n r e a c t i o n s o c c u r r i n g i n the s u r f a c e l a y e r s of the m o o n and the parent m e t e o r i t e f r a g m e n t , due to b o m b a r d m e n t b y c o s m i c r a y s . C o s m i c r a y s i n the 500 M e V range a r e capable of penet r a t i n g a p p r o x i m a t e l y lOOg/crri^ of m a t t e r .  T h i s i s the m e a n f r e e  path f o r the p r i m a r y p a r t i c l e s w h i c h induce s u c h s p a l l a t i o n r e a c t i o n s as Fe  5 6  + H ^ C l  3  6  + H + ZHS 3  + He  3  + 3 r f + 4 neutrons  T B o t h E u g s t e r et a l (1970b) and L u g m a i r (1970) r e p o r t e d e r r o r s of 0.01% (two s t a n d a r d d e v i a t i o n s of the mean) f o r l u n a r G d a n a l y s e s .  10  0.712 *  «  0.713 1  '  Integrated FIGURE 1-4.  1  15  J  1  thermal  0.714 !  1  1  10  I  neutron f l u x  0.715 I  I  '  (10  .  0.716 .  .  »  .  o  5  D  neutrons/cm )  THERMAL NEUTRON CAPTURE IN LUNAR GADOLINIUM  Gd 58/ d vs G d ^ / G d (normalized to G d ^ / G d .9361) f o r s e v e r a l l u n a r samples from A p o l l o 11. Lunar samples i n c l u d e s i x rocks, one "breccia, two core samples and s o i l . The Gd i s o t o p i c r a t i o s are c l e a r l y s h i f t e d ( r e l a t i v e t o t e r r e s t r i a l Gd) due t o neutron capture. Neutrons are c o n t i n u o u s l y produced d u r i n g bombardment o f the l u n a r s u r f a c e by cosmic r a y s . The c o r r e l a t i o n diagram and a l l data were p u b l i s h e d by Eugster e t a l (1970b). 1  1 6 0  G  1 6 0  1 6 0  =  11  H i g h e n e r g y neutrons a r e a product of these r e a c t i o n s . In the p r e s e n c e of light n u c l e i such as hydrogen, the neutrons a r e r e a d i l y t h e r m a l i z e d . ^ The t h e r m a l i z a t i o n p r o c e s s i n v o l v e s s l o w i n g down high energy p a r t i c l e s b y m u l t i p l e c o l l i s i o n s w i t h other n u c l e i , u n t i l e n e r g i e s i n the t h e r m a l , range (^20 C) have been reached. Since r e a c t i o n c r o s s s e c t i o n s a r e m u c h g r e a t e r f o r slow neutrons, the m a j o r i t y of the neutrons w i l l be captured only after r e a c h i n g thermal velocities.  In l o w density m e d i a , a s i g n i f i c a n t f r a c t i o n  of the neutrons m a y undergo r a d i o a c t i v e decay into protons and e l e c t r o n s , since the h a l f - l i f e of a neutron i s only 11 minutes. N e u t r o n decay w i l l be dominant only i n d i s p e r s e d m e d i a having m e a n d e n s i t i e s ~ 10-7  g/  c m  3 ( F o w l e r et a l , 1962).  The neutron  thermal-  i z a t i o n and capture p r o c e s s e s o c c u r o v e r a range of a p p r o x i m a t e l y 30 g/cm^.  It i s t h e r e f o r e evident that the s e c o n d a r y neutrons  s t r a y only a s h o r t distance f r o m the p r i m a r y c o s m i c r a y path and are confined to the v o l u m e i r r a d i a t e d by the high energy p a r t i c l e s . Although the p e n e t r a t i o n depth of c o s m i c r a y s v a r i e s w i t h the e n e r g y of the p r i m a r y p a r t i c l e s and the c o m p o s i t i o n of the i r r a d i a t e d m a t e r i a l , a range of 1 to 4 m e t e r s i s probable for *>r 500 M e V p a r t i c l e s p e n e t r a t i n g m a t e r i a l s c o m p a r a b l e to o r d i n a r y r o c k with a density of 2 to 4 g/cm^.  The s u r f a c e of the e a r t h i t s e l f i s  l a r g e l y s h i e l d e d f r o m c o s m i c r a y s by the 10^ g/cm^ of a i r i n its a t m o s p h e r e .  12  Outside the earth's a t m o s p h e r e the p r e s e n t - d a y f l u x of p r i m a r y c o s m i c r a y s having e n e r g i e s 2  cm  '  sec.  200 M e V  i s 1 to 10 p a r t i c l e s /  r  The b u l k of this f l u x i s of g a l a c t i c o r i g i n and appears  to have b e e n of u n i f o r m i n t e n s i t y throughout the p r e v i o u s h i s t o r y of our s o l a r s y s t e m . our sun.  Only a few per cent of the flux i s d e r i v e d f r o m  T h i s f r a c t i o n i s of v a r i a b l e i n t e n s i t y , with m a x i m a  o c c u r r i n g d u r i n g p e r i o d s of intense s o l a r f l a r e a c t i v i t y . A l t h o u g h the p r e s e n t - d a y high energy p a r t i c l e f l u x f r o m the sun i s s m a l l r e l a t i v e to the g a l a c t i c f l u x , there i s c o n s i d e r a b l e evidence to suggest that this has not a l w a y s been the case. S e v e r a l t h e o r i e s f o r the o r i g i n of the s o l a r s y s t e m propose the e x i s t e n c e of an intense high energy p a r t i c l e f l u x d u r i n g i t s e a r l y h i s t o r y ( B u r b i d g e et a l , 1957; 1965; (10  H a y a s h i , 1966). to 10  F o w l e r et a l , 1962;  Cameron,  Although the i r r a d i a t i o n t i m e s a r e s h o r t  y e a r s ) the p r o p o s e d values of the i n t e g r a t e d high energy  p a r t i c l e f l u x range f r o m 1 0 ^ to 1 0 ^ p r o t o n s / c m ^ .  T h i s may be 17  d i r e c t l y c o m p a r e d w i t h the i n t e g r a t e d c o s m i c r a y f l u x  of 10  to  10^ p r o t o n s / c m ^ s i n c e the f o r m a t i o n of the e a r t h and m e t e o r i t e 8  parent bodies a p p r o x i m a t e l y 4. 5 b i l l i o n y e a r s  ago.  If an intense high e n e r g y p r o t o n f l u x was e m i t t e d f r o m the p r o t o s u n d u r i n g the p e r i o d p r i o r to the f o r m a t i o n of the m a j o r p l a n e t a r y b o d i e s , then one would expect v a r y i n g d e g r e e s of i r r a d i a t i o n of the m a t e r i a l of the nebula depending upon i t s distance f r o m the protosun.  The high e n e r g y p a r t i c l e s would induce  spallation  13  r e a c t i o n s i n the m a t e r i a l of the nebula w h i c h , i n t u r n , would generate neutrons.  Where l o c a l condensation of the m a t e r i a l had  c o c c u r r e d , a l a r g e f r a c t i o n of the neutrons would be t h e r m a l i z e d and captured;  the r e m a i n d e r would decay.  The m a j o r i t y of the captured neutrons w o u l d r e a c t with H . O , F e , N i , S and S i n u c l e i w h i c h h a v e high absolute abundances, although t h e i r c r o s s s e c t i o n s are s m a l l (< 5 b a r n s ) .  These e l e m e n t s  are not i d e a l l y s u i t e d f o r subsequent detection of the i r r a d i a t i o n effects s i n c e the r e s u l t i n g changes i n i s o t o p i c c o m p o s i t i o n s m a l l (except f o r H).  are  The e l e m e n t s w h i c h show the l a r g e s t m e m o r y  effect a f t e r i r r a d i a t i o n are those having isotopes w i t h v e r y high t h e r m a l neutron capture c r o s s sections (e. g. Gd, Sm,  Eu) or  those having isotopes of low i n i t i a l abundance (e. g. K, V). i s o t o p i c abundances of s e v e r a l of the l i g h t e r elements B,C)  The  (H,He,Li,  a r e a l s o v e r y s e n s i t i v e to p a r t i c l e i r r a d i a t i o n , although the  n u c l e a r r e a c t i o n s i n w h i c h they p a r t i c i p a t e are m u c h m o r e d i f f i c u l t to r e s o l v e . 1.4  D e l i n e a t i o n of the P r e s e n t Study The i n v e s t i g a t i o n of Gd, Sm  was  and E u i n enstatite chondrites  f i r s t p r o p o s e d to the w r i t e r by Dr. M.  Institute, U n i v e r s i t y of Tokyo.  O z i m a of the  Geophysical  Dr. O z i m a was v i s i t i n g p r o f e s s o r  i n the D e p a r t m e n t of G e o p h y s i c s at the U n i v e r s i t y of B r i t i s h C o l u m b i a d u r i n g the p e r i o d May  to O c t o b e r , 1 9 6 9 .  He and Dr. M a b u c h i  14  of the D e p a r t m e n t of C h e m i s t r y , r e s e a r c h into this p r o b l e m .  U n i v e r s i t y of Tokyo, i n i t i a t e d  The p r e s e n t w r i t e r was i n v i t e d to  a s s u m e r e s p o n s i b i l i t y f o r the i s o t o p i c a n a l y s e s of Gd, Sm and E u because of h i s e x p e r i e n c e i n m a s s s p e c t r o m e t r y .  The c h e m i s t r y  of the r a r e e a r t h e l e m e n t s e p a r a t i o n s was u n d e r t a k e n b y M r . Y a n a g i t a under the d i r e c t i o n of Dr. Mabuchi.  This research is therefore  a joint p r o j e c t between the U n i v e r s i t y of Tokyo and our own l a b o r a t o r y . The p r e v i o u s r e s e a r c h of the w r i t e r was i n the f i e l d of ion o p t i c s at the U n i v e r s i t y of Toronto.  A c o m p u t e r p r o g r a m was  w r i t t e n to c a l c u l a t e i o n t r a j e c t o r i e s through electrostatic lenses.  two-dimensional  T h i s method was s u f f i c i e n t l y g e n e r a l to be  a p p l i e d to any r e a l i s t i c lens c o n f i g u r a t i o n encountered i n m a s s spectrometer  ion s o u r c e s o p e r a t i n g at low b e a m i n t e n s i t i e s .  Ion b e a m s w e r e computed f o r three conventional i o n s o u r c e s w h i c h w e r e shown to have l o w t r a n s m i s s i o n e f f i c i e n c i e s ( L o v e l e s s , 1967). The f i r s t y e a r of r e s e a r c h at the U n i v e r s i t y of B r i t i s h C o l u m b i a was devoted to the d e s i g n of a m o r e e f f i c i e n t ion s o u r c e .  This  l e d to a c l e a r d e f i n i t i o n of the fundamental l i m i t a t i o n to the degree of f o c u s s i n g w h i c h m a y be a c h i e v e d i n an ion s o u r c e .  In p a r t i c u l a r ,  it was p o s s i b l e to e s t i m a t e the m a x i m u m t r a n s m i s s i o n e f f i c i e n c y of an i d e a l i o n source i n t e r m s of b a s i c p a r a m e t e r s of the m a s s s p e c t r o m e t e r and the i o n i z a t i o n p r o c e s s ( L o v e l e s s and R u s s e l l , 1969). T h i s had not been r e p o r t e d b e f o r e i n the m a s s literature.  spectrometer  15  A s i m p l e ion s o u r c e having a p p r o x i m a t e l y  one half the  o p t i m u m t r a n s m i s s i o n e f f i c i e n c y was then designed and b u i l t . T h i s gave a f a c t o r of five i n c r e a s e i n s e n s i t i v i t y ( c o m p a r e d with the ion s o u r c e p r e v i o u s l y i n use) when tested on the a v a i l a b l e m a s s s p e c t r o m e t e r (with'.a 12.in r a d i u s .90 de_g. s e c t o r , s i n g l e stage m a s s a n a l y z e r ) .  A f u r t h e r f a c t o r of two gain i n s e n s i t i v i t y  was a c h i e v e d by attaching second o r d e r f o c u s s i n g s h i m s to the a n a l y z e r magnet, so that a b e a m of higher angular  divergence  could be t r a n s m i t t e d through the a n a l y z e r without l o s s of r e s o l u t i o n . These fundamental i m p r o v e m e n t s i n i n s t r u m e n t s e n s i t i v i t y , c o m b i n e d with the i m p l e m e n t a t i o n of d i g i t a l r e c o r d i n g of m a s s s p e c t r a , w e r e s i g n i f i c a n t c o n t r i b u t i o n s by the w r i t e r i n the f i e l d of m a s s s p e c t r o meter instrumentation.  These m o d i f i c a t i o n s s u b s t a n t i a l l y i n c r e a s e d  the p r e c i s i o n of i s o t o p i c analyses on the r a r e e a r t h elements. A 40 g r a m s a m p l e of the Abee enstatite chondrite was obtained f r o m D r . Douglas of the G e o l o g i c a l S u r v e y of Canada. T h i s s a m p l e was p a r t of s p e c i m e n 13b taken f r o m an e q u a t o r i a l s l i c e through the m e t e o r i t e . nearest fusion crust surface.  It was located 9-10 c m f r o m the (The m e t e o r i t e was o b s e r v e d to  f a l l on June 9, 19 52 and was found five days l a t e r i n a wheat f i e l d , n e a r A b e e , j u s t n o r t h of E d m o n t o n , A l b e r t a ( M i l l m a n , 1953). It had a r e c o v e r y m a s s of 107 kg and a s p e c i f i c g r a v i t y of 3. 5. The m a j o r constituents of the m e t e o r i t e a r e enstatite (MgSiC^) -  16  48%, k a m a c i t e - t a e n i t e ( n i c k e l - i r o n a l l o y s ) - 2 2 % , and t r o i l i t e (FeS) - 13%. mineralogy,  D a w s o n et al(19 6 0 ) r e p o r t f u r t h e r d e t a i l s of the c h e m i s t r y and p e t r o l o g y of the Abee m e t e o r i t e . )  It i s evident that s e v e r a l f a c t o r s c o m b i n e d f a v o u r a b l y to m a k e this r e s e a r c h p r a c t i c a l .  Although our l a b o r a t o r y has  had c o n s i d e r a b l e e x p e r i e n c e i n the i s o t o p i c a n a l y s i s of elements in the h i g h e r m a s s range (Rb, S r , Pb, U), the r a r e e a r t h elements had n e v e r been i n v e s t i g a t e d p r e v i o u s l y at this institution.  This  c h a l l e n g e , i n a d d i t i o n to the s i g n i f i c a n c e of the a s t r o p h y s i c a l p r o b l e m i t s e l f , p r o v i d e d the i n t e r e s t and m o t i v a t i o n for the w r i t e r ' s c o n t r i b u t i o n s to this study. The s p e c i f i c c o n t r i b u t i o n s of the w r i t e r to the c o m b i n e d r e s e a r c h project are: (a) I m p r o v e m e n t s i n i n s t r u m e n t a t i o n .  A n o r d e r of magnitude  i n c r e a s e i n s e n s i t i v i t y was a c h i e v e d b n the a v a i l a b l e m a s s spectrometer  through i m p r o v e m e n t s to the i o n o p t i c s .  The  i n s t r u m e n t was a l s o i n t e r f a c e d w i t h a magnetic tape r e c o r d e r for d i g i t a l r e c o r d i n g of m a s s s p e c t r a . (b) The development of p r o c e d u r e s . f o r a n a l y z i n g the i s o t o p i c c o m p o s i t i o n of 1 m i c r o g r a m quantities of Gd, Sm and E u w i t h i n a s t a n d a r d d e v i a t i o n of 0. 1% o r l e s s .  T h i s includes  e x p e r i m e n t a l and c o m p u t a t i o n a l techniques f o r r e m o v i n g the effects of t r a c e quantities of i n t e r f e r i n g s p e c t r a f r o m neighb o u r i n g e l e m e n t s , due to incomplete c h e m i c a l s e p a r a t i o n .  17  (c) The p r e c i s e i s o t o p i c a n a l y s i s of Gd,  Sm  and E u i n  t e r r e s t r i a l ores d e r i v e d f r o m two different geographic l o c a t i o n s (USA  and B r a z i l ) and i n the Abee enstatite chondrite.  The a n a l y s i s of the above s a m p l e s would r e v e a l any i s o t o p i c a n o m a l i e s as s m a l l as 0. 1 - 0. 3% due to v a r i a t i o n s i n p r e v i o u s e x p o s u r e to t h e r m a l neutrons.  The two t e r r e s t r i a l ores  w e r e studied to e s t a b l i s h the c o n s i s t e n c y of the t e r r e s t r i a l r a t i o s , and to p r o v i d e a r e l i a b l e standard against w h i c h the m e t e o r i t i c r a t i o s could be c o m p a r e d . The p r i m a r y objective of this r e s e a r c h i s to s e a r c h for an i r r a d i a t i o n a n o m a l y i n the Abee enstatite chondrite. evidence w h i c h suggests that the p r o t o s u n was  There is  a s t r o n g e m i t t e r of  high e n e r g y p a r t i c l e s d u r i n g the e a r l y h i s t o r y of the s o l a r s y s t e m , p r i o r to the f o r m a t i o n of the planets and parent m e t e o r i t e bodies. The e x i s t e n c e of an i r r a d i a t i o n a n o m a l y for Abee would give s t r o n g support to the hypotheses of M a s o n and M i y a s h i r o :  that v a r i a t i o n s  i n the o x i d a t i o n state of c h o n d r i t i c m e t e o r i t e s r e s u l t f r o m d i s t i n c t d i f f e r e n c e s i n the m e a n distance f r o m the sun of the p r i m i t i v e . m a t e r i a l f r o m w h i c h the c h o n d r i t e s w e r e d e r i v e d .  18  CHAPTER  A R E V I E W OF  THE  2  EVIDENCE CONCERNING IRRADIATION ANOMALIES  2. 1 Synthesis of the E l e m e n t s and H i g h E n e r g y P a r t i c l e I r r a d i a t i o n B u r b i d g e , B u r b i d g e , F o w l e r and H o y l e (1957) s u c c e s s f u l l y e x p l a i n e d the s y n t h e s i s of m o s t e l e m e n t s on the b a s i s of chains of thermal nuclear reactions ocurring in stellar interiors. is b a s e d on the a s s u m p t i o n that only h y d r o g e n is p r i m e v a l .  T h e i r theory Successive  stages of h y d r o g e n b u r n i n g , h e l i u m b u r n i n g , and m o r e c o m p l e x n u c l e a r p r o c e s s e s a r e capable of s y n t h e s i z i n g m o s t e l e m e n t s i n the i n t e r i o r of s t a r s .  H o w e v e r , the high t e m p e r a t u r e  and p r e s s u r e w h i c h p l a y  the e s s e n t i a l r o l e i n t h e i r t h e o r y would d e s t r o y d e u t e r i u m  (D),  l i t h i u m ( L i ) , b e r y l l i u m (Be) and b o r o n (B) r a t h e r than s y n t h e s i z i n g them.  E v e n at m o d e r a t e l y low t e m p e r a t u r e s these e l e m e n t s would  be r a p i d l y c o n v e r t e d to h e l i u m by p r o t o n b o m b a r d m e n t . I n o r d e r to e x p l a i n the e x i s t e n c e of these e l e m e n t s i n m e t e o r i t e s and the e a r t h (note the L i abundance i n F i g u r e 1-1) F o w l e r , G r e e n s t e i n and H o y l e ( F G H , of the s o l a r nebula was  1962) p r o p o s e d that the m a t e r i a l  subjected to h i g h e n e r g y p a r t i c l e i r r a d i a t i o n  f r o m the p r o t o s u n p r i o r to the f o r m a t i o n of l a r g e p l a n e t a r y bodies. The m o s t i m p o r t a n t i n t e r a c t i o n s would be those of high e n e r g y (*v 500 protons and a l p h a - p a r t i c l e s , w i t h abundant n u c l e i such as 0^, ~28 56 Si and Fe .  The elements D, L i , Be and B would be  Mg^,  products  Mev)  19  of such s p a l l a t i o n r e a c t i o n s .  The i s o t o p i c c o m p o s i t i o n of some of  these elements should d i f f e r f r o m the t e r r e s t r i a l v a l u e s , h o w e v e r , if they were p r o d u c e d by s p a l l a t i o n alone. t r i a l v a l u e s of L i / L i ^ and B ^ / B  In p a r t i c u l a r , the t e r r e s - •  * a r e 0.08 and 0.23  respectively,  w h e r e a s the p r e d i c t e d s p a l l a t i o n y i e l d s should give r a t i o s c l o s e to unity. ~i  "Fowler et a l (1962) d r e w attention to the l a r g e t h e r m a l neutron c r o s s s e c t i o n s f o r the r e a c t i o n s L i ^ ( n , 0<  and  B^(n,C<)Li^.  T h e y showed q u a n t i t a t i v e l y that an i n t e g r a t e d t h e r m a l neutron f l u x of 4 x l O ^ n e u t r o n s / c m ^ would be capable of p r o d u c i n g the o b s e r v e d 1  i s o t o p i c r a t i o s f o r L i and B.  The a s s u m p t i o n that t h e r m a l neutrons  e x i s t e d is e n t i r e l y l o g i c a l , s i n c e neutrons a r e d i r e c t products of s p a l l a t i o n r e a c t i o n s , and t h e r m a l i z a t i o n of neutrons w i l l o c c u r p r i o r to capture as long as there i s a m o d e r a t e e x c e s s of h y d r o g e n i n the irradiated material.  ( F o w l e r et al(1962) suggested a c o m p o s i t i o n  f o r the i r r a d i a t e d m a t e r i a l of I-^Oand the oxides of Mg,  S i and F e  i n the r a t i o two to one by volume.) The t e r r e s t r i a l D^/H  r a t i o of 1.5 x 10  4  was a l s o e x p l a i n e d  by the neutron capture p r o c e s s H^(n, e" )D^, by i m p o s i n g the a d d i t i o n a l condition that only 10% of t e r r e s t r i a l m a t t e r was exposed to the i r r a d i a t i o n . B u r n e t t , F o w l e r and Hoyle (1965) l a t e r r e v i s e d this e s t i m a t e to 5%. The f r a c t i o n of t e r r e s t r i a l m a t e r i a l which was i r r a d i a t e d was  not  r e l e v a n t to the d i s c u s s i o n of L i and B, since these elements were  20  a s s u m e d to have n e g l i g i b l e abundances p r i o r to i r r a d i a t i o n .  Their  p r o d u c t i o n was attributed e n t i r e l y to i r r a d i a t i o n p r o c e s s e s , w h e r e a s H  was v e r y abundant b e f o r e i r r a d i a t i o n . In o r d e r to s h i e l d m o s t of the t e r r e s t r i a l m a t t e r f r o m the  i r r a d i a t i o n , F o w l e r et a l (1962) p r o p o s e d that the m a t e r i a l of the nebula had condensed into s o l i d p l a n e t e s i m a l s of d i m e n s i o n s f r o m 1 to 50 m e t r e s .  The high-energy p a r t i c l e s c o u l d then penetrate the  n e a r - s u r f a c e m a t e r i a l to a depth of about one m e t r e .  The t e m p e r a -  ture of the p l a n e t e s i m a l s was e s t i m a t e d to be i n the range 130-200° K. The F G H t h e o r y f o r the s y n t h e s i s of D, L i , B e , and B i s supported b y the o b s e r v e d h i g h L i abundance i n young (T T a u r i ) stars. On the other hand, and c o n t r a r y to p r e d i c t i o n , variations  v e r y few  i n i s o t o p i c c o m p o s i t i o n have yet been o b s e r v e d , between  m e t e o r i t e s and the e a r t h , f o r those n u c l i d e s w h i c h have v e r y h i g h neutron capture c r o s s s e c t i o n s , low abundance ratios,.- o r high spallation yields.  A r e v i e w of the s e a r c h f o r such a n o m a l i e s  be g i v e n i n S e c t i o n 2. 2.  will  F o r the moment, h o w e v e r , we., suggest  that one of the following a l t e r n a t i v e s  m u s t be t r u e :  (a) The F G H p r o c e s s was o p e r a t i v e but there a r e no s i g n i f i c a n t i r r a d i a t i o n a n o m a l i e s between t e r r e s t r i a l and m e t e o r i t i c s a m p l e s because of u n i f o r m i r r a d i a t i o n and d i l u t i o n ( B u r n e t t et a l , 1965). (b) The F G H p r o c e s s was not o p e r a t i v e ; D, L i , B e and  21  B w e r e not f o r m e d p r i m a r i l y by h i g h energy p a r t i c l e i r r a d i a t i o n d u r i n g the e a r l y h i s t o r y of the s o l a r s y s t e m , or  (c) The s e a r c h f o r a n o m a l i e s has not been s u f f i c i e n t l y  • thorough. A n a l t e r n a t i v e to the F G H  t h e o r y f o r the s y n t h e s i s of  D, L i , Be and B has been g i v e n by C a m e r o n (1962, 1965).  He  suggests that these elements a r e p r o d u c t s of g a l a c t i c n u c l e o s y n t h e s i s and w e r e a l r e a d y p r e s e n t i n the i n t e r s t e l l a r m e d i u m f r o m w h i c h the s o l a r s y s t e m was f o r m e d .  Although C a m e r o n d i s m i s s e s the  p o s s i b i l i t y of intense i r r a d i a t i o n of the p r o t o p l a n e t a r y m a t e r i a l as r e q u i r e d by the F G H  t h e o r y , he does postulate the e x i s t e n c e of a  m o d e r a t e f l u x f o r the p r o d u c t i o n of enough r a d i o a c t i v e A l ^ t bodies of a s t e r o i d a l s i z e ( C a m e r o n , 1965). Mg^,  Q  rnelt  E v i d e n c e of ' f o s s i l '  f r o m the .72 m. y. decay of A l ^ , has r e c e n t l y been r e p o r t e d  f o r s e v e r a l m e t e o r i t e s by C l a r k e et a l (1970).  2. 2 A Review of the S e a r c h f o r I r r a d i a t i o n A n o m a l i e s The s e a r c h f o r i r r a d i a t i o n a n o m a l i e s has b e e n r e s t r i c t e d to t e r r e s t r i a l and m e t e o r i t i c s a m p l e s , although l u n a r r o c k s have b e c o m e a v a i l a b l e w i t h i n the past y e a r .  The only w e l l - e s t a b l i s h e d  a n o m a l i e s w h i c h have been a t t r i b u t e d to s p a l l a t i o n r e a c t i o n s d u r i n g (or p r i o r to) the f o r m a t i o n of the s o l a r s y s t e m w e r e o b s e r v e d i n the noble gases xenon and k r y p t o n (e.g. R e y n o l d s , 1963; M e r i h u e , 1963). A l t h o u g h these a n o m a l i e s a r e l a r g e i n a r e l a t i v e sense ( e n r i c h m e n t s  • -•• 2 2  of up to 600%), they a r e s m a l l i n an absolute sense (10 ^ to 10 ^ ppb of the m e t e o r i t e mass). M u r t h y (1960,1962) r e p o r t e d anomalous i s o t o p i c c o m p o s i t i o n s for s i l v e r (2 to 4%) and m o l y b d e n u m (up to 7%) i n c e r t a i n i r o n m e t e o r i t e s , and an e n r i c h m e n t (< 2%) i n the l i g h t isotopes of b a r i u m was r e p o r t e d b y Umemoto (1962).  These apparent a n o m a l i e s a r e a l l  l a r g e i n an absolute sense (0. 1 to 170 ppb) but s m a l l i n a r e l a t i v e sense.  R e c e n t attempts to v e r i f y these a n o m a l i e s have l a r g e l y  d i s c r e d i t e d the e a r l i e r o b s e r v a t i o n s on the b a s i s of m a s s s p e c t r o m e t e r f r a c t i o n a t i o n ( E u g s t e r et a l , 1969; C h a k r a b u r t t y et a l , 19 64). A n o m a l i e s among the l i g h t e r elements have a l s o been r e p o r t e d ( r e f e r e n c e s a r e given by A n d e r s , 1964).  Their interpretation  is ambiguous, however, because of t h e i r s u s c e p t i b i l i t y .to c h e m i c a l f r a c t i o n a t i o n , addition of m a t e r i a l f r o m the s o l a r wind, or bulk t r a n s f e r i n v o l a t i l e gases. A m o n g the n o n - v o l a t i l e e l e m e n t s , the isotopic r a t i o s Li /Li , K ?  6  4 0  /K  and E u ^ l / E u ^ ^  4 1  a  , V r  e  5 0  /V  5 1  , Gd  1 5 7  /Gd 8, G d ^ / G d ^ , 1 5  Sm^/Sm ^ 1  p a r t i c u l a r l y s e n s i t i v e to i r r a d i a t i o n . The isotopes  of L i , K and V a r e d i r e c t products of s p a l l a t i o n r e a c t i o n s , although t h e i r abundances a r e s i g n i f i c a n t l y a l t e r e d b y t h e r m a l neutron capture as w e l l .  F o r the r a r e e a r t h elements (Gd, Sm and E u ) the i s o t o p i c  abundances a r e m u c h m o r e s t r o n g l y influenced by t h e r m a l neutrons than by d i r e c t s p a l l a t i o n .  Since these elements a r e of p r i m a r y  23  i n t e r e s t i n the p r e s e n t r e s e a r c h , the r e s u l t s of i n v e s t i g a t i o n s on L i , K and V w i l l be i n t e r p r e t e d i n t e r m s of the t h e r m a l neutron f l u x , r a t h e r than the p r i m a r y high e n e r g y p a r t i c l e flux.  This interpreta-  t i o n i s b a s e d on the a s s u m p t i o n of a p p r o x i m a t e l y a one to one c o r r e s pondence between the t h e r m a l neutron flux and the p r i m a r y flux > 200 M e v  (after F o w l e r et a l , 1962).  F i g u r e 2-1 shows the r e l a t i v e s e n s i t i v i t y of the Gd, Sm  and  E u i s o t o p i c r a t i o s to d i f f e r e n t i a l i r r a d i a t i o n of m e t e o r i t i c and terrestrial material.  In this d i a g r a m 1^ ^[  and ~)f/ j? r e p r e s e n t  n  n  the i n t e g r a t e d t h e r m a l neutron flux i r r a d i a t i n g m e t e o r i t i c and t e r r e s t r i a l material respectively. v a l u e s of J ( ^  n  M  -  The c u r v e s i l l u s t r a t e the range of  7/ )/Y E| "YnE /  nE  /  n  and  f o r  w  h  i  c  h  anomaly  a  would be o b s e r v e d i n the r e l e v a n t i s o t o p i c r a t i o .  The a r e a above  each of these c u r v e s r e p r e s e n t s an a n o m a l y >0.1%. (The p r o c e d u r e f o r computing the c u r v e s i s o u t l i n e d i n A p p e n d i x I. )  Figure  2-2  i s a s i m i l a r d i a g r a m except that only the m o s t s e n s i t i v e r a t i o , ^  Gd  157  .  /Gd  ,  158  , has been i l l u s t r a t e d . F i g u r e 2-1 i l l u s t r a t e s a m o d e l f o r w h i c h only 5% of the  p r i m i t i v e m a t e r i a l was i r r a d i a t e d , while the r e m a i n d e r was s h i e l d e d in the i n t e r i o r of p l a n e t e s i m a l s s e v e r a l m e t e r s i n r a d i u s . T h i s m o d e l i s c o n s i s t e n t with the F G H  hypothesis when""^ ^, ; 4 x  10^  2  neutrons/cm .  It does not a s s u m e that a l l planete s i m a l s had the  same d i m e n s i o n s , but r a t h e r that the d i s t r i b u t i o n of p l a n e t e s i m a l  24  FIGURE 2-1.  MODEL FOR WHICH 5 % OF PRIMITIVE MATERIAL WAS  Differential irradiation \  n  IRRADIATED  nM ^ n E nE  0.1  0.01  » Region excluded * because of the \absence of a K \ isotopic y anomaly ( f o r Abee)  WMmmm  l l l l l l l l l l l l l  • : • : > : • Region of V ^ i n f l u e n c e of \ S S r e c e n t cosmic\ S r a y exposure \ (for Abee) ^ 0.001  •IS  Flux p r e d i c t e d FGH h y p o t h e s i s  mm 10 16  i  10  18  10  20  10  22  T i m e - i n t e g r a t e d neutron f l u x which i r r a d i a t e d d i a l m a t e r i a l from which the e a r t h was formed  nE primor(n/cm^).  Curves show the s e n s i t i v i t i e s o f the Gd, Eu and Sm i s o t o p i c r a t i o s t o a d i f f e r e n c e i n the thermal neutron f l u x e s which i r r a d i a t e d p r i m i t i v e m e t e o r i t i c (M) and t e r r e s t r i a l (E) matter p r i o r t o the f o r m a t i o n o f l a r g e p l a n e t a r y b o d i e s . The a r e a above each curve r e p r e s e n t s >0,1% i s o t o p i c anomaly.  25  FIGURE 2-2.  UNIFORM IRRADIATION MODEL  Differential irradiation  Feasible j-:-x-Upper l i m i t imposed byanomaly f o r £:V:;x the absence o f a K v  -ifnE nE  0.1  ~  O.Ol  0.001  T i m e - i n t e g r a t e d n e u t r o n f l u x which i r r a d i a t e d p r i m o r d i a l m a t e r i a l from w h i c h the e a r t h was formed (n/cm ) 2  The r e g i o n above t h e s o l i d l i n e r e p r e s e n t s a Gd^-^/Gd^^ anomaly ^0.1% due t o d i f f e r e n t i a l i r r a d i a t i o n o f p r i m i t i v e m e t e o r i t i c (M) and t e r r e s t r i a l (E) m a t t e r . The o t h e r r a t i o s are l e s s s e n s i t i v e t o n e u t r o n c a p t u r e (see F i g u r e 2 - 1 ) .  26  s i z e s and shapes was the same f o r both the p r e - m e t e o r i t i c and pre-terrestrial material.  P r e s u m a b l y , the p l a n e t e s i m a l s  later  a c c r e t e d to f o r m l a r g e r p l a n e t a r y bodies i n w h i c h m i x i n g of the i r r a d i a t e d and n o n - i r r a d i a t e d f r a c t i o n s could o c c u r . The  second m o d e l ( F i g u r e 2-2) a s s u m e s u n i f o r m i r r a d -  i a t i o n of p r i m i t i v e m a t e r i a l .  T h i s c o r r e s p o n d s to p l a n e t e s i m a l s  having a m a x i m u m r a d i u s of a p p r o x i m a t e l y one m e t r e .  F o r this  m o d e l an upper l i m i t f o r ")^ j£ of 2 x 1 0 ^ neutrons / c m ^ can be set n  on the b a s i s of the p r e s e n t - d a y G d " / G d " ^ r a t i o , 7  A higher f l u x  ICQ  would i m p l y a negative G d  abundance i n i t i a l l y .  F i g u r e s 2-1 and 2-2 a r e r e p r e s e n t a t i v e of s e v e r a l p o s s i b l e models;  another i m p o r t a n t p o s s i b i l i t y is that the f r a c t i o n i r r a d i a t e d  was d i f f e r e n t f o r p r i m o r d i a l m e t e o r i t i c and t e r r e s t r i a l m a t e r i a l . O n the b a s i s of the F G H hypothes i s , B u r n e t t et a l (1965) e s t i m a t e d that a given percentage v a r i a t i o n i n the i n t e g r a t e d neutron f l u x would produce a p p r o x i m a t e l y the same percentage v a r i a t i o n i n 7  6  Li /Li .  The fact that K r a n k o w s k y et a l (1964, 1967)  observed  no v a r i a t i o n s i n this r a t i o i n s e v e r a l m e t e o r i t e s ( i n c l u d i n g the Abee enstatite chondrite) w i t h i n an e x p e r i m e n t a l u n c e r t a i n t y of 2% was t h e r e f o r e i n t e r p r e t e d as p l a c i n g an upper l i m i t of 2 % on the value of T h i s i s only t r u e , h o w e v e r , i f m o s t of the t e r r e s t r i a l L i was p r o d u c e d b y s p a l l a t i o n w i t h i n the s o l a r system,  neutrons/cm .  27  The l i m i t s on the d i f f e r e n t i a l i r r a d i a t i o n h i s t o r y b a s e d on a 5 % i r r a d i a t i o n m o d e l ( F i g u r e 2-1) and a u n i f o r m i r r a d i a t i o n m o d e l ( F i g u r e 2-2) a r e given i n Table 2-1. A n i n v e s t i g a t i o n of t h e . V ^ / V ^ r a t i o i n s e v e r a l m e t e o r i t e s , i n c l u d i n g the Abee enstatite c h o n d r i t e , r e v e a l e d no a n o m a l i e s w i t h i n a n e x p e r i m e n t a l u n c e r t a i n t y of 2 % ( B a l s i g e r et a l , 1969). B u r n e t t et a l (1966) s e a r c h e d for an a n o m a l y i n the K ^/K 4  4 1  r a t i o i n m a n y c l a s s e s of m e t e o r i t e s , again i n c l u d i n g Abee,  T h e y o b s e r v e d no a n o m a l i e s , w i t h i n an e x p e r i m e n t a l u n c e r t a i n t y of 0.5%,which could be a t t r i b u t e d to anomalous i r r a d i a t i o n d u r i n g the e a r l y h i s t o r y of the s o l a r s y s t e m .  S m a l l a n o m a l i e s i n the  N o r t o n County m e t e o r i t e (a h i g h l y r e d u c e d a c h o n d r i t e ) , a n i r o n m e t e o r i t e , and a s t o n y - i r o n , were a t t r i b u t e d to r e c e n t c o s m i c r a y e x p o s u r e p r i o r to capture by the e a r t h .  E s t i m a t e s of the  upper l i m i t to the d i f f e r e n t i a l p a r t i c l e flux a s s o c i a t e d w i t h a 0.5% anomaly for K  4 0  /K  4 1  are given i n Table 2-1 (after B u r n e t t et a l , 1969).  The absence of an i r r a d i a t i o n a n o m a l y f o r K i n the Abee enstatite chondrite p l a c e s an upper l i m i t on the d i f f e r e n t i a l irradiation J t ^ ^ n  " "Xf" ^ | of  1 x 10 ^ n e u t r o n s / c m ^ for the 18  5 % i r r a d i a t i o n m o d e l and * 5 x 10 i r r a d i a t i o n model.  7  neutrons/cm  f o r the u n i f o r m  These a r e o r d e r of magnitude e s t i m a t e s only.  The shaded r e g i o n s on the r i g h t side of F i g u r e s 2-1 and 2-2 i n d i c a t e u n r e a s o n a b l e values of ^ y i and " V ^ E b a s e d on the above l i m i t s f o r n  28  TABLE 2-1.  MAXIMUM DIFFERENTIAL TERRESTRIAL  Uncertainty i n published experimental abundance measurements  Isotopic ratio  Li /I'i 7  K^U/K^  6  1  AND  >?5  o.5-l%  *b  Mev  Thermal  v  50/ 51 * c d  fo  v  Gd  l 5 7  /Gd  1 5 8  Gd 57/ dl58 1  G  Vfo  e  #f  Otl  f  #  Excluding a l l enstatite  a  Krankowsky e t a l  b  Burnett  c  Balsiger  e  Murthy  f  Eugster et a l  et a l  neut.  Thermal neut.  0  I n c l u d i n g Abee  enstatite  (1964).  (1966).  et a l  neut.  Thermal neut.  *  et a l  Mev  (1969).  (1963). (1970a).  2  2 x 10  > 50 Mev  2  ^  5fo i r r a d , model (part/cm )  £ 500 Mev  2-20  OF  PRE-  PRE-METEORITIC MATERIAL  Energy range of p a r t i c l e flux  2%  a  IRRADIATION  chondrite.chondrites,  6  x  19  10  19  Uniform irrad. model (part/cm )  1 x  10  3  x  10  18  18  1 x 10  20  5 x  10  18  6 x 10  19  3 x  10  18  8 x 10  18  4 x 10 4 x  3x  17  10  l6  10 5 1  29  I "VnM  " "l/ nE I ^' anC  * ^ n  t  ie  c  a  s  e  °^  t n e  uniform irradiation model,  on the m a x i m u m value of "ty'-j- consistent w i t h the p r e s e n t - d a y „ .157 ,„ ,158 Gd  /Gd  ratio. M u r t h y and S c h m i t t (1963) i n v e s t i g a t e d Gd,  Sm  andEu  i s o t o p i c r a t i o s f o r a recent H a w a i i n b a s a l t , three o r d i n a r y ( i . e . hypersthene and b r o n z i t e c h o n d r i t e s ) , and one chondrite.  chondrites  carbonaceous  T h e y found no s i g n i f i c a n t v a r i a t i o n i n the i s o t o p i c r a t i o s  w i t h i n an e x p e r i m e n t a l  u n c e r t a i n t y of 1%.  Although their  observations  suggest the absence of any l a r g e a n o m a l i e s among the c h o n d r i t e s , they do not r u l e out the p o s s i b i l i t y of f i n d i n g a s i g n i f i c a n t a n o m a l y f o r the Abee enstatite c h o n d r i t e , since this c l a s s of chondrites  was  not i n v e s t i g a t e d . M o r e r e c e n t l y , E u g s t e r et a l (1970) have m e a s u r e d the i s o t o p i c c o m p o s i t i o n of Gd i n s e v e r a l m e t e o r i t e s w i t h a p r e c i s i o n of 0.1%.  T h e i r r e s u l t s show a g r e e m e n t w i t h t e r r e s t r i a l Gd w i t h i n  the e x p e r i m e n t a l  u n c e r t a i n t y , except f o r the N o r t o n County achondrite 157, 158 . w h i c h showed a d e c r e a s e i n the Gd /Gd r a t i o of (0.27 i .04)% . T h e y a t t r i b u t e d this a n o m a l y to the v e r y l a r g e (230 m. y. ) c o s m i c r a y exposure age ( B e g e m a n n et a l , 1957) of the N o r t o n County meteorite.  A g a i n , it i s of s i g n i f i c a n c e to the p r e s e n t study that no  enstatite chondrites w e r e i n c l u d e d i n t h e i r i n v e s t i g a t i o n s . 2. 3 The E f f e c t of Recent C o s m i c I r r a d i a t i o n Any  i n t e r p r e t a t i o n of i r r a d i a t i o n a n o m a l i e s i n m e t e o r i t e s  30  r e q u i r e s a knowledge of the extent of r e c e n t exposure to c o s m i c r a y s , p r i o r to capture by the earth.  S e v e r a l methods have been  developed f o r d e t e r m i n i n g the c o s m i c r a y exposure age of m e t e o r ites.  M o s t methods e m p l o y two c o s m o g e n i c n u c l i d e s , one r a d i o a c t i v e  and one stable {e.g. A  3 9  -A  3 8  , Cl  3 6  -Ne  2 1  , Al  2 6  -Ne  2 1  and  K  4 0  -K  4 1  ).  S p a l l a t i o n r e a c t i o n s generate the n u c l i d e s at r e l a t i v e rates which can be e s t i m a t e d f r o m c o n t r o l l e d e x p e r i m e n t s with c y c l o t r o n beams. A s s u m i n g a constant c o s m i c r a y f l u x , the rate of p r o d u c t i o n of the r a d i o a c t i v e nuclide w i l l a p p r o a c h its decay rate after a few h a l f l i v e s of e x p o s u r e .  If this steady state had been r e a c h e d p r i o r to  capture by the e a r t h , then the p r e s e n t - d a y abundances of the n u c l i d e s p r o v i d e an e s t i m a t e of the total exposure age.  A c o m p a r i s o n of the  apparent e x p o s u r e ages using d i f f e r e n t nuclide p a i r s p r o v i d e s a check on the a c c u r a c y of the method and the long t e r m u n i f o r m i t y of the c o s m i c r a y flux. The c o s m i c r a y exposure" ages of m e t e o r i t e s a r e short 9  ( < 10  y e a r s ) c o m p a r e d to the s o l i d i f i c a t i o n and gas r e t e n t i o n ages of  the m e t e o r i t e , parent bodies.  M o s t stone m e t e o r i t e s have c o s m i c  r a y e x p o s u r e ages l e s s than 60 m.y.  ( A n d e r s , 1964).  Evidently,  the m e t e o r i t e s have spent m o s t of t h e i r existence (after c o d l i n g , but b e f o r e e x p o s u r e to c o s m i c r a d i a t i o n ) i n s i d e c o l d , i n e r t b o d i e s , or s i z e a b l e c o l l i s i o n f r a g m e n t s of these b o d i e s . The Abee enstatite chondrite has a c o s m i c r a y exposure  31  age of 13 m. y. (Begemann et a l , 1959). n e u t r o n p r o d u c t i o n rate of  If we a s s u m e a m e a n t h e r m a l  one n e u t r o n / c m ^ s e c (after E u g s t e r  et a l , 1970a, 1970b) then the i n t e g r a t e d t h e r m a l neutron f l u x generated i n Abee d u r i n g r e c e n t exposure to c o s m i c r a d i a t i o n i s 14 4 x 10  2 neutrons/cm  \/ .  T h i s f l u x would induce (n, 5 ) r e a c t i o n s  in m e t e o r i t i c m a t e r i a l w h i c h would be i n d i s t i n g u i s h a b l e f r o m i r r a d i a t i o n effects d u r i n g the early h i s t o r y of the s o l a r s y s t e m .  Since  the.earth i t s e l f i s s h i e l d e d f r o m such p r o c e s s e s , we may conclude that a d i f f e r e n t i a l i r r a d i a t i o n I ~\L , , - ~\f „ I l e s s than 4 x 1 0 ^ J Y nM " nE J neutrons/cm  , between the Abee m e t e o r i t e and the e a r t h c o u l d  not be i n t e r p r e t e d i n t e r m s of the hypotheses of M a s o n and M i y a s h i r o . T h i s l i m i t a t i o n i s i l l u s t r a t e d i n F i g u r e s 2-1 and 2-2.  32  CHAPTER 3  INSTRUMENTATION The a i m of the present r e s e a r c h i s to p r e c i s e l y  determine  the i s o t o p i c c o m p o s i t i o n of Gd, Sm, and E u i n a few s e l e c t e d s a m p l e s . P a r t i c u l a r attention w i l l be f o c u s s e d on those isotopes whose abundiii i ii i^ 155 ,156 ances would be changed by neutron i r r a d i a t i o n : G d , Gd , r - A  Gd  1  5  1  , Gd  , Sm149 , Sm150 ,„ E 151 u ,,-,153 Eu c  c  3.1 Ope r a t i o n a l P r i n c i p l e s of a M a s s  Spectrometer  The m o s t c o m m o n method of d e t e r m i n i n g the i s o t o p i c c o m p o s i t i o n of the m e t a l l i c e l e m e n t s i n g e n e r a l e m p l o y s a s o l i d source mass spectrometer.  The b a s i c o p e r a t i o n a l p r i n c i p l e s of  this i n s t r u m e n t a r e : (a) The element to be studied i s concentrated into a few drops of s o l u t i o n and deposited on one or two r i b b o n f i l a m e n t s , depending upon whether a single o r t r i p l e f i l a m e n t c o n f i g u r a t i o n is to be used.  The f i l a m e n t s a r e t y p i c a l l y made of high w o r k -  f u n c t i o n m e t a l s such as tungsten, r h e n i u m , tantalum or platinum. The d r o p s are evaporated to d r y n e s s b y p a s s i n g a c u r r e n t through ^ each f i l a m e n t while e x p o s e d to the a t m o s p h e r e .  This leaves  a s a l t , containing the element of i n t e r e s t , on the s u r f a c e of the f i l a m e n t ( s ) . (b) The f i l a m e n t ( s ) a r e then p o s i t i o n e d i n a m a s s  33  s p e c t r o m e t e r and the s y s t e m i s evacuated to a low p r e s s u r e ( £ I O " t o r r ). 7  (c) The s a m p l e i s s l o w l y e v a p o r a t e d and p a r t i a l l y i o n i z e d . at high f i l a m e n t t e m p e r a t u r e s .  If a single f i l a m e n t i s used,  both p r o c e s s e s o c c u r on the same s u r f a c e .  In the case of the  t r i p l e f i l a m e n t c o n f i g u r a t i o n (after I n g h r a m et a l , 19 53) the s a m p l e i s e v a p o r a t e d f r o m two side f i l a m e n t s and i o n i z e d m a i n l y by a t h i r d (center) f i l a m e n t w h i c h is at a m u c h higher temperature.  The type of ions p r o d u c e d , and t h e i r r e l a t i v e  abundances, w i l l depend upon the c o m p o s i t i o n of the s a l t , the f i l a m e n t t e m p e r a t u r e s (and w o r k function) , . the i o n i z a t i o n potentials of the components and the t o t a l geometry. (d) A n e l e c t r o s t a t i c lens i s used to a c c e l e r a t e the r e s u l t i n g p o s i t i v e ions through a p o t e n t i a l d i f f e r e n c e , w h i c h i s t y p i c a l l y a few k i l o v o l t s .  The lens a l s o f o c u s s e s and  c o l l i m a t e s the b e a m into a r i b b o n - s h a p e d s t r e a m of m o n o e n e r g e t i c ions.  The c o m b i n e d f i l a m e n t a s s e m b l y and e l e c t r o s t a t i c lens  is c o m m o n l y r e f e r r e d to as an ion source. (e) The m o n o e n e r g e t i c ion b e a m i s t r a n s m i t t e d t h r o u g h a m a g n e t i c f i e l d w h i c h separates the b e a m into s e v e r a l components with d i f f e r e n t c h a r g e - t o - m a s s r a t i o s .  F o r a s u i t a b l e value of  the magnetic f i e l d each component b e a m can be f d c u s s e d  indep-  endently onto a c u r r e n t sensing device (e.g. a F a r a d a y cup o r electron multiplier).  34  (f) B y s l o w l y v a r y i n g the a c c e l e r a t i n g voltage o r the m a g n e t i c f i e l d i n t e n s i t y , the r e l a t i v e i n t e n s i t i e s of the component ion b e a m s can be m o n i t o r e d . for G d  +  F i g u r e 3-1 shows a sample s p e c t r u m  ions ( o m i t t i n g the m a s s 152 peak).  In the absence  •A ;,of m a s s d i s c r i m i n a t i o n e f f e c t s , o r i n t e r f e r e n c e by other s p e c t r a , the r e l a t i v e peak heights a r e d i r e c t l y r e l a t e d to the r e l a t i v e i s o t o p i c abundance values i n the o r i g i n a l sample.  It i s often  ; _ n e c e s s a r y to apply a c o r r e c t i o n for the growth (or decay) of the i o n b e a m i n t e n s i t y as a function of time.  This is sometimes  a c c o m p l i s h e d by u s i n g a p o l y n o m i a l r e p r e s e n t a t i o n of the t o t a l b e a m i n t e n s i t y and t r a n s f o r m i n g a l l m e a s u r e d peak heights to t h e i r e f f e c t i v e values at a c o m m o n point i n time. The above d i s c u s s i o n outlines the b a s i c o p e r a t i o n a l p r i n c i p l e s of a single-stage m a s s s p e c t r o m e t e r .  A s with any  e x p e r i m e n t a l technique there a r e m a n y p r a c t i c a l d i f f i c u l t i e s a s s o c i a t e d w i t h e a c h new a p p l i c a t i o n .  T h e r e a r e a l s o fundamental  l i m i t a t i o n s of the i n s t r u m e n t i t s e l f r e g a r d l e s s of the p r o b l e m under investigation.  Some of the p r o b l e m s w h i c h r e c e i v e d p a r t i c u l a r  attention d u r i n g the present r e s e a r c h w i l l now be r e v i e w e d .  3. 2 Ion O p t i c s "The  ion s o u r c e i s c l e a r l y the 'heart' of the (mass)  s p e c t r o m e t e r and i s , as m i g h t be expected, the p a r t e x h i b i t i n g the g r e a t e s t c o m p l e x i t y of action. "  ( B a r n a r d , 1953, p. 47)  time  (sec)  n  - r  100  75  50  25  A  160  158  157  L  156  155  F I G U R E 3-1. C O N T I N U O U S  154  155  156  j 157  u  -158  SCANNING OVER THE GADOLINIUM S P E C T R U M  160  36  P r i o r to i n v e s t i g a t i n g the r a r e e a r t h e l e m e n t s , the w r i t e r ' s i n t e r e s t s were d i r e c t e d t o w a r d the i o n o p t i c a l p r o p e r t i e s of ion s o u r c e s , i . e . t h e i r a b i l i t y to focus and t r a n s m i t ions. To study t h i s p r o b l e m , i o n t r a j e c t o r i e s were c a l c u l a t e d through several two-dimensional ion sources.  Laplace's equation was  s o l v e d b y a f i n i t e - d i f f e r e n c e method and i o n paths w e r e computed by n u m e r i c a l i n t e g r a t i o n of the t r a j e c t o r y equation ( L o v e l e s s , 1967). T h e s e i n v e s t i g a t i o n s l e d to the design of the lens s y s t e m shown i n F i g u r e 3-2.  The t r a j e c t o r i e s i l l u s t r a t e d w e r e c a l c u l a t e d  a s s u m i n g an i n i t i a l ion energy k T = .015 eV (cor responding to T = 1750* K on the c e n t e r i o n i z i n g f i l a m e n t - .001 x .030 i n c h ribbon) and angles of i n c l i n a t i o n to the lens axis f r o m 0 to ^ 75 deg. T h i s lens was shown to be capable of t r a n s m i t t i n g 7 0 % of the ions produced at the c e n t e r f i l a m e n t (for the m o s t c r i t i c a l f o c u s s i n g plane i l l u s t r a t e d i n F i g u r e 3-2).  L o v e l e s s and R u s s e l l (1969)  c o m p a r e d the p r o p e r t i e s of this lens with those of an i d e a l l e n s , u s i n g the concepts of b e a m e m m i t t a n c e and i n s t r u m e n t acceptance w h i c h had not been a p p l i e d before to m a s s s p e c t r o m e t e r i o n s o u r c e s . T h r o u g h the use of these concepts it was p o s s i b l e to c l e a r l y define the l i m i t a t i o n to b e a m f o c u s s i n g and t r a n s m i s s i o n e f f i c i e n c y i n t e r m s of b a s i c i n s t r u m e n t p a r a m e t e r s .  A n understanding of this  l i m i t a t i o n w i l l be of c o n s i d e r a b l e value i n the future d e s i g n of m a s s s p e c t r o m e t e r s , and w i l l a v o i d w a s t e d effort i n attempting to achieve what i s t h e o r e t i c a l l y i m p o s s i b l e .  37  5000 V  * y  5000 V  F I G U R E 3-2.  ION B E A M P R O D U C E D B Y NEW ION S O U R C E  38  The ion source i n F i g u r e 3-2 i s a s i m p l e design, with s t r o n g f o c u s s i n g p r o p e r t i e s (because of the use of t h i c k e l e c t r o d e s ) w h i c h has p r o v e d i t s e f f e c t i v e n e s s i n p r a c t i c e and has consistentlyp r o v i d e d s a t i s f a c t o r y peak shape and r e s o l u t i o n when the f o c u s s i n g e l e c t r o d e s a r e tuned to give m a x i m u m beam i n t e n s i t y .  It r e p l a c e s  a c o m p l e x s t a c k of t h i n s l i t s w h i c h were acting e s s e n t i a l l y as a collimating system, with m i n i m a l focussing capability.  The new  source gave a f a c t o r of five i n c r e a s e i n s e n s i t i v i t y , w i t h c o m p a r able r e s o l u t i o n . T h i s a c h i e v e m e n t was coupled with a f a c t o r of two i n c r e a s e i n s e n s i t i v i t y through the use of second o r d e r f o c u s s i n g s h i m s w h i c h enabled a b e a m of higher angular divergence to be used.  T h i s method  of a c h i e v i n g second o r d e r f o c u s s i n g i s a t t r a c t i v e because i t uses only a s i n g l e p a i r of s h i m s on one side of the a n a l y z e r magnet, and r e q u i r e s no m o d i f i c a t i o n of the b a s i c magnet. It i s d i s c u s s e d i n A p p e n d i x II. The i n c r e a s e d s e n s i t i v i t y r e s u l t i n g f r o m fundamental i m p r o v e m e n t s i n the ion optics of the a v a i l a b l e m a s s  spectrometer  r e p r e s e n t s a s i g n i f i c a n t c o n t r i b u t i o n by the present w r i t e r . Although the a n a l y s i s of the r a r e e a r t h elements would s t i l l have b e e n p o s s i b l e without these m o d i f i c a t i o n s , the p r e c i s i o n of the r e s u l t s would have been somewhat p o o r e r . 3.3 The I o n i z a t i o n P r o c e s s  39  The m o s t i n e f f i c i e n t p r o c e s s o c c u r r i n g i n the m a s s s p e c t r o m e t e r was the c o n v e r s i o n of n e u t r a l atoms into p o s i t i v e l y c h a r g e d ions w h i c h a n a l y z e r magnet.  can be a c c e l e r a t e d and i n j e c t e d into the W i t h the t r i p l e (rhenium) f i l a m e n t a r r a n g e -  ment, only about 1 5 % of the e v a p o r a t e d atoms succeeded i n hitting the hot i o n i z i n g f i l a m e n t ;  Of these, only .1% or l e s s w e r e  e m i t t e d f r o m the center f i l a m e n t as G d  +  ions.  The i o n i z a t i o n  e f f i c i e n c y was g r e a t e r f o r Sm and E u . T h r e e p o s s i b l e ways of i m p r o v i n g the i o n i z a t i o n e f f i c i e n c y w i l l be b r i e f l y r e v i e w e d . (a) The s i n g l e f i l a m e n t technique has been shown, at l e a s t i n c e r t a i n c o n f i g u r a t i o n s , to produce an ion b e a m w h i c h i s an o r d e r of magnitude m o r e intense than that p r o d u c e d by the t r i p l e f i l a m e n t method ( E u g s t e r et a l , 1970a).  Both Eugster  et a l (1970a, 1970b) and L u g m a i r (1970) have e x t e n s i v e l y u s e d s i n g l e , zone-refined (high p u r i t y ) r h e n i u m f i l a m e n t s f o r the i s o t o p i c a n a l y s i s of Gd.  Since t h e i r w o r k has been p u b l i s h e d ,  the p r e s e n t w r i t e r has used the single f i l a m e n t method f o r s e v e r a l G d a n a l y s e s , i n c l u d i n g Abee.,  Although comparable  s e n s i t i v i t y was a c h i e v e d f o r both the single and t r i p l e f i l a m e n t techniques, the f o r m e r d i d not appear to be s i g n i f i c a n t l y b e t t e r , even when z o n e - r e f i n e d r h e n i u m f i l a m e n t s w e r e used.  Since  v e r y few s i n g l e f i l a m e n t analyses w e r e p e r f o r m e d , however,  40  this m a y r e f l e c t a l a c k of e x p e r i e n c e with this technique. A l t e r n a t i v e l y , the poor t r i p l e f i l a m e n t s e n s i t i v i t y o b s e r v e d by E u g s t e r et a l (1970) may r e s u l t f r o m l o w t r a n s m i s s i o n e f f i c i e n c y i n t h e i r i o n s o u r c e when u s i n g this f i l a m e n t c o n f i guration. One m i n o r c o m p l i c a t i o n w i t h the single f i l a m e n t technique is the fact that G d O ions.  +  ions a r e m u c h m o r e abundant than G d  +  F o r this r e a s o n the G d O * s p e c t r u m was a n a l y z e d ,  and a c o r r e c t i o n was made f o r the i s o t o p i c c o m p o s i t i o n of oxygen.  T h i s a c t u a l l y p r o v e d to be an advantage i n the analys  of the Abee s a m p l e w h i c h p r o d u c e d i n t e r f e r i n g L a O the G d  +  +  ions i n  s p e c t r u m but no s i g n i f i c a n t i n t e r f e r e n c e i n the G d O ^  spectrum. The magnitude of m a s s f r a c t i o n a t i o n i s g e n e r a l l y g r e a t e r f o r single f i l a m e n t a n a l y s e s , m a k i n g this m e t h o d l e s s p r e c i s e f o r the d e t e r m i n a t i o n of absolute i s o t o p i c abundances.  (Mass  f r a c t i o n a t i o n , o r d i s c r i m i n a t i o n , c o m m o n l y o c c u r s on hot f i l a m e n t s u r f a c e s due to p r e f e r e n t i a l e v a p o r a t i o n of the l i g h t e r isotopes of an element. ) (b) The use of a p o r o u s f i l a m e n t s u r f a c e (e. g. G o r i s , 1 9 6 2 ) o r the r e t e n t i o n of the s a m p l e i n a s i l i c a g e l ( C a m e r o n et a l , 1 9 6 9 ) a r e p r o m i s i n g ways of i n c r e a s i n g the y i e l d of p o s i t i v e ions f r o m a s i n g l e f i l a m e n t .  The w r i t e r has not  41  i n v e s t i g a t e d these techniques because of the h i g h l y s p e c i a l i z e d f i l a m e n t t r e a t m e n t o r sample p r e p a r a t i o n r e q u i r e d . (c) A t h i r d p o s s i b l e i m p r o v e m e n t i n v o l v e s the use of two p a r t i a l l y - c l o s e d (canoe-shaped) s a m p l e f i l a m e n t s w i t h s l i t openings a i m e d d i r e c t l y at the centre i o n i z i n g f i l a m e n t .  This  method should g r e a t l y i n c r e a s e the p r o b a b i l i t y that an evaporated atom w i l l s t r i k e the centre f i l a m e n t .  S e v e r a l attempts w e r e  made by the w r i t e r to use this technique, but no s i g n i f i c a n t i m p r o v e m e n t was evident.  A p p a r e n t l y , this method has p r o m i s e  but r e q u i r e s c l o s e c o n t r o l d u r i n g each stage of p r e p a r a t i o n : m a k i n g a V - s h a p e d c r e a s e along the length of the f i l a m e n t , shaping and mounting i t on p o s t s , outgassing it without l o s s of d u c t i l i t y , d e p o s i t i n g and evaporating the sample inside the 'canoe', c l o s i n g the f i l a m e n t sides without l o s s of s a m p l e , and c a r e f u l l y a l l i g n i n g them w i t h r e s p e c t to the centre f i l a m e n t . It i s b e l i e v e d that f r a c t i o n a t i o n would not be enhanced b y this technique. A l t h o u g h the i o n i z a t i o n p r o c e s s i s r e c o g n i z e d as being the least e f f i c i e n t p r o c e s s o c c u r r i n g i n the i n s t r u m e n t , the w r i t e r has not made s i g n i f i c a n t p r o g r e s s on this p r o b l e m .  The t r i p l e  f i l a m e n t technique was used f o r m o s t a n a l y s e s because an e l e m e n t a l Gd"*" ion b e a m of s u f f i c i e n t i n t e n s i t y c o u l d be p r o d u c e d b y this method, and m a s s d i s c r i m i n a t i o n effects were expected to be l e s s .  42  3.4  D i g i t a l R e c o r d i n g of M a s s S p e c t r a No m o d i f i c a t i o n s w e r e made to the e x i s t i n g c o l l e c t o r s y s t e m  of the m a s s s p e c t r o m e t e r .  The e l e c t r o n i c c i r c u i t r y and the e l e c -  trode c o n f i g u r a t i o n s were designed by other w o r k e r s i n our l a b o r a t o r y ( R . D. R u s s e l l , J . B l e n k i n s o p , J.S. Stacey, and J.M. Ozard} The entrance s l i t of the c o l l e c t o r had a width of 0. 0 2 0 inch.  A s t a n d a r d F a r a d a y cup and 1 0 ^ ohm r e s i s t o r s e r v e d as an  i o n detector.  The voltage developed a c r o s s the r e s i s t o r was  . a m p l i f i e d b y an adjustable gain (5 r a n g e s ) , h y b r i d d. c. a m p l i f i e r , and fed both to a c h a r t r e c o r d e r and to a d i g i t a l v o l t m e t e r .  The  c h a r t r e c o r d e r p r o v i d e d a v i s u a l d i s p l a y of the b e a m i n t e n s i t y at the c o l l e c t o r while the d i g i t a l v o l t m e t e r was used p r i m a r i l y to output data i n a d i g i t a l ( b i n a r y - c o d e d d e c i m a l ) f o r m a t . A n i n t e r f a c i n g device was designed and built by the w r i t e r to enable continuous r e c o r d i n g of s p e c t r a on a seven track magnetic tape r e c o r d e r .  The data c o u l d then be p r o c e s s e d by computer  (see S e c t i o n 3. 5 and Chapter 5 ) . The output f r o m the m a s s s p e c t r o m e t e r  measuring  s y s t e m was i n t e g r a t e d over A* ^ second i n t e r v a l s b y the d i g i t a l v o l t m e t e r and t r a n s f e r r e d t o m a g n e t i c tape at the rate of 7 data/sec.  E a c h datum c o n s i s t e d of 6 c h a r a c t e r s :  3^ binary-  coded d e c i m a l digits f r o m the d i g i t a l v o l t m e t e r , 1 c o n t r o l c h a r a c t e r _ g i v i n g shunt i n f o r m a t i o n and s c a n d i r e c t i o n (up, down o r s t a t i o n a r y ) , and an end-of-word c h a r a c t e r .  43  + The Gd  s p e c t r u m of F i g u r e 3-1 was taken by continuous  m a g n e t i c s c a n n i n g over the m a s s ranges 160-154 and 154-160 i n succession.  H o w e v e r , because of the steady decay of the  Gd  +  b e a m as a f u n c t i o n of t i m e , this does not r e p r e s e n t an e f f i c i e n t way of u t i l i z i n g the a v a i l a b l e beam.  L i t t l e t i m e i s spent m e a s u r -  ing the p e a k i n t e n s i t i e s , w h i l e m u c h t i m e i s lost between the peaks m e a s u r i n g b a s e l i n e s w h i c h are r e l a t i v e l y stable. M o r e efficient use of the ion b e a m i s i l l u s t r a t e d i n F i g u r e 3-3.  In this case the b a s e l i n e s are m e a s u r e d f o r compu-  t a t i o n a l p u r p o s e s at p o s i t i o n s \ m a s s above and \ m a s s below the highest and lowest m a s s e s r e s p e c t i v e l y .  T h i s i s done f o r both  the down-mass and up-mass p o r t i o n s of the scan. the magnet c u r r e n t i s advanced manually. ning m a y  B e t w e e n peaks  Slow continuous s c a n -  then be e m p l o y e d over the peaks only.  T h i s scanning  technique, h e r e a f t e r r e f e r r e d to as discontinuous s l o w scanning, was u s e d f o r a l l a n a l y s e s r e p o r t e d here. E a c h s c a n c o m p r i s e s the f o l l o w i n g r e c o r d s : (a) M e a s u r e m e n t of b a s e l i n e s on each shunt f o r a m i n i m u m of 5 seconds at a p o s i t i o n  \ m a s s above the h i g h - m a s s peak  i n the s p e c t r u m to be r e c o r d e d .  (Scan d i r e c t i o n :  stationary).  (b) Discontinuous, slow s c a n down the s p e c t r u m to a p o s i t i o n fiS \ m a s s below the l o w - m a s s peak.  (Scan d i r e c t i o n :  down).  (c) M e a s u r e m e n t of b a s e l i n e s on each shunt f o r a m i n i m u m  44  time ( s e c ) <~T  100  FIGURE 3-3.  \  1  75  50  -l  25  j o  DISCONTINUOUS SCANNING OVER THE GADOLINIUM SPECTRUM  The diagram r e p r e s e n t s one s c a n of the G d spectrum. .In this t h e s i s a s c a n i s d e f i n e d as one down-mass sweep of the spectrum p l u s one up-mass sweep.. B a s e l i n e s are measured above -the high-mass peak of the spectrum and - below the low-mass peak. +  45  of 5 seconds.  (Scan d i r e c t i o n :  stationary).  (d) D i s c o n t i n u o u s slow s c a n up the s p e c t r u m to a p o s i t i o n *ss \ m a s s above the h i g h - m a s s peak.  (Scan d i r e c t i o n :  up).  v.  (e) Same as (a). D u r i n g subsequent r e d u c t i o n of the data, b a s e l i n e s w e r e r e c o g n i z e d by the fact that the s c a n d i r e c t i o n was  stationary.  The effective peak i n t e n s i t y and the time of m e a s u r e m e n t could r e a d i l y be deduced f r o m the r e c o r d e d data (see Chapter V). 3.5  Noise Rejection Using a Low-Pass F i l t e r It i s evident f r o m F i g u r e 3-3 that high f r e q u e n c y noise i s  s u p e r i m p o s e d on the m a s s s p e c t r u m .  Since we are i n t e r e s t e d i n  computing peak heights, r a t h e r than the a r e a under each peak, i t is p a r t i c u l a r l y i m p o r t a n t to smooth out the effect of noise by u s i n g a suitable f i l t e r .  The l o c a l m a x i m a of the f i l t e r e d s p e c t r u m w i l l  then be a t r u e r r e p r e s e n t a t i o n of the s i g n a l i n t e n s i t y of the s p e c t r a l peaks. A low-pass f i l t e r was designed f o r this s p e c i f i c a p p l i c a t i o n by R. D. R u s s e l l and J . B l e n k i n s o p .  The f r e q u e n c y r e s p o n s e  function of the f i l t e r i s i l l u s t r a t e d i n F i g u r e 3-4.  The data window  has a width of 2. 4 sec, w h i c h i s c o n s i s t e n t w i t h the m i n i m u m width of peak-tops of 3 sec.  The f i l t e r e d data c o n s i s t s of - data p o i n t s / s e c . 6  The f i l t e r i n g p r o c e s s was only the f i r s t stage i n the r e d u c t i o n of the r e c o r d e d data.  A l l p r o c e s s i n g was p e r f o r m e d on an  IBM  360 c o m p u t e r through the f a c i l i t i e s of the C o m p u t i n g C e n t r e at U. B. C.  Frequency response 1.0  -  F i l t e r i n g sequence (a) Convolve with 7-point b o x - c a r f i l t e r . (.5 1.0 1.0 1.0 1.0 1.0 .5)  0.8  (b) Reject e v e r y second point. (c)  Convolve with 3-point b o x - c a r f i l t e r . (.5 1.0 .5) (d) Convolve with 5-point b o x - c a r f i l t e r . (.5 1.0 1.0 1.0 .5)  0.6  0.4  (e) R e j e c t 2 out of 3 points. Width of data window  0.2  Nyquist frequency  2.4 sec. 3.5 cps.  0.0 0.7  1.4  r  2.1  2.8  3.5  F r e q u e n c y (cps) F I G U R E 3-4. F R E Q U E N C Y R E S P O N S E O F D I G I T A L F I L T E R The f i l t e r was designed b y R. D. R u s s e l l and J. B. B.lenkinsop to r e j e c t high frequency noise f r o m the r e c o r d e d data.  47  CHAPTER 4  • PREPARATION OF SAMPLES  T e r r e s t r i a l ore s a m p l e s were obtained i n the f o r m of reagent G d 0 , > Sm a , and E u O, f r o m Matheson C o l e m a n and ?  B e l l , C i n c i n n a t i , Ohio, and the Shinetsu K a g a k u Co. , T o k y o , Japan.  A l l reagents had a s p e c i f i e d p u r i t y of 99-9%.  A sample  name was a s s i g n e d to each of the reagents f o r convenience of r e f e r e n c e (see Table 4-1). A b a s a l t s a m p l e f r o m K i t a m a t s u - u r a , Japan (JB1) was u s e d to gain e x p e r i e n c e i n the c h e m i c a l s e p a r a t i o n and m a s s spect r o m e t r y of the r a r e e a r t h elements.  Although the e x p e r i m e n t a l  techniques were t e s t e d on this s a m p l e , no p r e c i s e a n a l y s e s were p e r f o r m e d to d e t e r m i n e i t s i s o t o p i c c o m p o s i t i o n . The c o n c e n t r a t i o n of the r a r e earths was an o r d e r of magnitude g r e a t e r i n the b a s a l t than i n the Abee m e t e o r i t e .  In  o r d e r to c o n s i s t e n t l y use m i c r o g r a m s a m p l e s of G d i n a l l m a s s s p e c t r o m e t e r a n a l y s e s a p p r o x i m a t e l y 10 g r a m s of the m e t e o r i t e w e r e p r o c e s s e d f o r a n a l y s i s , whereas only 1 g r a m s a m p l e s of the b a s a l t w e r e used.  4. 1 S e p a r a t i o n of the R a r e E a r t h E l e m e n t s The e x t r a c t i o n of Gd, Sm and E u f r o m the b a s a l t and m e t e o r i t e s a m p l e s was p e r f o r m e d by S. Yanagida and Dr. H. M a b u c h i  48  TABLE 4-1.  DESCRIPTION OF SAMPLES STUDIED  Sample  Specifications  Gd- US  Gd 0^  a  Eu- US  Eu 0  a  Sm- US  Sm 03  a  F o l k s t o n , G e o r g i a , U.S.A.  Gd- J  Gd 0^  a  S h i n e t s u Kagaku Co.,  Eu- J  Eu 0^  a  - d e r i v e d from a.mine near  Sm- J  Sm 0-^  2  2  Matheson Coleman & B e l l , U.S.A - d e r i v e d from Humphrey's Mine  3  2  2  2  Rio  a  2  ^ l  de J a n e i r o ,  Japan  Brazil.  ppm Gd >y  4 JB1  \  Source  b  K i t a m a t s u - u r a , Japan  ppm Eu  5° ppm Sm ) 0.3°  Abee  J  0.06° 0.2  C  ppm Gd \ ppm Eu ppm Sm i  G e o l o g i c a l Survey o f Canada >  p a r t o f specimen 13b  a  A l l t e r r e s t r i a l samples were i n the form o f r e a g e n t s having a s p e c i f i e d p u r i t y of 9 9 . 9 $ .  b  Abundances c o r r e s p o n d t o t e r r e s t r i a l mean v a l u e s f o r diabases.  c  Based on a n a l y s e s o f the Abee m e t e o r i t e by Shima and Honda (196?).  49  of the D e p a r t m e n t of C h e m i s t r y , U n i v e r s i t y of Tokyo.  The m a s s  s p e c t r o m e t r y was undertaken by the p r e s e n t w r i t e r i n the Department of G e o p h y s i c s , U n i v e r s i t y of B r i t i s h C o l u m b i a . The r a r e e a r t h elements i n the lanthanide s e r i e s ( L a , Ce, P r , Nd, P m , Sm, E u , Gd, Tb, Dy, Ho, E r , Tm, Yb, Lu) have s i m i l a r c h e m i c a l p r o p e r t i e s , m a k i n g the s e p a r a t i o n of one element, f r o m the r e m a i n d e r of the elements i n the series, d i f f i c u l t . In o r d e r to a s s e s s whether s e p a r a t i o n of these elements was e s s e n t i a l , the f i r s t a n a l y s e s on Gd, E u and Sm w e r e p e r f o r m e d on s a m p l e s containing the complete lanthanide s e r i e s of elements ( s a m p l e s JB1-1 and JBl-2).  A l t h o u g h Gd, Sm and E u ion b e a m s  w e r e s u c c e s s f u l l y produced i n the m a s s s p e c t r o m e t e r , the abundance of i n t e r f e r i n g ion s p e c t r a i n h i b i t e d r e l i a b l e i n t e r p r e t a t i o n of the i s o t o p i c c o m p o s i t i o n of these e l e m e n t s .  T a b l e s 4-2 and 4-3 indicate  some of the ions w h i c h w e r e i d e n t i f i e d d u r i n g the m a s s s p e c t r o meter analyses.  A p a r t f r o m the r a r e e a r t h elements and t h e i r  o x i d e s , the m o s t c r i t i c a l contaminant was b a r i u m w h i c h p r o d u c e d Ba  , BaF  and B a C l  ion beams.  The abundance of these ions  was a t t r i b u t e d to the low i o n i z a t i o n potential of b a r i u m and its compounds.  E v e n s m a l l t r a c e s of b a r i u m were s u f f i c i e n t t o generate  intense i o n beams. S e v e r a l m o d i f i c a t i o n s w e r e made to the s e p a r a t i o n techniques u n t i l a t h r e e - s t a g e ion exchange p r o c e s s was adopted.  This technique  50  TABLE 4-2.  INTERFERING IONS NEAR GD, EU AND SM SPECTRA .  Mass  Ions  143  Nd(12.17)  144  Sm(3.09),Nd(23.85) .  145  Nd(8.20)  146  Nd(17.22),BaO(.11)  147  Sm(l4.97)  148  Sm(11.24),Nd(5-73),BaO(.10)  149 '  Sm(13.83),BaF(.11)  150  Sm(7.44),Nd(5.62),BaO(2.4l)  151  Eu(47.82),BaO(6.58),BaF(.10)  152  Sm(26.72),Gd(.20) CeO(.19),BaO(7.84)  153  Eu(52.l8),BaO(11.22),BaF(2.42)  154  Sm(22.?l),Gd(2.15),LaO(.09),Ceo(.25),BaO(71.55),BaF(6. 59)  155  .  f  . Gd(l4.73),LaO(99.67),BaO(.05),BaF(7.85)  156  Gd(20.47) ,Dy( .05) ,Ce'0(88.27) ,LaO( .04) ,BaO(.15) , B a F ( l l .23)  157  Gd(15.68),PrO(99.76),CeO(.03),LaO(.20),BaF(71.70)  158  Gd(24.87),Dy(.09),Ce0(11.23),Nd0(27.04),Pr0(.04)  159  Tb(100.00),Nd0(12.15),Pr0(.20)  160  Gd(2l.90),Dy(2.29),Nd0(23.85),Sm0(3.08),Ce0(.02)  161  Dy(l8.88),NdO(8.3D  Each type of i o n i s l i s t e d w i t h i t s percentage abundance as g i v e n i n the Handbook of Chemistry and P h y s i c s (1970). Barium abundances were taken from E u g s t e r e t a l (1969). A l l oxygen i s o t o p e s were c o n s i d e r e d when c a l c u l a t i n g the abundances of the o x i d e . i o n s . Some of the abundances may be i n e r r o r by as much as 1-2%.  Sample & Fil. Filament Current B a C o n f i g u r a t i o n (amps) 138 +  JBl-la tantalum c l o s e d canoe JBl-lb tantalum flat  JBl-2a tantalum part, closed  JBl-2b rhenium flat  2.1 3.0  4.0  1.4 2.6 2.8  2.0 >10 .'•  1.3 2.6 3.0  1.1 1.5 1.8 1.9 2.2  BaF + 157  LaO 155  CeO 156  .05  < .003 .1  .02  .02 .08  .2  .001 <.0005  .2 .2 .2  .04 10.  .008 .01  +  Eu 153  Sm + 152  .04 .05  .002 .05  <!.005  GdO (174)~ .07  .02 .1  .02 .08  <£.003  Nd (144) = .01  .001 .04 .06  .01 .03  «.l  Tb  .02 .1  Gd 158  +  (159) < .001  Cs (133) = .05 GdO (174) = .01 D y ( l 6 2 ) = .03 GdO (174) < .005:. +  <.0005  +  +  <.003 >10.  Other  >10.  1.  +  4.  T A B L E 4-3. E x a m p l e s of beam i n t e n s i t i e s o b s e r v e d (using v a r i a t i o n s of the t r i p l e f i l a m e n t technique) when Gd, E u and Sm were not separated f r o m the other r a r e e a r t h elements. B e a m intensities a r e s p e c i f i e d i n units of 10"^ amps. F i l a m e n t c u r r e n t s cannot be d i r e c t l y c o m p a r e d between analyses because of different configurations and sample h i s t o r i e s . B l a n k e n t r i e s in the table do not i m p l y the absence of an ion beam.  52  attempted complete s e p a r a t i o n of Gd, E u and Sm  into three d i f f e r e n t  f r a c t i o n s f o r independent a n a l y s i s i n the m a s s s p e c t r o m e t e r . a n a l y t i c a l scheme i s s u m m a r i z e d i n T a b l e The 9.5 g r a m Abee s a m p l e was HF  The  4-4. completely dissolved in  and HCIQ^, e v a p o r a t e d to d r y n e s s , and r e d i s s o l v e d i n 0.5 N  The s o l u t i o n was D i a i o n SK#1  HC1.  loaded onto a c a t i o n exchange c o l u m n c o n t a i n i n g  resin.  M o s t of the r a r e earths w e r e s e p a r a t e d f r o m  a l l . m a j o r constituents by e l u t i o n w i t h 2 N HC1 f o l l o w e d by 6 N  HC1.  A second c o l u m n was then u s e d to separate Gd, E u and f r o m the other r a r e earths u s i n g 0.5 M  Sm  2-methyl l a c t i c a c i d .  The r a r e e a r t h e l e m e n t s w e r e e l u t e d i n o r d e r of d e c r e a s i n g a t o m i c n u m b e r (or i n c r e a s i n g i o n i c r a d i u s ) . c o l u m n was  C a l i b r a t i o n of the exchange  p e r f o r m e d u s i n g a s o l u t i o n containing a r t i f i c i a l l y -  p r o d u c e d r a d i o a c t i v e isotopes of the r a r e e a r t h e l e m e n t s .  The  a c t i v i t y of the eluate was m o n i t o r e d to y i e l d the e l u t i o n c u r v e s i n F i g u r e 4-1.  .  -  A f t e r s e p a r a t i o n of Gd, E u and Sm  into three  separate  f r a c t i o n s , a t h i r d i o n exchange p r o c e s s ( s i m i l a r to the f i r s t  one)  was u s e d to f u r t h e r reduce the c o n c e n t r a t i o n of a l k a l i s and a l k a l i n e earths i n the i n d i v i d u a l f r a c t i o n s .  The  s e p a r a t e d Gd, E u and  Sm  f r a c t i o n s w e r e then converted into p e r c h l o r a t e s a l t s i n p r e p a r a t i o n for l o a d i n g i n the m a s s  spectrometer.  Although the s e p a r a t i o n technique succeeded, the e l u t i o n  53  T A B L E 4-4.  E X T R A C T I O N OF  GD,  EU  AND  SM  FROM  ABEE  1. D i s s o l v e 9.5 g r a m Abee sample i n 300 m l H F and 20 m l HCIO4 and evaporate to d r y n e s s . 2. D i s s o l v e i n 20 m l 6 N HC1 and evaporate to d r y n e s s . 3. D i s s o l v e i n 400 m l 0.5 N HC1 and store i n polyethylene bottle. 4. P a c k r e s i n i n 45 x 1.6 cm c o l u m n and p r e c o n d i t i o n with 1500 m l 6 N HC1 and 1500.ml H O to a-height-of 43 cm. 5. L o a d 400 m l s o l u t i o n (containing sample) onto column. 6. E l u t e with 840 m l 2 N HC1 to r e m o v e m a j o r constituents. 7. E l u t e w i t h 150 m l 6 N HC1 and c o l l e c t eluate containing a l l r a r e earth elements. 8. E v a p o r a t e to d r y n e s s and d i s s o l v e i n a few drops of HNO3. 9. R e p e a t p r o c e d u r e 8. 10. D i s s o l v e i n 2.5 m l H2O i n p r e p a r a t i o n f o r loading on second i o n -exchange column. 11. P r e c o n d i t i o n r e s i n to r e m o v e t r a c e s of r a r e e a r t h e l e m e n t s and b a r i u m . Wash with a p p r o x i m a t e l y 1000 m l 6 N HC1 and then r e m o v e a c i d with 3x d i s t i l l e d H £ 0 . C o n v e r t r e s i n to NH^"^ f o r m by adding 500 m l 7.5 N NH^OH. W a s h with H O. Preconditioning gives a r e s i n height of 22 cm. 12. The s a m p l e s o l u t i o n ( f r o m 10 above) was loaded onto the second (25 x 0.4 cm) c a t i o n exchange column. 13. E l u t e with 0.5 M 2-methyl l a c t i c a c i d (pH of 3.20). 14. C o l l e c t the Gd, E u and Sm f r a c t i o n s f r o m the eluate as s p e c i f i e d in F i g u r e 4-1. 15. A d d 1 N HC1 to each f r a c t i o n to l o w e r the pH to 2.5 ; then dilute (by a f a c t o r of two) by adding an equal volume of H2O. 16. L o a d each f r a c t i o n onto a t h i r d c o l u m n to r e m o v e the 2-methyl lactate f r o m the s o l u t i o n s . F o r each f r a c t i o n i n t u r n , p r e c o n d i tion r e s i n with 100 m l 6 N HC1 and 50 m l H2O to a r e s i n h e i g h t of 10 c m i n a 15 x 0.4 cm c a t i o n exchange column. L o a d the f r a c t i o n onto the c o l u m n and w a s h with 5 m l 2 N HC1. E l u t e with 10 m l 6 N HC1 to r e l e a s e the r a r e e a r t h content of the column. 17. E v a p o r a t e e a c h f r a c t i o n to d r y n e s s and r e d i s s o l v e i n a drop of H C I O 4 . E v a p o r a t e to d r y n e s s i n a teflon b e a k e r . z  G a d o l i n i u m , e u r o p i u m and s a m a r i u m were s e p a r a t e d f r o m the Abee sample by i o n exchange, using a three c o l u m n operation. Diaion SK#1, 100-200 m e s h r e s i n was used i n a l l c o l u m n s ; the c a t i o n exchange columns were made of a c r i l . The s e p a r a t i o n was p e r f o r m e d by S. Y a n a g i t a , Department of C h e m i s t r y , U n i v e r s i t y of Tokyo.  54  LOG(ACTIVITY) (arbitrary  units)  i  50  l 100  ml  FIGURE 4-1.  1  i  150  I  200  ! I  250  ^.  ELUATE  ELUTION CURVES FOR TB, GD, EU AND SM  A r t i f i c i a l l y - p r o d u c e d r a d i o a c t i v e i s o t o p e s were used to c a l i b r a t e the second i o n exchange process (see Table 4-4). The e l u t i o n curves were determined by m o n i t o r i n g the a c t i v i t y o f the e l u a t e and i d e n t i f y i n g i n d i v i d u a l r a d i o a c t i v e i s o t o p e s of Tb, Gd, Eu and Sm. During the subsequent Abee s e p a r a t i o n , the Gd-AB, Eu-AB and Sm-AB f r a c t i o n s correspond t o the 4 0 - 7 0 , ? 0 - l 4 0 and 140-240 ml f r a c t i o n s of the e l u a t e . The c a l i b r a t i o n was performed by S. Yanagita, Department o f Chemistry, U n i v e r s i t y o f Tokyo.  55  c u r v e s f o r the m e t e o r i t e s a m p l e and the c a l i b r a t i o n r u n were not i d e n t i c a l , p o s s i b l y because of d i f f e r e n t c o n c e n t r a t i o n s of the r a r e e a r t h elements i n the two s o l u t i o n s .  The three f r a c t i o n s G d - A B ,  E u - A B and S m - A B w e r e s e l e c t e d f r o m the eluate to c o i n c i d e w i t h the c a l i b r a t e d e l u t i o n c u r v e s f o r Gd, E u and Sm (see F i g u r e 4-1).  respectively  H o w e v e r , subsequent a n a l y s e s i n the m a s s  s p e c t r o m e t e r showed that m o s t of the Gd f r o m the m e t e o r i t e was c o l l e c t e d i n the f r a c t i o n E u - A B , and a l a r g e p o r t i o n of the E u was in the f r a c t i o n Sm-AB.  The G d - A B f r a c t i o n contained some Gd  ( e s t i m a t e d to be <• .1 j*g) as w e l l as Tb. S m a l l quantities of E u , Sm, Nd, Ce, L a and B a w e r e o b s e r v e d i n a l l three f r a c t i o n s .  S p e c i a l c a r e was r e q u i r e d d u r i n g  the m a s s s p e c t r o m e t e r a n a l y s e s to m i n i m i z e the effect of i n t e r f e r i n g elements on the s p e c t r a of Gd, E u and Sm.  4. 2 P r e p a r a t i o n of F i l a m e n t s A l t h o u g h tantalum f i l a m e n t s w e r e used f o r some of the p r e l i m i n a r y a n a l y s e s l i s t e d i n T a b l e 4-3, a l l other a n a l y s e s w e r e performed with rhenium filaments. s p e c i f i e d p u r i t y of 99-9%.  The r h e n i u m m e t a l had a  Two single f i l a m e n t a n a l y s e s ( G d - J Z l  and G E - A B ) made use of z o n e - r e f i n e d r h e n i u m of v e r y high p u r i t y . F o r these two a n a l y s e s , G d O  +  ions w e r e o b s e r v e d i n the m a s s spec-  c t r o m e t e r at m u c h l o w e r t e m p e r a t u r e s (  1350 C) than was o b s e r v e d  when the other (99.9% purity) r h e n i u m was used (<*•• 1700* C).  56  The m o s t c r i t i c a l i m p u r i t i e s i n the r h e n i u m m e t a l were B a , L a and Ce.  These were reduced to t o l e r a b l e l e v e l s by  outgassing a l l f i l a m e n t s at 2 0 0 0 ° C f o r one hour.  This eliminated  L a and Ce c o m p l e t e l y while B a was reduced to a c o n t a m i n a t i o n l e v e l w h i c h was only a s m a l l f r a c t i o n of the B a c o n t a m i n a t i o n i n the s a m p l e s t h e m s e l v e s .  The p u r i t y of a l l f i l a m e n t s used f o r a n a l y s e s  on the Abee m e t e o r i t e was checked i n the m a s s s p e c t r o m e t e r , at t e m p e r a t u r e s w e l l above o p e r a t i n g c o n d i t i o n s , p r i o r to l o a d i n g the samples. A l l reagent a n a l y s e s were p e r f o r m e d w i t h m i c r o g r a m quantities of Gd, Sm or E u .  The e l e m e n t s w e r e studied s e p a r a t e l y ,  except f o r Sm and E u w h i c h w e r e o c c a s i o n a l l y a n a l y z e d together because of the l a c k of i s o b a r s f o r these elements.  The  samples  were d e p o s i t e d on outgassed f i l a m e n t s as c h l o r i d e s , and then conv e r t e d to p e r c h l o r a t e s a l t s on the f i l a m e n t s .  F o r Gd single f i l a m e n t  p r e p a r a t i o n s , the f i l a m e n t s w e r e subsequently heated i n a i r to a d u l l r e d glow ( i n a d a r k room) to c o n v e r t the salt to the oxide f o r m , t h e r e b y i n c r e a s i n g the p r o d u c t i o n of G d O  +  ions i n the m a s s spec-  trometer. The Abee f r a c t i o n s w e r e taken up i n a drop of 3x d i s t i l l e d w a t e r , d e p o s i t e d on outgassed f i l a m e n t s , and e v a p o r a t e d to d r y n e s s .  The complete S m - A B f r a c t i o n was loaded f o r one t r i p l e  f i l a m e n t a n a l y s i s of Sm.  M o s t of the E u - A B f r a c t i o n was loaded  f o r one t r i p l e f i l a m e n t a n a l y s i s of E u (at low side f i l a m e n t t e m p e r a t u r e s ) ,  57  f o l l o w e d by an a n a l y s i s of Gd i n the same f r a c t i o n (at higher t e m p e r a t u r e s ) . A p p r o x i m a t e l y one half of the G d - A B f r a c t i o n was loaded f o r one t r i p l e f i l a m e n t a n a l y s i s of Gd.  The r e m a i n d e r of f r a c t i o n s E u - A B  and G d - A B was c o m b i n e d f o r one s i n g l e f i l a m e n t a n a l y s i s of Gd. T h i s l a t t e r a n a l y s i s w i l l be r e f e r r e d to as G E - A B .  4. 3 P r o d u c t i o n of Ion S p e c t r a w i t h M i n i m u m I n t e r f e r e n c e O p e r a t i n g p r e s s u r e s of 10 --  mass spectrometer analyses.  torr were maintained for a l l  P r e s s u r e s approaching 2 x 10  _g  t o r r w e r e a c h i e v e d f o r m a n y a n a l y s e s on the Abee f r a c t i o n s b y thoroughly outgassing the i n s t r u m e n t , and by a l l o w i n g s e v e r a l h o u r s f o r b a r i u m and other contaminants at low f i l a m e n t t e m p e r a t u r e s .  to be d r i v e n out of each s a m p l e  T h e r e was no substitute for slow  heating of the f i l a m e n t s to i n c r e a s e the p u r i t y of the Gd, Sm and E u i n each s a m p l e . None of the contaminants  i n the reagent s a m p l e s caused  s i g n i f i c a n t s p e c t r a l i n t e r f e r e n c e w h i c h c o u l d not be e l i m i n a t e d by c a r e f u l heating of the s a m p l e f i l a m e n t s i n the m a s s s p e c t r o m e t e r . F o r the m e t e o r i t e a n a l y s e s , h o w e v e r , it was not a l w a y s p o s s i b l e to e l i m i n a t e a l l i n t e r f e r i n g ions i n this way. Nd~*" ions i n t e r f e r e d with the m e a s u r e m e n t of the S m  LaO  and  spectrum  +  d u r i n g a n a l y s i s S m - A B , and LaO"^ ions i n t e r f e r e d w i t h the Gd~^ s p e c t r u m d u r i n g a n a l y s e s E u - A B and G d - A B . p o s s i b l e to m o n i t o r the i n t e n s i t y of the N d  F o r t u n a t e l y , it was  and L a O  +  ion beams  58  a t m a s s e s 146  a n d 155  r e s p e c t i v e l y , so that a c o r r e c t i o n c o u l d b e  m a d e to s e p a r a t e these s p e c t r a f r o m the Sm tionally.  The  • i n C h a p t e r 5. ( a t m a s s 155)  spectrum  computa-  m e t h o d of a p p l y i n g these c o r r e c t i o n s i s d i s c u s s e d A  similar  w i t h the G d  correction for L a O spectrum  +  interference  c o u l d not be m a d e b e c a u s e of  the s i m i l a r m a s s r a n g e s f o r b o t h i o n s .  The only way  i n w h i c h the  155 Gd  i s o t o p i c abundance c o u l d b e d e t e r m i n e d f o r the m e t e o r i t e  s a m p l e was  b y u s i n g the G d 0 ;  +  spectrum  single filament analysis G E - A B .  produced during  the  59  CHAPTER 5  C O M P U T E R REDUCTION OF C O M P L E X  SPECTRA  5.1 E v a l u a t i o n of P e a k Heights The m e t h o d of r e c o r d i n g s p e c t r a , and the d e f i n i t i o n of a s c a n (the unit r e c o r d on magnetic tape) were d i s c u s s e d i n Section 3. 4.  A r e p r e s e n t a t i v e s c a n of the G d * s p e c t r u m was shown  in F i g u r e 3-3.  A c o m b i n e d E u * and S m  +  s p e c t r u m is i l l u s t r a t e d  in F i g u r e 5-1. Data was r e c o r d e d i n d i g i t a l f o r m and a l o w - p a s s f i l t e r was u s e d to r e m o v e high f r e q u e n c y noise f r o m the s p e c t r u m (see S e c t i o n 3. 5).  A f t e r f i l t e r i n g the data, a m o v i n g 5-point a v e r a g e  was computed f o r the purpose of l o c a t i n g the t i m e c o o r d i n a t e s of the v a r i o u s peaks.  A t a p o s i t i o n where the 5-point average was a  m a x i m u m , the a s s o c i a t e d peak height (without the b a s e l i n e r e m o v e d ) was chosen to be the l a r g e s t value of the five points c o m p r i s i n g the m o v i n g a v e r a g e .  The true peak height was then computed by-  s u b t r a c t i n g the r e c o r d e d b a s e l i n e w h i c h was  represented, for  each shunt i n t u r n , by the b e s t l e a s t - s q u a r e s s t r a i g h t line f i t t i n g a l l (baseline) data (within a single scan) w h i c h was r e c o r d e d w h i l e the s c a n d i r e c t i o n was s t a t i o n a r y .  The c o r r e c t e d peak height and  its a s s o c i a t e d t i m e coordinate w e r e subsequently used to r e p r e s e n t the i n t e n s i t y and t i m e of m e a s u r e m e n t of the. component i o n b e a m  60  152 153 150  i >  .151  i  !  154  time ( s e c )  100  "T"  - — r — —  75  50  0  25  i  147  i  •149 144  148  Sm  ;Sm  FIGURE 5-1.  Sm  Sm  Sm  Eu  Sm  Eu  Sm  SLOW DISCONTINUOUS SCANNING  Down-mass p o r t i o n o f a s c a n s h o w i n g t h e E u and Sm  spectra.  61  at a p a r t i c u l a r m a s s number. The r e d u c t i o n of data f r o m a single scan gave two m e a s u r e m e n t s of the intensity of each peak i n the s p e c t r u m . B y r e t a i n i n g a running t i m e coordinate it was p o s s i b l e to combine m e a s u r e m e n t s d e r i v e d f r o m a set of s e v e r a l s u c c e s s i v e scans and apply a c o r r e c t i o n f o r the continuous growth or decay of each type of i o n d u r i n g the set of scans.  5. 2 L e a s t Squares R e d u c t i o n of a Set of Scans P a s t e x p e r i e n c e i n our m a s s s p e c t r o m e t e r l a b o r a t o r y has shown that a t h i r d o r d e r p o l y n o m i a l p r o v i d e s a f a i r l y good r e p r e s e n t a t i o n of the i n t e n s i t y of a single peak as a function of t i m e t , i . e. 2 I(t) =  a i  + a t + a t 2  3  3 + a t  (5-1)  4  In o r d e r to m i n i m i z e the e r r o r s i n t r o d u c e d by this type of approxi m a t i o n i t i s d e s i r a b l e to make use of the fact that the i s o t o p i c c o m p o s i t i o n of each type of i o n r e m a i n s e s s e n t i a l l y constant d u r i n g a set of scans at f i x e d f i l a m e n t t e m p e r a t u r e s . 151 the i n t e n s i t i e s 1^ and I ^ of the E u may be r e p r e s e n t e d i n the f o r m  A s an e x a m p l e ,  .153 and E u  component i o n b e a m s  62  I (t) = r (b + b t + b t 1 l i 2 3  2  3 + b t ) 4 ' (5-2)  I (t)= r ( b + b t + b t 2  2  2  3  2  + b t  3  4  where r ^ and r ^ a r e p r o p o r t i o n a l to the r e l a t i v e i s o t o p i c abundance values (one of w h i c h must be a r b i t r a r i l y f i x e d , e. g. r^ sr 1. 0).  These p a r a m e t e r s a r e constant f o r the d u r a t i o n of  a set of scans at f i x e d f i l a m e n t t e m p e r a t u r e s , except for m i n i m a l f r a c t i o n a t i o n effects w h i c h a r e g e n e r a l l y s m a l l ( < 0.05% p e r unit m a s s d i f f e r e n c e ) c o m p a r e d with s t a t i s t i c a l e r r o r s .  The t e r m i n  b r a c k e t s i s a p o l y n o m i a l r e p r e s e n t a t i o n of the growth or decay 151  153  c h a r a c t e r i s t i c s of both the E u  and E u  ion beams.  Repres-  entation (5-2) r e q u i r e s the d e t e r m i n a t i o n of only 5 p a r a m e t e r s w h e r e a s 8 p a r a m e t e r s would have been r e q u i r e d i f the constancy of the i s o t o p i c r a t i o had not been used as a constraint. A suitable method f o r d e t e r m i n i n g the o p t i m u m values of the p a r a m e t e r s r ^ , b^, b_>, b^, b ^ i s one w h i c h m i n i m i z e s the v a r i a n c e between the m e a s u r e d peak heights and t h e i r  polynomial  r e p r e s e n t a t i o n i n equations (5-2).  If p ^ a n d t ^ a r e the peak 151 th height and t i m e of m e a s u r e m e n t of the E u peak d u r i n g the k m e a s u r e m e n t of that peak, and p  ,t 2k  a r e s i m i l a r quantities f o r 2k  153 the E u  peak, then the function t o b e m i n i m i z e d may be  i n the f o r m  represented  63  k= l,2n I  L  where n i s the number of scans i n the set. T h e r e a r e s e v e r a l p o s s i b l e s o u r c e s of e r r o r when e x p e r i m e n t a l l y m e a s u r i n g any s i n g l e peak i n t e n s i t y on the m a s s spectrometer:  overshooting  a peak when m a n u a l l y advancing the  magnet c u r r e n t , m a k i n g an e r r o r i n shunt s e l e c t i o n , sudden line t r a n s i e n t s , etc.  These can r e a d i l y be r e c o g n i z e d on the c h a r t  r e c o r d d u r i n g an a n a l y s i s .  P r o v i s i o n has t h e r e f o r e been made f o r  e l i m i n a t i n g faulty peak m e a s u r e m e n t s f r o m the r i g h t hand side of equation (5-3).  Such e r r o r s w e r e , however, infrequent i n p r a c t i c e .  The conditions w h i c h m u s t be s a t i s f i e d i n o r d e r f o r g(r ,b ,b ,b ,b ) to be a m i n i m u m a r e :  ^ 2~ * r  (5-4) b  l "  * 3""^ 4~ b  b  These give r i s e to five n o n - l i n e a r equations i n the v a r i a b l e s A s o l u t i o n i s p o s s i b l e by a p e r t u r b a t i o n method p r o v i d e d a good f i r s t a p p r o x i m a t i o n m a y be obtained.  64  T h i s i s p o s s i b l e since the i s o t o p i c r a t i o r ^ / r ^ i s known to w i t h i n a few per cent.  If r ^ i s i n i t i a l l y t r e a t e d as a constant =: r ,  then four l i n e a r equations may b^, b^, b^, b^.  be obtained i n the v a r i a b l e s :  T h e s e can e a s i l y be s o l v e d d i r e c t l y to give good  e s t i m a t e s of b^, b , b^,  b^.  A t this stage, equations (5-4) may the new v a r i a b l e s A r ^ ,  r  2=  r  2  +  bjSbj  b  2  = b  2  .  A r  2  +  A.bj  Ab  +  A  b^,  b  ^b^,  3 =  .  b  3  +  b =  b  4  ^3'  A  +  4  b  where  3 Ab  (5-5)  4  2  The l i n e a r i z e d equations (5-4) may e s t i m a t e of r , b ^ b 2  A  be l i n e a r i z e d u s i n g  2 >  b^, b .  p e r f o r m e d i n this way u n t i l r  be s o l v e d to get an i m p r o v e d  S e v e r a l i t e r a t i o n s may  4  2 >  s u f f i c i e n t p r e c i s i o n ( < .01% was  b^, b , b^, b 2  4  be  are a l l known w i t h  c o n s i d e r e d m o r e than adequate)..  F i g u r e 5-2 shows a d i r e c t c o m p a r i s o n between the m e a s u r e d peak heights and the s o l u t i o n p o l y n o m i a l s I^(t) and ^ ( t ) f o r a set of scans of the E u  spectrum.  The quality of the p o l y n o m i a l fit i s  quite good, even when the b e a m i n t e n s i t y shows s t r o n g growth and/or decay c h a r a c t e r i s t i c s .  In p r a c t i c e , h o w e v e r , the b e a m  i n t e n s i t y u s u a l l y changed m o n o t o n i c a l l y d u r i n g a set of s c a n s , with a t o t a l v a r i a t i o n of 2 0 % or l e s s .  Ion b e a m intensity  Upper curve:  I ^ a r ^ b j + b t + b^t  L o w e r curve:  2 3 I (t) = r (b + b t + b t + b _t ) 1 1 1 2 3 4  (10" amp) 12  2  2  + b t ) 3  4  ;  4.0  153 Eu r  = 1.0 scan  r _ , b, , b , b _ , a n d b . w e r e 2 1 , 2 3 4 3.5  =  j Eu  .x  (Pzk-Vzk))'  *  •*  Eu  4  Eu  1  x  151  I  time t  10  153  ;  15  20  F I G U R E 5-2. P O L Y N O M I A L R E P R E S E N T A T I O N O F I O N B E A M 1  5  1  l  L e a s t s q u a r e s p o l y n o m i a l fitI .—I ^  JL  The E u  r  M e a s u r e d p e a k h e i g h t p j ^ at  • . ..x •  2.5  151  x*  k = I, 12  k = 1,12  6  determined  b y m i n i m i z i n g the f u n c t i o n g(r2»b)  3.0  x.  25  Time (min)  INTENSITY  a n d E u " i o n b e a m s h a v e b e e n r e p r e s e n t e d b y p o l y n o m i a l s o f d e g r e e 3 f o r a s e t of s c a n s . 3  66  The above e x a m p l e i l l u s t r a t e s the b a s i c n u m e r i c a l p r o c e dure e m p l o y e d i n this r e s e a r c h to compute the l e a s t s q u a r e s p o l y n o m i a l r e p r e s e n t a t i o n of the v a r i o u s peak i n t e n s i t i e s a s s o c i a t e d with ions of a single element.  F o r the m a j o r i t y of a n a l y s e s i t  was not n e c e s s a r y to apply a c o r r e c t i o n f o r the effect of i n t e r f e r i n g ion spectra.  H o w e v e r , whenever s p e c t r a l i n t e r f e r e n c e was  o b s e r v e d , c a r e was taken to r e c o r d a d d i t i o n a l peaks i f p o s s i b l e , so that the growth o r decay c h a r a c t e r i s t i c s of each a d d i t i o n a l ion s p e c i e s c o u l d be d e t e r m i n e d and subsequently r e m o v e d . W h e r e s e v e r a l ion s p e c t r a o c c u r i n a set of scans the to be s o l v e d a r e m o r e complex. be t r e a t e d as v a r i a b l e s .  equations  A l s o , not a l l i s o t o p i c r a t i o s should  Where i n t e r f e r i n g ions f o r m only a s m a l l  percentage of the t o t a l beam, i t i s m o r e r e l i a b l e to f i x t h e i r i s o t o p i c c o m p o s i t i o n at p u b l i s h e d values than to t r e a t them as a d d i t i o n a l v a r i a b l e s i n the l e a s t s q u a r e s solution. In the g e n e r a l c a s e , where there a r e m e l e m e n t s and q d i f f e r e n t peaks, the function w h i c h r e p l a c e s (5-3) as the sum of s q u a r e s to be m i n i m i z e d i s  (5-6)  where p ^ i s the k** m e a s u r e m e n t of the j ^ 1  1  peak at t i m e  67  t. •; r . i s the r e l a t i v e isotopic abundance a s s o c i a t e d with peak j and e l e m e n t i ; and b ^ , b ^ ' b ^ , b ^  the c o e f f i c i e n t s of  a r e  the g r o w t h p o l y n o m i a l for element i . ( i = l,m; w = 1, 4)  The v e c t o r b r e p r e s e n t s a l l b. 1W  while the v e c t o r  r r e p r e s e n t s only those abundance values  w h i c h a r e to be t r e a t e d as v a r i a b l e s i n the least squares solution. F o r convenience, a new l o g i c a l function f^j may be defined such that f-- i s false only when r.. i s to be t r e a t e d as a v a r i a b l e .  This  means that r r e p r e s e n t s a l l r ^ ( i s: l,m; j = l,q) f o r w h i c h f — is f a l s e .  (Note that equation (5-6) includes the c o m m o n s i t u a t i o n  where isotopes of m o r e than one element e x i s t at a given m a s s number The function g(r,b) i s a m i n i m u m when i = l.m;  14-= 0  «b.  w = 1, 4  1W  and  (5-7)  \_g_  jr.-  _  o  j=l,q;  i ~ \ , m ;  ij  = false  These equations reduce to  (.P  j = 1, q  k  k = 1, 2n u =: 1, m;  S L  1  T -> ( P  k = 1, 2n  v k  v  SL  p..  i = 1, m  b. F- )T  SL  1  .ty,- = 0 1  w = 1 ";4  v ; 1,4  b. t ^ X ^ b t\\=0 .i -= ^l , m i v w = r~ . iw v k , , uw v k l,4 w = 1, 4 k  1  u=l,m;  v  =  l,q;  f  =  uv  false  (5-8) ' v  68  E q u a t i o n s (5-8) m a y be l i n e a r i z e d and s o l v e d by a p e r t u r b a t i o n method s i m i l a r to the p r e v i o u s example, i n o r d e r to obtain solution vectors  r and  b.  5. 3 C a l c u l a t i o n of Means and Standard D e v i a t i o n s of Isotopic R a t i o s f o r a Set of Scans It i s evident f r o m the d i s c u s s i o n i n the p r e v i o u s s e c t i o n that i s o t o p i c r a t i o s r e s u l t e d d i r e c t l y f r o m the s o l u t i o n of equations (5-8).  In the case of the e x a m p l e given e a r l i e r one could r e a d i l y  c a l c u l a t e the i s o t o p i c r a t i o E u * ^ / E u  == r ^ / r ^ .  H o w e v e r , there  is no e a s y way to estimate the e r r o r i n this r a t i o apart f r o m comp a r i n g r e s u l t s f r o m s e v e r a l sets of scans.  T h i s was  considered  inadequate because of l a r g e v a r i a t i o n s i n the s t a b i l i t y of an ion b e a m f r o m one set of scans to another.  Differences in sample purity  and abundance, and v a r i a t i o n s i n the operating c h a r a c t e r i s t i c s of the m a s s s p e c t r o m e t e r a l s o contribute to r e a l v a r i a t i o n s i n the p r e c i s i o n achieved.  (When i s o t o p i c r a t i o s and standard deviations  w e r e c a l c u l a t e d by the method to be d e s c r i b e d i n this s e c t i o n , a test f o r homogeneity of v a r i a n c e f r o m one set of scans to another showed c o n v i n c i n g l y that there w e r e r e a l - v a r i a t i o n s i n the v a r i a n c e . ) A method was t h e r e f o r e d e v i s e d f o r e s t i m a t i n g the standard d e v i a t i o n of each i s o t o p i c r a t i o f o r a set of scans. The p o l y n o m i a l r e p r e s e n t a t i o n of the i n t e n s i t y of peak j as a f u n c t i o n of t i m e i s  69  I.(t) = <^ r.. > _ i = l,m w = J  1 J  -b. t l , 4  (5-9)  1 W  Suppose the dominant component i n this peak belongs to the e l e m e n t r e p r e s e n t e d by s u b s c r i p t i = s.  The i s o t o p i c abundance c o e f f i c i e n t  a s s o c i a t e d with peak j and element s i s j -  If  r  g  w  e  a s s u m e that,  on the k** m e a s u r e m e n t of peak j , the d i f f e r e n c e between the measurec 1  peak height p., and the p o l y n o m i a l value I.(t. ) may be r e g a r d e d as a J J- j k -  semi-independent m e a s u r e m e n t of j > then we can t r a n s f o r m r  s  equation (5-9) into the f o r m  p.,. = r'(s,j,k)5>"~ bsw tj^ k" + J> jk ^-—r1  w=  r'(s,j,k) ( =  ~ i=l, m i jd s  1,4  -21  P J k  r  i = l,m i ptf s  1  £Z__ J  w=l,4  r.. ij ^  b. t '* aw j k w  w = 1,  4  b. 1  W  J  b J  k  w = l , 4  s  w  J  k  (5-10)  where r'(s, j,k) i s a semi-independent m e a s u r e m e n t of the c o e f f i c i e n t r j at t i m e tj^,. g  C o n s i d e r the case where we a r e i n t e r e s t e d i n m e a s u r i n g the i s o t o p i c r a t i o R =: r . / r . , the r a t i o of isotopes of element s 1 2 S J  o c c u r r i n g at peaks j and j .  S J  B e c a u s e of the s y m m e t r y of s p e c t r a  w i t h i n a s i n g l e s c a n it i s r e a s o n a b l e to compute one e s t i m a t e of R for e a c h scan.  L e t R^ (i == 1, 2, ... n) be the computed e s t i m a t e of  70  R d u r i n g scan i .  Then  '  r H s . j p l ) + r ' ( s , j , 2) r (s,j ,l) + r'(s,j 1  2  2)  2 >  r ' ( s , j ,3) + r ' ( s j , 4 ) J  :  R  z=  1  : r'(s,j ,3) + r ' ( s j , 4 ) 2  3  (5-11)  2  etc.  T h i s method of computing semi-independent e s t i m a t e s of each i s o t o p i c r a t i o was a p p l i e d to a l l r a t i o s of i n t e r e s t f o r the three e l e m e n t s E u , Sm and Gd.  The c o m b i n e d data, f o r each  i s o t o p i c r a t i o , was then p r e s e n t e d i n the f o r m  i = 1, n ( ^~ (R -R) /(n-l))^ i = 1, n 2  i  qr =  <r~ / f n  (5-12)  w h e r e R i s the m e a n value of the i s o t o p i c r a t i o , ^~ i s an e s t i m a t e of the s t a n d a r d d e v i a t i o n of each m e a s u r e m e n t R^, and ^P" i s an e s t i m a t e of the s t a n d a r d d e v i a t i o n of the mean.  71  5. 4 C a l c u l a t i o n of F r a c t i o n a t i o n - C o r r e c t e d P a r a m e t e r s In the p r e c i s e ^determination of i s o t o p i c abundances, c a r e m u s t be taken to account f o r any p r o c e s s e s w h i c h m i g h t cause m a s s d i s c r i m i n a t i o n i n the s a m p l e at any stage p r i o r to, o r d u r i n g , the m e a s u r e m e n t of an i o n b e a m i n the m a s s s p e c t r o m e t e r .  In the  p r e s e n t r e s e a r c h , m a s s f r a c t i o n a t i o n was detected by c o m p a r i n g i s o t o p i c r a t i o s f o r s e v e r a l sets of scans obtained at one or m o r e filament temperatures.  The l a r g e s t f r a c t i o n a t i o n effect o b s e r v e d  was 0.3% p e r unit m a s s d i f f e r e n c e .  T h i s r e p r e s e n t s the change i n  i s o t o p i c c o m p o s i t i o n of the i o n b e a m over the e x t r e m e s of s a m p l e filament temperatures produced.  f o r w h i c h a m e a s u r e a b l e ion b e a m could be  F o r a l l three e l e m e n t s , E u , Sm,  p r o c e d e d t o w a r d the e n r i c h m e n t  and Gd, the p r o c e s s  of the h e a v i e r isotopes i n the i o n  b e a m (or the d e p l e t i o n of the l i g h t e r isotopes) w i t h i n c r e a s i n g filament temperatures  and time.  In a g r e e m e n t w i t h s i m p l e t h e o r y (see O z a r d and R u s s e l l , 1970) and c o m m o n e x p e r i e n c e , the f r a c t i o n a t i o n effect f o r a p a r t i c u l a r e l e m e n t was o b s e r v e d to s a t i s f y the equations  X  = X (1 + ( m - m ) ^ 2  N  1  Y = Y (1 + (m -m )^ N  4  3  )  )  (5-13)  where X and Y a r e m e a s u r e d i s o t o p i c r a t i o s (for the same element)  72  c o r r e s p o n d i n g to the m a s s r a t i o s m,/m . 1.  2  and m  3  /m  4  r e s p e c t i v e l y ; X^ and Y_ are n o r m a l i z e d (or f r a c t i o n a t i o n N N T  corrected) isotopic ratios; ^  i s the f r a c t i o n a l change i n the i s o -  topic r a t i o s per unit m a s s d i f f e r e n c e (due to f r a c t i o n a t i o n ) . ^ «-l. The quantity ^  may  be e l i m i n a t e d f r o m equations  (5-13) to give Y (m -m )^ N  ,  Y = Y  N  4  +  3  U-X )  :  (5-14)  N  X  N  {  m  2" l m  )  F i g u r e 5-3 shows the c l o s e a g r e e m e n t between the slope p r e d i c t e d by equation (5-14) and the m e a s u r e d i s o t o p i c r a t i o s . The s o l i d line i l l u s t r a t e d i n this plot was f o r c e d to pass through the point (.9361, .6769) adopted by E u g s t e r et a l (1970a) f o r t e r r e s t r i a l gadolinium.  The p u b l i s h e d s t a n d a r d d e v i a t i o n of the  l a t t e r c o o r d i n a t e i s 0.01% w h i l e the r a t i o G d  1 5 6  /Gd  l 6  ° =  .9361  is the value which they u s e d f o r n o r m a l i z a t i o n of a l l other Gd isotopic ratios.  The n o r m a l i z a t i o n p r o c e s s i n v o l v e s m e a s u r i n g  two i s o t o p i c r a t i o s , X and Y, a s s i g n i n g a s p e c i f i c value to 156 X-^ (e.g. X^j —  Gd  ;  /Gd  160 rr .9361) and s o l v i n g equation (5-14)  forY . N  A n a l t e r n a t i v e method may  be used to r e m o v e the effect  of f r a c t i o n a t i o n f r o m m e a s u r e d i s o t o p i c r a t i o s . may  be r e w r i t t e n i n the f o r m  E q u a t i o n (5-14)  73 F I G U R E 5-3.  MASS S P E C T R O M E T E R F R A C T I O N A T I O N  .155 Y =  .688  .684  .680  .676  ~  .672  .930  .9 34  .938  .942  .946  The data points r e p r e s e n t m e a s u r e d i s o t o p i c r a t i o s f o r i n d i v i d u a l sets of scans (usually 6 scans per set). Data a r e shown f o r one single f i l a m e n t a n a l y s i s (after c o r r e c t i o n f o r the i s o t o p i c c o m p o s i t i o n of oxygen) and f o r one t r i p l e f i l a m e n t a n a l y s i s on the same sample. The s o l i d line has the t h e o r e t i c a l f r a c t i o n a t i o n slope and p a s s e s through the point (.9361, .6769).  74  X-X Y N ( - ) = l+ * ( £-) Y X y  N  N  N  X  N  Y ^— X  =  = X  Y = constant  (5-15)  N  where <X = (m^-m^J/^m^-m^).  B y m e a s u r i n g a p p r o p r i a t e i s o t o p i c r a t i o s f o r the e l e m e n t s G d and Sm i t i s p o s s i b l e to evaluate p a r a m e t e r s w h i c h are c o r r e c t e d f o r f r a c t i o n a t i o n and are a l s o v e r y s e n s i t i v e to ^^55, ^ , 1 5 6 ,-.,157, v , , the neutron capture p r o c e s s e s G d (n, $ )Gd , Gd (n, <j ) v  Gd^^  and S r a ^ V i ^ )Sm  Three parameters which satisfy  these conditions a r e given i n Table 5-1. - E a c h p a r a m e t e r i s sens i t i v e to only one capture p r o c e s s .  The value of a l l three  p a r a m e t e r s ( A , B and C) w i l l be g r e a t e r f o r s a m p l e s w h i c h have been exposed to a l a r g e r neutron flux.  They should be constant,  h o w e v e r , f o r a l l a n a l y s e s p e r f o r m e d on the same sample.  Since  there are only two isotopes of E u , no c o r r e c t i o n f o r f r a c t i o n a t i o n was p o s s i b l e . The method of computing a n o r m a l i z e d r a t i o  Y^. o r  75  TABLE 5-1.  FRACTIONATION-CORRECTED  c< =  ± m-m. 2  Gd 156 Gdl55  G d  15?  G d ^  1  158  2  Sm ^ ml50  1 2  G d  Sm Sm^9  1 5 0  Each  1 4  G d  1  S  1  a G d  Gd^^°  /Sm  150  5  ^  2  r  ^  Gd^8 G d  l60  | .  ;  1  |  j  S  m  9  1  ^  6  S m  parameters capture  A,B and process.  a r e somewhat  (X i n above t a b l e )  ,  Gd  l 5 7  /Gd  Q *5 5 a r e ^ , ^ and ^ r e s p e c t i v e l y .  and Sm^^  irradiation.  5  \  Sm ^ Sml50 l4 ^ 152  The s e n s i t i v i t i e s 1  G*££* l60  G d  8  i5?  •  than the simple r a t i o s  the i s o t o p i c r a t i o s G d l4Q  Gd ^  R  o f the parameters  which they are d e r i v e d .  (  G d  to a p a r t i c u l a r neutron  sensitivities  greater  Sm  _ G d ^ 155  o f the f r a c t i o n a t i o n - c o r r e c t e d  C i s sensitive The  \ = Constant  1  gd 156  1 6 0  Gd^!  PARAMETERS  from  r e l a t i v e to  1 5 8  and  The  isotopes  a r e n o t s i g n i f i c a n t l y changed by n e u t r o n  76  Y w i l l now be given.  afractionation-corrected parameter Let  and  be the a p p r o p r i a t e isotopic r a t i o s computed f o r j  s c a n i ( i = i , 2,. . . n) f r o m equations (5-11).  The  corresponding  n o r m a l i z e d r a t i o Y j ^ . (or p a r a m e t e r  \ .) was computed f r o m  equation (5-14) (or equation (5-15)).  The m e a n Y ^ (or ^ ) and  standard deviations \f~ and C~ were then computed f r o m  equations  (5-12). 5.5 C o r r e c t i o n f o r the Isotopic C o m p o s i t i o n of Oxygen A l l t r i p l e f i l a m e n t a n a l y s e s were p e r f o r m e d on elemental E u  , Sm  or Gd  ions.  w e r e p e r f o r m e d using the GdO  A few s i n g l e f i l a m e n t analyses  s p e c t r u m , and a subsequent 17  c o r r e c t i o n was r e q u i r e d f o r the s m a l l c o n t r i b u t i o n of the O 18 and O  isotopes.  The c o r r e c t i o n s r e q u i r e d f o r a l l isotopes 16 +  at m a s s e s where GdO  ions w e r e m e a s u r e d are given by the  equations (168/176) c  o  r  r  o  r  r  ((170/176) c  (171/176) 'corr . (172/176) corr (173/176) corr (174/176) 'corr v  v  r 1.00232 x (168/176) meas = 1.00213 x (170/176) meas - 1.00226 x (171/176) 'meas =1.00183 x (172/176) meas =0.99990x (173/176) meas = 1.00040 x (174/176) meas v  (5-16)  77  The  r a t i o s i n d i c a t e d by the s u b s c r i p t 'meas' a r e the m e a s u r e d  r a t i o s ( u n c o r r e c t e d f o r m a s s f r a c t i o n a t i o n ) while those i n d i c a t e d by the s u b s c r i p t ' c o r r ' a r e the r a t i o s  w h i c h would have been  m e a s u r e d i f oxygen were m o n o i s o t o p i c ( i . e . only G d O ^ present).  ions  The c o r r e c t i o n s i n equations (5-16) a r e b a s e d on the  isotopic ratios 0 N i e r (1950).  +  1 8  /0  1 6  = .00204 and 0  1 7  /0  1 6  = .00037 given by  E u g s t e r et a l (1970a) have e s t i m a t e d that the  m a x i m u m e r r o r i n any of the c o r r e c t i o n f a c t o r s in equations (5-16) i s l e s s than 0. 005%, the e x p e r i m e n t a l  w h i c h i s n e g l i g i b l e c o m p a r e d to  uncertainty.  78  CHAPTER 6  I N T E R P R E T A T I O N OF M E T E O R I T I C AND T E R R E S T R I A L ISOTOPIC RATIOS  6. 1. T e r r e s t r i a l G a d o l i n i u m The r e s u l t s of s e v e r a l i s o t o p i c a n a l y s e s on t e r r e s t r i a l s a m p l e s G d - J and Gd-US a r e s u m m a r i z e d i n Table 6-1 and F i g u r e s 6-1 and 6-2. in Appendix III).  (Complete data f o r i n d i v i d u a l scans i s g i v e n  W i t h i n the e x p e r i m e n t a l u n c e r t a i n t y of the  m e a s u r e d i s o t o p i c r a t i o s (shown as two s t a n d a r d d e v i a t i o n s of the mean) there i s no s i g n i f i c a n t d i f f e r e n c e between the t e r r e s t r i a l s a m p l e s d e r i v e d f r o m two different geographic l o c a t i o n s ( e a s t e r n US and R i o de J a n e i r o ) .  F u r t h e r m o r e , the good a g r e e m e n t w i t h  the t e r r e s t r i a l r a t i o s obtained by E u g s t e r et a l (1970a) supports the v i e w that t e r r e s t r i a l G d i s of f a i r l y u n i f o r m i s o t o p i c c o m p o s i t i o n when s a m p l e s r e p r e s e n t i n g l a r g e segments of the e a r t h a r e c o m p a r e d . It i s i n t e r e s t i n g to note that the d i s p l a c e m e n t between the m e a n t e r r e s t r i a l v a l u e s obtained by E u g s t e r et a l (1970a) and those obtained by the p r e s e n t w r i t e r ( F i g u r e s 6-1 and 6-2) i s s i m i l a r to that w h i c h would be p r o d u c e d by a d i f f e r e n c e i n exposure to t h e r m a l neutrons.  (The c a l c u l a t i o n of the slope of each of the t h e o r e t i c a l  c o r r e l a t i o n l i n e s f o r t h e r m a l neutron capture i s g i v e n i n Appendix IV. ) In the w r i t e r ' s opinion, no s i g n i f i c a n c e can be attached to this apparent  79  TABLE 6-1. ISOTOPIC RATIOS OF GD IN TERRESTRIAL SAMPLES  Gd ^ 1  Gd  Analyses  *  1 6 0  *  Gd ^ 1  Gd  Gd^ Gd  l 6 U  i 6  *  8  °  A -  B  ±.00044  1.36030 ±.00027  1.69083 ±.00067  +.00014  .71593  1.13566 +.00021  1.36029 +.00054  I.69038 +.00054  .67693 +.00022  .71575 +.00029  1.13586 +.00063  1.36001 +.00042  1.6910 1.0013  Gd-JZl single f i l . z o n e - r e f . Re  .67702 ±.00025  .71591 . I . I 3 5 7 O ±.00023 ±.00018  1.35999 ±.00050  1.69070 ±.00069  Gd-US triple  .67682 ±.00015  ±.00048  .71639  1.13595 ±.00034  1.36026 ±.00027  1.6900 ±.0016  +.00008  .67688  .71601 +.00009  1.13576 +.00012  1.36021 +.00016  I.6906O  .67692 ±.00009  •71589 ±.00004  1.13590 ±.00009  1.36024 ±.00018  1.69108 ±.00031  .683  .717  1.124-  1.348  1.662  Gd-Jl,j4,J5  .67686 ±.00015  .71620 ±.00016  Gd-j6 triple f i l .  .67685 +.00026  Gd-JSl,JS2 single f i l .  triple f i l .  fil.>  Average ( t h i s work) Eugster e t a l (1970a) single f i l . z o n e - r e f . Re Collins  (1956)  *  eta l  Fractionation  A l l single correction  correction  1.13620  normalized  +.00034  Gd 56 / jl60 _ 0/361,1  to  /  G(  #  f i l a m e n t a n a l y s e s u s e d t h e Gd,0 s p e c t r u m , a n d a was made f o r t h e i s o t o p i c c o m p o s i t i o n o f oxygen. +  The f r a c t i o n a t i o n - c o r r e c t e d , A and B have t h e v a l u e s  G d  l 5 5 Gdi°o  i r r a d i a t i o n - s e n s i t i v e parameters  B  O n l y s t a t i s t i c a l e r r o r s a r e shown. s t a n d a r d d e v i a t i o n s o f t h e mean.  Gd  157  They  G d  l60  correspond  t o two  80  FIGURE 6 - 1 .  ISOTOPIC ANALYSES ON TERRESTRIAL GADOLINIUM  ,158  Gd" Gd  A  *  1 6 0  Gd-Jl,J4,J5  (triple filament)  1.1365  Value adopted by E u g s t e r e t a l (19?Oa)  Gd-JSl,JS2 (single f i l ament)  1.1360  I  Gd-US (triple filament)  I  I T I  Weighted mean (all analyses)  Gd-JZl (single f i l ament; zoner e f i n e d Re)  1.1355  \  Gd-J6  ( t r i p l e filamen  Theoretical correlation l i n e f o r thermal neutron, c a p t u r e (see Appendix IV)  1  1.1350 .7150  Gd  1 5 8  /Gd ° l 6  .7160  .7155  *" v s G d  1 5 ?  I  .7165  '.  Gd 1 5 7 T7JO Gd  L_  .7170  /Gd °'* for a l l terrestrial  analyses.  l 6  *. F r a c t i o n a t i o n c o r r e c t i o n n o r m a l i z e d t o G d ^ ^ / c d ^ ^ 1  .9361.  81  FIGURE  CORRELATION  6-2.  /ft  Gd  B  =  1.693  BETWEEN  AND A  FOR  TERRESTRIAL  GD  158 I  158  ^3? Hdi^o (  G  B  )  Gd-JSl,JS2  ( s i n g l e  1.692  f i l a m e n t )  Value computed from d a t a o f E u g s t e r e t  al  (1970a)  Gd-Jl,J4,J5 ( t r i p l e  1.691 Gd-JZl ( s i n g l e f i l a m e n t ; z o n e r e f i n e d Re)  f i l a m e n t )  \^Gd-j6  ( t r i p l e  I.690  f i l a m e n t )  Weighted mean ( a l l a n a l y s e s )  Gd-US  1.689  ( t r i p l e  f i l a m e n t )  T h e o r e t i c a l c o r r e l a t i o n l i n e f o r thermal n e u t r o n capture (see A p p e n d i x IV)  1.688  Gd Gd  1.687 1.3590  1.3594  1.3598  1.3602  C o r r e l a t i o n "between i r r a d i a t i o n - s e n s i t i v e A f o r a l l t e r r e s t r i a l a n a l y s e s .  156  156  Gd"  nA  155 l60 v  Gd  1.3606 parameters  B  and  i )  82  c o r r e l a t i o n since a l l the i s o t o p i c r a t i o s agree w i t h i n the 95% confidence l i m i t s shown.  A l s o , the s p e c i f i e d e r r o r s c o r r e s p o n d  to s t a t i s t i c a l e r r o r s only.  The a n a l y s e s were p e r f o r m e d on  different m a s s s p e c t r o m e t e r s using s i g n i f i c a n t l y d i f f e r e n t t e c h niques f o r scanning, r e c o r d i n g and r e d u c i n g m a s s s p e c t r a .  The  good a g r e e m e n t f o r a l l i s o t o p i c r a t i o s suggests that any e x t e r n a l e r r o r s m u s t be s m a l l ; h o w e v e r , the existence of any b i a s in m e a s u r e ment on e i t h e r i n s t r u m e n t could only i n c r e a s e the absolute  error  of the i s o t o p i c r a t i o s . F o r the purpose of c o m p a r i s o n between G d in t e r r e s t r i a l and m e t e o r i t i c s a m p l e s , t e r r e s t r i a l G d w i l l be a s s u m e d to have the i s o t o p i c c o m p o s i t i o n given b y the average values in Table 6-1. The  mean  r  and standard d e v i a t i o n  w e r e computed, f o r each  c o l u m n in t u r n , f r o m the equations  is  2  1,  n  i = 1,  n  2  i (6-1)  where x- and ^P. a r e the m e a n and s t a n d a r d d e v i a t i o n for the th i entry i n the column.  83  TABLE 6-2.  ADDITIONAL Gd  RESULTS FOR TERRESTRIAL GADOLINIUM  152 *+  Gd  Gd 160  This  Gd 16*0  .00918 i.00005 .00928 ±.00002  work  Eugster et a l (1970a)  154 *+ Gd ^ 1  .09961 ±.0000?  b  .943  ±.004  #  c e n t abundances o f t e r r e s t r i a l  152  Gd  154  Gd  155  Gd  156  Gd  Collins  et  .205  Gd  '2.23  15.1  20.6  c o r r e c t i o n normalized  24.80  15.7  24.5  *  Fractionation  +  E r r o r s r e p r e s e n t two  #  Normalized  a  B a s e d on a l l t r i p l e f i l a m e n t a n a l y s e s s a m p l e s G d - J and Gd-US.  b  Average o f three data s e t s analyses)  (representing three  c  Average  (more t h a n  d  Simple  e.  One  f  Based  to G d  1 5 6  standard  /Gd  l 6  °=  of eight data sets average  standard on  158  160  2.1809 14.800 20.466 15.652 24.835 21.863  .2029  15.67  Gd  2.191  14.86  20.52  157  Gd  .203  et  a l (1956)  1  (adopted arbitrarily)  Eugster  a l (1970a)  e  .9361  Per  T h i s work  d  .09975 1.00002  Value c u r r e n t l y used f o r normaliz a t i o n of a l l other r a t i o s  Gd  meas  0  f o r 23  1  1  21.6  .9361.  d e v i a t i o n s o f t h e mean.  .943  data sets  d e v i a t i o n of a s e t  single  to Gd ^^/Gd ^°=  21.76  . on  one  terrestrial different  analysis),  (several analyses), (not o f the  filament analyses.  mean),  84  A n a l y s e s on the t e r r e s t r i a l sample G d - J a l s o showed the absence of any detectable b i a s between data obtained b y the t r i p l e f i l a m e n t method (using the G d data (using the G d O  +  +  s p e c t r u m ) and single f i l a m e n t  s p e c t r u m and c o r r e c t i n g f o r the i s o t o p i c  c o m p o s i t i o n of oxygen).  F u r t h e r m o r e , there was no s i g n i f i c a n t  i n t e r - a n a l y s i s e r r o r w h i c h could be a t t r i b u t e d to loading the same sample ( G d - J i n s o l u t i o n f o r m ) onto d i f f e r e n t f i l a m e n t s . T h e r e a r e two a d d i t i o n a l Gd isotopes w h i c h w e r e s e l d o m 154  152 m e a s u r e d : Gd  and G d  r e p o r t e d i n T a b l e 6-2. Gd^^/Gd ^ 1  r a  .  T h e i r t e r r e s t r i a l abundances a r e  A l s o , the m e a n value of the u n n o r m a l i z e d  t i o i s r e p o r t e d i n Table 6-2 f o r a l l t r i p l e f i l a m e n t  a n a l y s e s on t e r r e s t r i a l Gd.  T h i s value i s 0.7% h i g h e r than the  value c u r r e n t l y used f o r n o r m a l i z a t i o n of a l l other r a t i o s (after M u r t h y et al,1970 , and E u g s t e r et a l , 1970); the c u r r e n t value was d e t e r m i n e d f r o m single f i l a m e n t a n a l y s e s only.  The t r i p l e f i l a -  ment value i s undoubtedly c l o s e r to the absolute abundance r a t i o , and should t h e r e f o r e be used when c a l c u l a t i n g the absolute abundance of the G d isotopes (see T a b l e 6-2). 6. 2 G a d o l i n i u m i n the Abee M e t e o r i t e T h r e e separate a n a l y s e s w e r e p e r f o r m e d on G d f r o m the Abee m e t e o r i t e .  Two of these used the t r i p l e f i l a m e n t con-  f i g u r a t i o n and the G d ^ s p e c t r u m .  B e c a u s e of L a O  +  interference  at m a s s 155 (see T a b l e s III - 6 and III - 7 of A p p e n d i x III) it was not  85  p o s s i b l e to m e a s u r e the Gd «-,157 Gd  ,160 * /Gd  mined.  abundance.  _ ,158/>~ J 6 0 * , Gd  /Gd  B e c a u s e the G d O  ,  only ,  0  and the p a r a m e t e r B c o u l d b e  The t h i r d a n a l y s i s was  technique.  Consequently,  +  deter-  p e r f o r m e d by the single f i l a m e n t s p e c t r u m was e m p l o y e d , there  was  no i n t e r f e r e n c e f r o m L a O * i o n s , and it was p o s s i b l e to d e t e r m i n e 155 both the Gd  160 * /Gd  r a t i o and the p a r a m e t e r A.  The  average  values f o r the three a n a l y s e s w e r e computed f r o m equations (6-1), and a r e shown i n Table  6-3.  It i s apparent that none of the Gd i s o t o p i c r a t i o s d i f f e r s by m o r e than 0. 0 6 % f r o m the t e r r e s t r i a l value.  This also applies  to the i r r a d i a t i o n - s e n s i t i v e p a r a m e t e r s A and B. A plot of ,157,_,l60 * 158.„,160* ,_. , , , . ' Gd /Gd vs. Gd /Gd ( F i g u r e 6-3) shows that the r  +  9 5 % confidence l i m i t s f o r the Abee and t e r r e s t r i a l data o v e r l a p , and the slope of the line p a s s i n g through the two points i s not the same as the slope of the t h e o r e t i c a l c o r r e l a t i o n line f o r t e r m a l neutron capture.  On a plot of B vs A, the data points f o r Abee  and f o r t e r r e s t r i a l Gd appear to be d i s t i n c t l y d i f f e r e n t on the b a s i s of the 9 5 % confidence l i m i t s .  H o w e v e r , the slope of the line through  the two points i s a l m o s t p e r p e n d i c u l a r to the> t h e o r e t i c a l c o r r e l a t i o n line for t h e r m a l neutron capture. W i t h i n the e x p e r i m e n t a l u n c e r t a i n t y , there i s no s i g n i f i cant t h e r m a l neutron i r r a d i a t i o n a n o m a l y f o r Gd i n the Abee sample ..(when c o m p a r e d w i t h the earth).  T h i s r e s u l t i s d i s a p p o i n t i n g because  of its i m p l i c a t i o n s f o r the c l a s s of enstatite c h o n d r i t e s as a whole.  86  The f a i l u r e to observe an a n o m a l y means that there i s no d e f i n i t i v e e x p e r i m e n t a l evidence to support the t h e o r i e s of M i y a s h i r o and Mason: that d i f f e r e n c e s i n the o x i d a t i o n state of c h o n d r i t e s r e s u l t f r o m d i f f e r e n c e s i n t h e i r mean d i s t a n c e s f r o m the sun d u r i n g the e a r l y h i s t o r y of the s o l a r s y s t e m .  A t the same  t i m e , t h e i r t h e o r i e s have not been d i s p r o v e d since other e s s e n t i a l conditions m u s t a l s o have been s a t i s f i e d i n o r d e r to produce a detectable i r r a d i a t i o n anomaly.  These w i l l be d i s c u s s e d i n  Section 6.6.. P r e c i s e a n a l y s e s on m e t e o r i t e s have a l s o been p e r f o r m e d by E u g s t e r et a l (1970a). and 6-4. means of  T h e i r r e s u l t s are plotted i n F i g u r e s 6-3  (The data f o r P a s a m o n t e and N o r t o n County a r e weighted several  . published analyses. )  Only the N o r t o n  County achondrite contains G d of s i g n i f i c a n t l y d i f f e r e n t i s o t o p i c c o m p o s i t i o n f r o m t e r r e s t r i a l Gd.  It l i e s along the t h e o r e t i c a l  c o r r e l a t i o n line f o r t h e r m a l neutron capture.  An irradiation  a n o m a l y c l e a r l y e x i s t s ; it has been a t t r i b u t e d to the long c o s m i c r a y exposure age of the N o r t o n County m e t e o r i t e .  The W e e k e r o o  Station i r o n has an even longer c o s m i c r a y exposure age, but i t s s m a l l e r m a s s and i n f e r i o r m o d e r a t i n g p r o p e r t i e s p r e s u m a b l y r e s u l t e d i n the p r o d u c t i o n of v e r y few t h e r m a l neutrons ( E u g s t e r et a l 1970a).  The other m e t e o r i t e s include another i r o n (Copiapo),  an achondrite ( P a s a m o n t e ) and a b r o n z i t e chondrite ( F o r e s t City).  87  TABLE 6 - 3 .  Chemical fraction  ISOTOPIC  Gd 155 * Gd I"&~o  COMPOSITION OF GD  Gd 157  *  Gd  IN THE ABEE  METEORITE  158 *  Gd  Gd T6~0  Eu-AB triple f i l .  .71614 +.00024  1.13533 ±.00035  1.68987 +.00065  Gd-AB triple f i l .  ,..71607 1.00032  1.1351 3.0011  1.6878 ±.0036  .67652  .71598  s i n g l e f i l . +.00024 z o n e - r e f . Re  +.00012  GE-AB  1.13557  + .OOO31.  1.36092 +.00044  1.68991 +.00058  ( t h i s work)  +.00024  +.00010  .71602  1.13545  I.36092  1.68986  Terrestrial  .67688 +.00008  .71601 +.00009  1.13576 +.00012  I.36021 +.00016  I.69060 +.00034  Average  ( t h i s work)  .67652  B  +.00023  +.00043  Fractionation  a  The f r a c t i o n Eu-AB c o n t a i n e d most o f t h e Gd as w e l l as t h e Eu f r o m t h e o r i g i n a l 9 . 5 g Abee s a m p l e . T h e r e was no s p e c t r a l i n t e r f e r e n c e b e t w e e n Eu+ and Gd+ i o n s .  b  The f r a c t i o n Gd-AB c o n t a i n e d v e r y l i t t l e be < 0.1p«.g on t h e b a s i s o f t h e o b s e r v e d sity).  c  N o t a l l o f t h e Eu-AB and G d - A B _ _ f r a c t i o n s were l o a d e d f o r the f i r s t two a n a l y s e s a b o v e . The r e m a i n d e r , a p p r o x i m a t e l y 20% o f f r a c t i o n Eu-AB and 30% o f f r a c t i o n Gd-AB, was combined on a s i n g l e ( z o n e - r e f i n e d ) r h e n i u m f i l a m e n t f o r a n a l y s i s GE-AB. the f o o t n o t e s  to Table  6-1  to Gd^"^/Gd"^° =  .9361.  *  See  correction normalized  +.00044  Gd ( e s t i m a t e d t o Gd+ beam i n t e n -  for definitions  o f A and B.  88  FIGURE 6-3.  Gd Gd  CORRELATION. OF GD ISOTOPES IN METEORITES  158 * 150  1.1370  Pasamonte  1.1365 Terrestrial ( t h i s work)  1.1360  Forest City Copiapo  1.1355  1.1350  Theoretical correlation l i n e f o r thermal neutron capture (see Appendix IV)  Weekeroo Station  1.1345 .7140  .7150  7160  ±  .7170  G d ^  .7180  Gd / G d ° * vs G d / G d ° * f o r . A b e e ( t h i s w o r k ) and p r e v i o u s m e t e o r i t e a n a l y s e s ( E u g s t e r e t a l , 1970a). 1 5 8  *  l 6  Fractionation  l 5 7  l 6  correction normalized  to G d ^ ^ / G d ^ ^  ^361,  89  6-4.  FIGURE  A  CORRELATION BETWEEN A AND  Gdl57 l60 l  B IN  METEORITES  J  Gd  Norton County  I.696  1.694  1.692  Forest City -  Weekeroo Station  1.690  1  Pasamonte Terrestrial ( t h i s work)  -a—  I  Abee  Copiapo  1.688  1.686  Theoretical correlation l i n e f o r thermal neutron capture (see Appendix IV) A -  Gd  1 5 6  ,Gd  ,  1 5 6  — \ — Gd i 6 o Gd  )  1 5 5  1.684 1.358  JL  1.359  J_  1.360  1.361  -  1  >  1.362  C o r r e l a t i o n between i r r a d i a t i o n - s e n s i t i v e p a r a m e t e r s B and A f o r Abee ( t h i s work) and p r e v i o u s m e t e o r i t e a n a l y s e s ( E u g s t e r e t a l , 1970a).  90  6.3  Do Abee and T e r r e s t r i a l G a d o l i n i u m have S i m i l a r  Composition?  The apparent d i f f e r e n c e between the values of A and B f o r the Abee and t e r r e s t r i a l s a m p l e s may  be s i g n i f i c a n t .  e x p l a i n e d by t h e r m a l neutron capture alone.  It cannot be  Nor can it be the  r e s u l t of m a s s f r a c t i o n a t i o n d u r i n g the c h e m i c a l p r e p a r a t i o n of the Abee s a m p l e , p r o v i d e d the f r a c t i o n a t i o n p r o c e s s was  governed  by a s i m p l e m a s s dependence. It i s p o s s i b l e that the Abee m e t e o r i t e was d e r i v e d f r o m a r e g i o n where the r e l a t i v e abundance of the n u c l i d e s was ferent f r o m that of p r e - t e r r e s t r i a l matter.  slightly dif-  Apart from meteorite  a n a l y s e s , the only other Gd i s o t o p i c studies on e x t r a - t e r r e s t r i a l s a m p l e s are the l u n a r a n a l y s e s of E u g s t e r et a l (1970b) and L u g m a i r (1970).  T h e i r l u n a r and t e r r e s t r i a l data c l e a r l y lie along the c o r r e -  l a t i o n l i n e f o r t h e r m a l neutron capture (see F i g u r e 1-4).  T h i s i s to  be expected if the e a r t h and moon are g e n e t i c a l l y r e l a t e d , and w e r e produced f r o m the same p o o l of n u c l i d e s .  The m e t e o r i t e s  may,  h o w e v e r , have been p r o d u c e d f r o m s l i g h t l y d i f f e r e n t s o u r c e m a t e r i a l . A l t e r n a t i v e l y , the apparent d i f f e r e n c e between the t e r r e s t r i a l and Abee values of A and B may  be the r e s u l t of poor s t a t i s t i c a l  e s t i m a t e s of the standard d e v i a t i o n s f o r the Abee data, or they  may  r e s u l t f r o m a d d i t i o n a l s o u r c e s of e r r o r . T h e r e a r e three p o s s i b l e s o u r c e s of e r r o r w h i c h may  have  i n t r o d u c e d a b i a s i n the m e a s u r e m e n t of Abee Gd r e l a t i v e to t e r r e s t r i a l  91  Gd.-  T r a c e amounts of one or m o r e i n t e r f e r i n g ions may  b e e n . s u p e r i m p o s e d upon the Gd probable ions are B a F , B a C l +  or GdO +  , LaCl  +  spectra.  The  and LaC>2 . +  r a t i o s was  most  None of these '  alone would give the o b s e r v e d a n o m a l y , however, and no change i n any of the Gd 155 15 8 /Gd lAf) '*  have  systematic  observed.  N e v e r t h e l e s s , the p o s s i b i l i t y of ion i n t e r f e r e n c e cannotbe r u l e d out completely. A second p o s s i b i l i t y i s that the b a s e l i n e was over the m a s s range 155-160 (or 171-176 f o r G d O  +  not  ions).  constant Since  b a s e l i n e s w e r e m e a s u r e d only at the ends of each s p e c t r u m (see Section 3.4), the e x i s t e n c e of any b a s e l i n e i r r e g u l a r i t y w i t h i n the s p e c t r a l range would not have been detected.  Baselines were  r o u t i n e l y c h e c k e d between peaks at the beginning of each a n a l y s i s , but d i s t o r t i o n of up to 0.1 to Q..2%  would not have been detected d u r i n g  a v i s u a l r e a d i n g of the d i g i t a l v o l t m e t e r . The m o s t probable causes of b a s e l i n e d i s t o r t i o n are s e c o n d a r y e l e c t r o n s and/or s c a t t e r e d ions (most l i k e l y B a ^ )  i n the v i c i n i t y of the ion detector.  B a s e l i n e d i s t o r t i o n was analyses (see S e c t i o n 6. 5); i t was the m a s s range of Sm Ba  +  a m a j o r s o u r c e of e r r o r f o r a t t r i b u t e d to B a  is m u c h c l o s e r to that of B a ;  +  ions.  Sm*  However,  distortion from  ions should be s i g n i f i c a n t l y l e s s i n the higher Gd m a s s range.  A l s o , Gd was . i t was  v a p o u r i z e d at higher f i l a m e n t t e m p e r a t u r e s than  t h e r e f o r e p o s s i b l e to b u r n off m o r e of the r e s i d u a l B a  Sm;  before  92  producing a G d  +  ion beam.  Although b a s e l i n e d i s t o r t i o n m a y be  a s o u r c e of e r r o r d u r i n g Gd a n a l y s e s , it does not e x p l a i n the s i m i l a r i s o t o p i c r a t i o s obtained f o r Abee when u s i n g the G d * and G d O * spectra. A t h i r d source of e r r o r i s p r e s s u r e s c a t t e r i n g i n the mass spectrometer.  T h i s c r e a t e s a slight t a i l on e i t h e r side of  the s p e c t r a l peaks.  D i r e c t m e a s u r e m e n t of peak p r o f i l e s showed  that the height of the t a i l at a p o s i t i o n \ m a s s unit above o r below a s p e c t r a l peak.was < 0.1% of the peak height f o r operating p r e s s u r e s <  1.0 x 10  away was  <  torr.  The c o r r e s p o n d i n g t a i l height 1 m a s s unit  0. 0 3 % .  These e r r o r s r e p r e s e n t a s i g n i f i c a n t b i a s  in m e a s u r e m e n t w h i c h v a r i e d with the operating p r e s s u r e .  The  m a x i m u m e r r o r s , w h i c h could be a t t r i b u t e d to p r e s s u r e s c a t t e r i n g , i n the r a t i o s G d  1 5 5  /Gd  l 6  ° * , Gd  1 5 7  /Gd  1 6 0  * , Gd  1 5 8  /Gd  l 6  ° * ,  A and B w e r e 0.01%, 0.04%,0.Q3%, 0.05% and 0.01% r e s p e c t i v e l y . The slope of the e r r o r line a s s o c i a t e d w i t h p r e s s u r e s c a t t e r i n g would be s i m i l a r to that of a line p a s s i n g through the Abee and Copiapo data points i n F i g u r e s 6-3 and 6-4.  The i n t e r s e c t i o n s of these  e r r o r l i n e s w i t h the t h e o r e t i c a l c o r r e l a t i o n l i n e s f o r t h e r m a l capture a r e below the t e r r e s t r i a l points.  neutron  T h i s would suggest,  i f anything, that Abee was exposed to fewer t h e r m a l neutrons d u r i n g its h i s t o r y than the e a r t h was.  H o w e v e r , the m o s t s e n s i t i v e  p a r a m e t e r B would not change by m o r e than 0.01%, and the d i f f e r e n c e  93  between the m e a n t e r r e s t r i a l and Abee values would not exceed 0.06%.  6.4 I n t e r p r e t a t i o n of E u r o p i u m Data When the p r e s e n t study of the r a r e earths was i n i t i a t e d , the extent of m a s s s p e c t r o m e t e r f r a c t i o n a t i o n was not known f o r the t r i p l e f i l a m e n t technique. (i.e.  The magnitude of the f r a c t i o n a t i o n effect  the d e v i a t i o n f r o m the absolute r a t i o s ) was l e s s than that which  was o b s e r v e d w i t h the single f i l a m e n t technique, but the v a r i a t i o n d u r i n g an a n a l y s i s was the same f o r both techniques:  approximately  0.3% v a r i a t i o n p e r unit m a s s d i f f e r e n c e (see F i g u r e 5-3). Since E u has only two i s o t o p e s , a s i m p l e n o r m a l i z a t i o n p r o c e s s c o u l d not be used.  Table 6-4 shows the m e a n values obtained  for a l l a n a l y s e s on t e r r e s t r i a l ( E u - J and Eu-US) and Abee ( E u - A B ) samples.  Two s e l e c t i v e methods of combining the data d e c r e a s e d  the d i s p e r s i o n due to f r a c t i o n a t i o n , but it was not c o m p l e t e l y e l i m i nated.  The f i r s t method c o m b i n e d data which was obtained f o r the  i n i t i a l 10-20% of the s a m p l e (based on the t i m e - i n t e g r a t e d b e a m intensity).  The second method used only the r e s u l t s of a n a l y s e s  for w h i c h the data-taking i n t e r v a l s w e r e u n i f o r m l y d i s t r i b u t e d over the l i f e t i m e of the E u * i o n beam.  R e c o r d i n g data f o r the f u l l d u r a t i o n  of the s a m p l e m i n i m i z e d b i a s due to f r a c t i o n a t i o n and p r o v i d e d the m o s t r e l i a b l e e s t i m a t e of the absolute i s o t o p i c r a t i o f o r each sample. It i s apparent that there i s no g r o s s d i f f e r e n c e between " t e r r e s t r i a l and Abee Eu.  The s l i g h t l y h i g h e r r a t i o f o r the Abee sample  TABLE 6-4.  ISOTOPIC COMPOSITION  OF TERRESTRIAL AND  A v e r a g e Ev?~^/Eu^^ ta selection  D  a  Eu-J (Brazil ore)  Average f o r a l l analyses  .91? 1.001  Average f o r s e t s obtained with i n i t i a l 10-20% o f sample o n l y  1.0006  Eu-US (U.S. o r e )  (20)  .9182  isotopic  (5)  .914 (20) 1.004 .9183  1.0010  (4)  ABEE EU  ratio Eu-AB (Abee)  .918 1.002  (38)  .9195 1.0004  (24)  # Average f o r a n a l y s e s f o r w h i c h d a t a was obtained f o r the f u l l d u r a t i o n o f t h e sample  .9155  1.0009  (9)  .9160  1.0018  (9)  .9173  1.0022  (11)  E r r o r s c o r r e s p o n d t o one s t a n d a r d d e v i a t i o n o f a s e t . The s t a n d a r d d e v i a t i o n o f t h e mean w o u l d n o t be a s u i t a b l e r e p r e s e n t a t i o n o f the e r r o r because i t would n o t a c c o u n t f o r r e a l v a r i a t i o n s i n t h e i s o t o p i c r a t i o i n t h e E u i o n beam due t o mass f r a c t i o n a t i o n . +  The numbers i n b r a c k e t s r e p r e s e n t t h e t o t a l ( u s u a l l y 6 scans p e r s e t ) used t o c a l c u l a t e standard d e v i a t i o n s .  number o f s e t s t h e means and  M u r t h y e t a l (1970) f o u n d t h a t E u 5 / E u = f i l a m e n t a n a l y s e s on t e r r e s t r i a l e u r o p i u m .  .9147 f o r s i n g l e  1  #  1  1 5 3  The a n a l y s i s o f t h e Eu-AB f r a c t i o n e x t e n d e d o v e r t h r e e days. S i n c e t h e mass s p e c t r o m e t e r was t u r n e d o f f o v e r n i g h t , s a m p l e e m i s s i o n was n o t c o n t i n u o u s . This appears t o have i n t r o d u c e d a n e r r o r i n d e t e r m i n i n g t h e sample d e p l e t i o n f r o m t h e t i m e - i n t e g r a t e d beam i n t e n s i t y .  95  p r o b a b l y r e f l e c t s a b i a s i n m e a s u r e m e n t : m o s t data sets f o r a n a l y s i s E u - A B w e r e obtained with the i n i t i a l p o r t i o n of the sample.  In view  of the l a r g e v a r i a t i o n caused by f r a c t i o n a t i o n (0.6%) d u r i n g the a n a l y s e s , no s i g n i f i c a n c e can be a s s i g n e d to the apparent d i f f e r e n c e of 0.14 - 0.17% between t e r r e s t r i a l and Abee Eu.  6. 5 I n t e r p r e t a t i o n of S a m a r i u m Data The c o m b i n e d r e s u l t s of s e v e r a l a n a l y s e s of s a m p l e s Sm-J  and Sm-US are r e c o r d e d i n Table 6-5.  E a c h e n t r y i s the  weighted m e a n of a m i n i m u m of s i x sets of scans of the S m  +  spectrum.  The two t e r r e s t r i a l s a m p l e s have the same i s o t o p i c c o m p o s i t i o n w i t h i n the e x p e r i m e n t a l u n c e r t a i n t y (shown as two s t a n d a r d deviations of the mean).  The average t e r r e s t r i a l value was  calculated for  e a c h r a t i o i n t u r n f r o m equations (6-1). A c o m p a r i s o n between t e r r e s t r i a l and Abee Sm i n T a b l e 6-6-  i s shown  Although the i r r a d i a t i o n - s e n s i t i v e p a r a m e t e r D  appears to have the same value f o r both s a m p l e s , i s o t o p i c r a t i o s appear to d i f f e r .  s e v e r a l of the  The apparent d i f f e r e n c e cannot be  a t t r i b u t e d to t h e r m a l neutron capture. The d i s c r e p a n c y between the t e r r e s t r i a l and Abee r e s u l t s is a l m o s t c e r t a i n l y due to b a s e l i n e d i s t o r t i o n .  A d e p r e s s i o n of  the b a s e l i n e i n the m a s s range 144 - 147, and a m o d e r a t e enhancement at h i g h e r m a s s e s , was c l e a r l y o b s e r v e d i n a s s o c i a t i o n w i t h an intense B a * i o n b e a m .  H o w e v e r , the c h a r a c t e r i s t i c shape and  T A B L E 6-5, I S O T O P I C C O M P O S I T I O N O F S M IN T E R R E S T R I A L S A M P L E S  Sample  S r n ^  Sm  1 4 8  Sm  Sm  1 5 2  1 5 2  *  Sm ? *  Sm  1 5 0  ,Sm <  Sm  1 5 2  1 4  152  *  Sm ^ * 1  Sm  1 5 2  Sm^_° S m . °"sm  1 4  ? Sm (  1 5  # Sm-J ( B r a z i l ore)  .11647 1.00015  .42323 + 00013  .51943 t.00013  .27725 t. 00009  .84836 + 00026  .28106 + 00014  Sm-US (U.S. o r e )  .11689 -.00031  .42322 +.00025  .51936 +.00031  .27728 +.00023  .84852 +.00080  .28096 +.00040  .11655 +.00014  .42323 +.00012  .51942 . +.00012  .27725 +.00008  .84838 + 00025  .28105 +.00013  #  # Average  Murthy et a l (1963)  .118  .426  .522  .280  147  x- F r a c t i o n a t i o n c o r r e c t i o n n o r m a l i z e d to Sm analyses f o r this work). #  .858  .283  152  /Sm  °  1 5 2  = .5650 (mean value for a l l t e r r e s t r i a l  B a s e l i n e d i s t o r t i o n was r e c o g n i z e d to be a source of b i a s i n m e a s u r e m e n t f o r a l l of these analyses. This was substantiated by c o m p a r i n g the values obtained f o r the p a r a m e t e r D when using the two s p e c t r a l ranges 149-152 and 144-154. In both cases the b a s e l i n e was m e a s u r e d at positions \ m a s s above the high m a s s peak and \ m a s s below the low m a s s peak only. No subsequent c o r r e c t i o n for the d i s t o r t i o n was p o s s i b l e without having m e a s u r e d b a s e l i n e s between intermediate peaks.  * >  T A B L E 6-6,.  ISOTOPIC COMPOSITION OF S A M A R I U M IN T H E A B E E M E T E O R I T E  Sm ,  Sample  bm  c  Abee* (Sm-AB)  152  +  Sm _  Sm  1 4 8  *  Sm ? *  Sm  _  „  1 4  152  om  152  Sm  *  1 5 0  152  Sm c  1 5 4  152  Sm  *  Sm D  =  _  149  Sm  (  .42284 t. 00016  .51871 +.00022  .27709 +.00017  .84976 +.00061  .28116 +.00023  .11655 .00014  .42323 +.00012  .51942 +.00012  .27725 +.00008  .84838 +.00025  .28105 +.00013  a  F r a c t i o n a t i o n c o r r e c t i o n n o r m a l i z e d to Sm  /Sm  a  b  Sm  1 5 0  .11585 00009 a  Terrestrial*  jfc  *  1 4 4  c  152  Sm  a  = . 5650 . 138  #  B a s e l i n e d i s t o r t i o n (mainly secondary e l e c t r o n s f r o m B a ions) i s the probable cause of the m a j o r d i s c r e p a n c i e s between Abee and t e r r e s t r i a l samples. The d i s t o r t i o n was greatest i n the l o w e r m a s s range 144-147.  a  A f t e r computationally (Nd+/Sm+< 0.02).  b  A f t e r computationally subtracting the LaO+ s p e c t r u m w h i c h was m o n i t o r e d at m a s s 155. ( L a O + / S m + < 0.005).  subtracting the Nd+ s p e c t r u m which was m o n i t o r e d at m a s s 146.  1 5 0  )  *  98  magnitude of the d i s t o r t i o n w e r e h i g h l y v a r i a b l e f r o m one a n a l y s i s to another, so that a c o r r e c t i o n could not be made without having m e a s u r e d b a s e l i n e s between i n d i v i d u a l peaks.  The good a g r e e m e n t •  obtained for the two t e r r e s t r i a l s a m p l e s i s p r o b a b l y a r e s u l t of their s i m i l a r chemical composition. a b i a s i n m e a s u r e m e n t , but i t was  Baseline distortion introduced  a p p r o x i m a t e l y the same f o r  both s a m p l e s . The e x i s t e n c e of b a s e l i n e d i s t o r t i o n was a n a l y z i n g t e r r e s t r i a l Sm  s a m p l e s u s i n g only the s h o r t e r m a s s range  149-152, and computing the p a r a m e t e r D. the values .28038  +  p r o v e n by  .00019 f o r Sm-J  T h i s was  and .28036  +  o b s e r v e d to have  .00010 f o r Sm-US.  The d i f f e r e n c e between these v a l u e s , w h i c h are undoubtedly c l o s e r to the true v a l u e , and the a v e r a g e value i n Table 6-6 i s s i g n i f i c a n t . It can only be e x p l a i n e d by the fact that the b a s e l i n e s w e r e m e a s u r e d at d i f f e r e n t p l a c e s f o r the two s p e c t r a l ranges. The e x i s t e n c e of b a s e l i n e d i s t o r t i o n would l e a d to a f u r t h e r s o u r c e of e r r o r for the Abee a n a l y s e s . b e a m was  A c o r r e c t i o n for the Nd +  made by m o n i t o r i n g the i n t e n s i t y of the peak at m a s s 146.  Since b a s e l i n e d i s t o r t i o n was the i n t e n s i t y of the N d  +  g r e a t e s t i n the v i c i n i t y of this peak,  ion b e a m would not have been c o r r e c t l y  d e t e r m i n e d , and a subsequent e r r o r would r e s u l t w h e r e v e r i s o b a r s of Nd and Sm m a s s 144.  occurred.  The l a r g e s t e r r o r f o r Sm .  would o c c u r at  99  6. 6 C o n c l u s i o n s A n i n v e s t i g a t i o n of Gd, E u and Sm i n the Abee enstatite c h o n d r i t e , and i n two t e r r e s t r i a l o r e s , has shown that there are no s i g n i f i c a n t i s o t o p i c a n o m a l i e s f o r Abee which c o u l d be a t t r i b u t e d 155 to one o r m o r e of the t h e r m a l neutron capture p r o c e s s e s : G d /(n, X ^^ r ^ l 5 8 , Sm149, 150 ,„E u 151, (n,yfl. ) 0 \ )Gd , r-J G di (n, )C 0 \)Gd '(n, y(xc)Sm 1 5 6  51  c  „ u153,(n, I Y ) \ i rE u154. E^ u 152, E Gd  1 5 7  /Gd  1 5 8  , Sm  1 4 9  /Sm  1 5 0  „d,155,-,156 The i.s o t.o p i.c r a t i o s G /Gd , and E u  1 5 1  /Eu  a r e i d e n t i c a l f o r the  1 5 3  three s a m p l e s studied w i t h i n m a x i m u m e x p e r i m e n t a l u n c e r t a i n t i e s of 0.15%, 0.10%, 0.3% and 0.3% r e s p e c t i v e l y .  These e s t i m a t e s  include s t a t i s t i c a l e r r o r s , as w e l l as e r r o r s i n t r o d u c e d by p o s s i b l e s o u r c e s of b i a s i n m e a s u r e m e n t (e.g. b a s e l i n e d i s t o r t i o n , p r e s s u r e s c a t t e r i n g and/or u n c o r r e c t e d f r a c t i o n a t i o n ) . On the b a s i s of the G d ^ / G d ^ 7  8  r a t i o , we m a y conclude  that the m a x i m u m d i f f e r e n t i a l t h e r m a l neutron i r r a d i a t i o n between the t e r r e s t r i a l and Abee s a m p l e s i s 3 x 10 _ n e u t r o n s / c m u n i f o r m i r r a d i a t i o n m o d e l of F i g u r e 2-2.  f o r the  It i s r e a s o n a b l e to a s s u m e  that this c o n c l u s i o n a p p l i e s to the Abee m e t e o r i t e as a whole, when c o m p a r e d w i t h the e a r t h .  In t e r m s of the i r r a d i a t i o n m o d e l s  d i s c u s s e d i n Chapter 2, we m a y exclude the r e g i o n s above the c u r v e s i n F i g u r e s 2-1 and 2-2 c o r r e s p o n d i n g to a G d ^ / G d ^ 7  ^  8  anomaly  0.1% f r o m the f a m i l y of p o s s i b l e solutions f o r the i r r a d i a t i o n  ... h i s t o r i e s of the Abee m e t e o r i t e and the e a r t h .  100  If the t i m e - i n t e g r a t e d t h e r m a l neutron flux a s s o c i a t e d w i t h the i r r a d i a t i o n phase was as l a r g e as the F G H dieted ( i . e. \L  nE  r 4 x 10  21  neutrons/cm  2  hypothesis p r e -  ), then the Gd  157  /Gd  r a t i o i s m o r e s e n s i t i v e to the f r a c t i o n of the m a t e r i a l which  158  was  i r r a d i a t e d than to the d i f f e r e n t i a l i r r a d i a t i o n of the p r i m i t i v e c h o n d r i t i c and t e r r e s t r i a l m a t t e r ( F o w l e r et a l , 1962; B u r n e t t et a l , 1966; M u r t h y et a l , 1963).  The absence of a Gd  anomaly  >• 0.1% i m p l i e s that the i r r a d i a t e d f r a c t i o n s of the Abee and t e r r e s t r i a l s o u r c e m a t e r i a l were i d e n t i c a l w i t h i n 2%. to the F G H  T h i s applies  m o d e l only, where it has been a s s u m e d that a p p r o x i -  m a t e l y 5% of the p r i m i t i v e t e r r e s t r i a l m a t t e r was  irradiated.  A p a r t f r o m u n i f o r m i r r a d i a t i o n and/or d i l u t i o n of the s o u r c e m a t e r i a l , a l t e r n a t i v e explanations could account f o r the 157 158 absence of s i g n i f i c a n t Gd  /Gd  anomalies: efficient shielding  of m o s t p l a n e t a r y m a t e r i a l inside l a r g e bodies having d i a m e t e r s »  50 m e t e r s ;  the l a c k of s u f f i c i e n t hydrogen to t h e r m a l i z e  s p a l l a t i o n - p r o d u c e d neutrons; or the absence of an i r r a d i a t i o n phase of s u f f i c i e n t i n t e n s i t y to generate detectable a n o m a l i e s . A p a r t f r o m the h i g h l y r e d u c e d state of the Abee m e t e o r i t e , there i s no c l e a r evidence to suggest that this e x t r a - t e r r e s t r i a l object o r i g i n a t e d i n the i n n e r r e g i o n of the s o l a r s y s t e m .  If any-  thing, the i s o t o p i c c o m p o s i t i o n of Gd i n the m e t e o r i t e suggests that Abee r e c e i v e d l e s s i r r a d i a t i o n than the e a r t h d u r i n g its p r e v i o u s  101  history.  T h i s evidence would t h e r e f o r e favour the v i e w that Abee  had i t s o r i g i n at a g r e a t e r distance f r o m the sun, p o s s i b l y i n the a s t e r o i d belt. Since the a s t e r o i d belt is a n a t u r a l source of m e t e o r i t e s , any explanation of the gradation i n the o x i d a t i o n states of chondrites would r e c e i v e s t r o n g e r support f r o m the a v a i l a b l e evidence i f i t a s s u m e d an o r i g i n i n this r e g i o n of the s o l a r s y s t e m .  One  inter-  esting p o s s i b i l i t y w h i c h has r e c e i v e d c o n s i d e r a b l e attention r e c e n t l y (see L a r i m e r et a l , 1970) i s that v a r i a t i o n s i n the o x i d a t i o n state of chondrites r e s u l t f r o m d i f f e r e n c e s i n their s p a t i a l d i s t r i b u t i o n d u r i n g g r a v i t a t i o n a l s e t t l i n g of p r i m o r d i a l m a t e r i a l t o w a r d the o r b i t a l plane of the nebula.  102  APPENDIX I E F F E C T OF N E U T R O N C A P T U R E  O N GD, S M A N D  EU  The p r o b a b i l i t y of a n u c l e a r p r o c e s s i s g e n e r a l l y e x p r e s s e d i n t e r m s of a c r o s s s e c t i o n \T~ which has the d i m e n sions of an a r e a . -24  defined as 10  The unit c o m m o n l y used i s the b a r n , which i s 2  cm .  A s i m p l e d e f i n i t i o n of the t h e r m a l neutron capture c r o s s 157 s e c t i o n of a s p e c i f i c nuclide (e. g. Gd c l a s s i c a l theory.  ) may be deduced f r o m  C o n s i d e r a b e a m of t h e r m a l neutrons s t r i k i n g  a thin t a r g e t i n which the b e a m i s attenuated only i n f i n i t e s i m a l l y . If R^ i s the number of capture p r o c e s s e s o c c u r r i n g i n the t a r g e t per unit t i m e f o r n u c l i d e i then the c o r r e s p o n d i n g r e a c t i o n c r o s s s e c t i o n i s defined by the equation R. 1  zz  In. q ~ d i 1  where I i s the number of incident t h e r m a l neutrons p e r unit t i m e n^ i s the number of t a r g e t n u c l e i of n u c l i d e i p e r unit v o l u m e <jr" i s the t h e r m a l neutron capture c r o s s s e c t i o n f o r n u c l i d e i d i s the t a r g e t t h i c k n e s s . In this t h e s i s we a r e i n t e r e s t e d i n t h e r m a l neutron capture p r o c e s s e s o c c u r r i n g i n the p r i m i t i v e m a t e r i a l of the s o l a r s y s t e m . The s i z e of the p l a n e t e s i m a l s , the d u r a t i o n of the a s s u m e d i r r a d i a t i o n p e r i o d , and the magnitude of the neutron flux a r e a l l unknown.  103  H o w e v e r , f o r heavy isotopes w i t h l a r g e t h e r m a l neutron capture c r o s s s e c t i o n s it i s p o s s i b l e to e s t a b l i s h a c l e a r r e l a t i o n s h i p between i s o t o p i c c o m p o s i t i o n and the t i m e - i n t e g r a t e d t h e r m a l n e u t r o n flux. Let N  Q  be the i n i t i a l atom abundance of a p a r t i c u l a r n u c l i d e  i n p r i m o r d i a l m a t e r i a l which l a t e r a c c r e t e d to f o r m a planetarybody (e. g. the e a r t h or the parent body of the Abee m e t e o r i t e ) . The quantity N  Q  r e p r e s e n t s the total n u m b e r of atoms of the nuclide  p r i o r to i r r a d i a t i o n .  If t h e r m a l neutron capture i s the dominant  p r o c e s s c a u s i n g a change i n the abundance of the n u c l i d e d u r i n g i r r a d i a t i o n , then the atom abundance INL after i r r a d i a t i o n by n neutrons m a y be deduced f r o m the d i f f e r e n t i a l equation (after F o w l e r et a l , 1962)  (1-1) D  N  N  A  A where  i s the t h e r m a l neutron capture c r o s s s e c t i o n f o r the  nuclide under c o n s i d e r a t i o n f  i s the f r a c t i o n of the incident neutrons w h i c h i s c a p t u r e d  by the i r r a d i a t e d m a t e r i a l .  The r e m a i n i n g f r a c t i o n ( l - f ) i n c l u d e s r  those neutrons w h i c h escape f r o m the i r r a d i a t e d m a t e r i a l or undergo ^  decay. ^  CTT N  A A  r e p r e s e n t s the total c r o s s s e c t i o n a l t a r g e t a r e a p r e s e n t e d  by a l l n u c l i d e s .  104  Let irradiation.  N  f - o N  n t o t a  2 be the total number of neutrons produced during  Then, i n t e g r a t i o n of equation (I-l) y i e l d s  or  e x p  An r  A  total A A  (1-2)  w h e r e N.^ i s the f i n a l atom abundance of the n u c l i d e i w h i c h has been depleted by neutron capture. So f a r , no mention has been made of the energy d i s t r i b u t i o n of the neutrons.  In the t h e r m a l range, the neutron capture c r o s s  s e c t i o n s of m o s t n u c l i d e s are i n v e r s e l y p r o p o r t i o n a l to the v e l o c i t y of the incident neutrons.  Hence, it i s evident f r o m equation (1-2)  that the v e l o c i t y t e r m s would c a n c e l out, l e a v i n g  essentially  independent of the neutron energy d i s t r i b u t i o n , p r o v i d e d it i s i n the t h e r m a l range.  F o r the purposes of this t h e s i s , however,  we  have chosen to separate theexponent i n equation (1-2) into two f a c t o r s C . ^ . i and"!/' 7 n  We  have chosen  i to be the c r o s s s e c t i o n f o r  neutron c a p t u r e , a s s u m i n g a t h e r m a l neutron v e l o c i t y of 2200 m/sec (or 20°C).  The p a r a m e t e r  i s defined as  f n r total A A  A  (1-3) 20°C  T h i s i s s i m p l y the t i m e - i n t e g r a t e d t h e r m a l neutron flux (at 20°C)  105  f o r a l l neutrons w h i c h undergo capture.  We  may  now  write  equation  (1-2) i n the f o r m  N - N 1  o  = -  N  o  £  l-exp(- ^~ Ifn  J  (1-4)  B y i n t r o d u c i n g another p a r a m e t e r , the d i l u t i o n f a c t o r (after F o w l e r et a l , 1962), we may that only a f r a c t i o n The  r e m a i n d e r , was  account f o r the p o s s i b i l i t y  of the p r i m o r d i a l m a t e r i a l was i r r a d i a t e d . e f f e c t i v e l y s h i e l d e d w i t h i n p l a n e t e s i m a l s having  d i m e n s i o n s of the o r d e r of one m e t e r or g r e a t e r .  Subsequent  m i x i n g of the two f r a c t i o n s d u r i n g a c c r e t i o n to f o r m a l a r g e r p l a n e t a r y body would y i e l d  A  N =  N,-N i o  N - >  [l-e*P(-^.  " d  V>>]  ^  n  E q u a t i o n (1-5) gives the net d e c r e a s e i n the abundance of a nuclide undergoing neutron capture. (which i n c l u d e s a l l r e a c t i o n s on Gd, Sm  F o r (n, \ ) r e a c t i o n s and E u under c o n s i d e r a t i o n  h e r e ) , equation (1-5) a l s o gives the net i n c r e a s e i n the neighbouring higher m a s s nuclide r e s u l t i n g f r o m this p r o c e s s .  Let R  q  and  be the i n t i t i a l and f i n a l i s o t o p i c abundance r a t i o s f o r m a s s e s m Then  R and m +  106  N R r  N  im  f,m 1  N  F  R=  F  d V d +  R  _  o  IT ) f l - e x p ( - \ b  d «-  om  ( N  ) "1  m fn ' J  l+^rn^L -^"^^] 1  No^ +  +  tz>  om  L^-T^VnO  R [l-exp(-^ y )J m  0  d-6)  n  155 E q u a t i o n (1-6) m a y be applied to each of the i s o t o p i c r a t i o s Gd / . 156 157 158 149 150 Gd , Gd /Gd and Sm /Sm . Two independent (n, g ) 151 r e a c t i o n s m a y contribute to a change i n the E u (see T a b l e 1-1).  F  d V  R  o  153 /Eu  ratio  T h e r e f o r e , f o r this r a t i o we have  I> -  F - [l-exp(d  exp(  ^Tsi^l  <r V„>]  (I  i53  "  7)  It i s evident f r o m equations (1-6) and(T-7)that each of the p r e s e n t day i s o t o p i c r a t i o s mentioned above may be r e g a r d e d as a function of R  o  ,F  d  and "\U .  The c r o s s sections a r e a l l known.  Let  R g and R ^ be the p r e s e n t - d a y v a l u e s of a p a r t i c u l a r r a t i o i n t e r r e s t r i a l o r e s and i n m e t e o r i t i c m a t e r i a l r e s p e c t i v e l y .  If these  materials  were exposed to p r e v i o u s i r r a d i a t i o n by integrated n e u t r o n fluxes oi^)U and "7/' .where the d i l u t i o n f a c t o r s "nE HnM r e s p e c t i v e l y , then  were F  dE  and F  dM  107  R(R , F o dE  ' nE  )= R  R(R , F ,7 / )= o dM * nM :  R  E  (1-8)  (1-9)  M  In equations (1-8) and (1-9) i t has been a s s u m e d that the i n i t i a l i s o t o p i c r a t i o s were the same for both p r e - t e r r e s t r i a l and p r e meteoritic  materials.  C o n s i d e r the case where it i s just p o s s i b l e to detect a difference  ofo.l% between  the t e r r e s t r i a l and m e t e o r i t i c  r a t i o s R^. and R ^ r e s p e c t i v e l y .  isotopic  The l o w e r l i m i t of detection i s  given b y  R  M  -R R  E  o  dM  nM R(R , F o dE  E  = .001  Vn> E  ( r: 0.1% detectable d i f f e r e n c e )  (1-10)  F i g u r e s 2-1 and 2-2 i l l u s t r a t e p a r t i c u l a r solutions of equations (1-8) and (1-10) f o r s p e c i f i c values of the d i l u t i o n f a c t o r s .  The  c u r v e s f o r F i g u r e 2-1 r e p r e s e n t the s o l u t i o n of the equations  (1-11)  E ( = .632 for the G d  1 5 7  /Gd  1 5 8  ratio)  108  and  R(R  o  ,20,  1/  J-R(R « nM  ,20,  o  X  " l / .„) • n£  = .001 R  (  o '  R  The p a r a m e t e r R  q  2  0  '  (1-12)  V E> n  was e l i m i n a t e d f r o m equations (I-11) and (1-12),  and the s o l u t i o n curve f o r * ^ £ n  | 'VnM-VnEi'VnEl  VS  and Y n M  P i t t e d i n the f o r m  w a s  YnE ' 157  The s e n s i t i v i t y of the Gd  158  /Gd  r a t i o to d i f f e r e n t i a l  i r r a d i a t i o n of p r e - m e t e o r i t i c and p r e - t e r r e s t r i a l m a t e r i a l is i l l u s t r a t e d f o r the case F • •= F ,, , r: 1 i n F i g u r e 2-2. The dE dM r e q u i r e m e n t that ^ * ^ '0, c o m b i n e d w i t h the present-day r a t i o R =r .632, p l a c e s an upper l i m i t on the value of 'Xf' which w i l l E * nE 5  0  s a t i s f y the equation  R(R  o  ,1, 1>  1  nE  ) =  .632  This is illustrated in Figure  (1-13)  2-2.  109  A P P E N D I X II S E C O N D O R D E R F O C U S S I N G SHIMS  I n c r e a s e d s e n s i t i v i t y , without l o s s of r e s o l u t i o n m a y b e achieved b y ."imp:roving the f o c u s s i n g p r o p e r t i e s of the a n a l y z e r magnet to p e r m i t the use of an ion b e a m with l a r g e r angular  divergence.  F i r s t o r d e r f o c u s s i n g r e s u l t s f r o m using plane s u r f a c e s at the pole faces w h e r e the i o n b e a m enters and leaves the magnet (at n o r m a l incidence and emergence).  B y s u i t a b l y contouring one o r both of  these s u r f a c e s it i s possible to produce a s h a r p e r focus at the collector.  A v a r i e t y of methods f o r a c h i e v i n g second o r d e r  f o c u s s i n g have been r e v i e w e d by B a r n a r d  (1953).  One s i m p l e method i n v o l v e s the use of c y l i n d r i c a l r a t h e r than plane s u r f a c e s f o r the magnet pole faces.  F o r a symmetric  90 deg s e c t o r magnet, s i m i l a r to the one used by the p r e s e n t w r i t e r , the i d e a l r a d i u s of c u r v a t u r e of the pole faces i s i d e n t i c a l to the o r b i t a l r a d i u s of the c e n t r a l ion t r a j e c t o r y w i t h i n the magnet.  An  a l t e r n a t i v e p r o c e d u r e adopted by the w r i t e r i s to use one plane s u r f a c e and one c y l i n d r i c a l s u r f a c e , where the l a t t e r has a r a d i u s equal to one h a l f the o r b i t a l r a d i u s ( F i g u r e II-1). Two c y l i n d r i c a l s h i m s w e r e designed to fit onto the e x i s t i n g pole face on the c o l l e c t o r side of the m a i n electromagnet. The s h i m s w e r e p o s i t i o n e d on either side of the a n a l y z e r tube, and p r o v i s i o n was made f o r a d j u s t i n g them to enable p r e c i s e f o c u s s i n g  *  a=34.li.2  1  COLLECTOR  '  >  ( m e a s u r e d ) ^  APEX  s  ; "at  m  c  E  .9±.2 'specification)  #/  ion trajectory w i t h i n i t i a l angle  ioi.01 (computed)  . My/  /  ^  l  V  •  .  ' /  •  antral rav .  s f  a t o the central ray  10- 01  J^\>  .-/TV  ,  \  ,'  \  ...  ^( c o m p u t e d ) shim positioning  \  (compute ' 3  +-..»'  X  NOT DRAWN TO SCALE  screw FIG. I I - l  MODIFICATION OF MAIN ELECTROMAGNET TO ACCOMMODATE FOCUSSING  The d o t t e d c o n t o u r shows t h e o r i g i n a l magnet p o s i t i o n . magnet, a n d a d d i n g two c y l i n d r i c a l central  r a y remains unchanged.  SHIMS  By moving t h e main  s h i m s (7.0 i n c h r a d i u s ) t h e p a t h o f t h e  A l ldistances are specified  i ninches.  Ill  experimentally.  No a l t e r a t i o n of the m a i n magnet was  necessary  except f o r r e p o s i t i o n i n g to compensate f o r the a d d i t i o n a l t h i c k n e s s of the magnet shims. In o r d e r to apply a c o r r e c t i o n f o r the effect of the f r i n g i n g m a g n e t i c f i e l d s on both s i d e s of the magnet, the f i e l d i n t e n s i t y was d e t e r m i n e d i n the m e d i a n plane along the path of the central ray.  The t h e o r e t i c a l method of C o g g e s h e l l (1947) was  u s e d to c a l c u l a t e the f i e l d c l o s e to the magnet, and m e a s u r e d f i e l d v a l u e s w e r e e m p l o y e d at distant points.  The r e s u l t i n g contour of  the f r i n g i n g f i e l d i s shown i n F i g u r e I I - 2 . W h i l e the magnet was i n the p o s i t i o n of best focus f o r f i r s t o r d e r f o c u s s i n g , the p a r a m e t e r s were m e a s u r e d .  a and d (see F i g u r e II-1)  The l a r g e e r r o r of .2 i n c h i n each m e a s u r e m e n t  r e f l e c t s the u n c e r t a i n t y i n d e t e r m i n i n g the effective p o s i t i o n s of m i n i m u m b e a m width at the source and c o l l e c t o r .  T a k i n g into  account the u n c e r t a i n t i e s i n a and d, the t r a j e c t o r y equation was i n t e g r a t e d through the known m a g n e t i c f i e l d , and the r a d i u s of c u r vature of the c e n t r a l r a y was computed to be r = 12.1 -.1 inch. T r a j e c t o r i e s were computed f o r s e v e r a l ions having i n i t i a l angles c i n c l i n a t i o n to the c e n t r a l r a y i n the range -4° <.©<'< 4°.  F o r eac  t r a j e c t o r y the r e q u i r e d change i n path length (within the magnetic field) w h i c h would b r i n g the t r a j e c t o r y to p e r f e c t focus at the c o l l e c t o r , was computed.  T h i s l e d to the d e t e r m i n a t i o n of the  112 R e l a t i v e magnetic f i e l d intensity 1.0  Theoretical f i e l d values  Smooth t r a n s i t i o n between t h e o r e t i c a l and e x p e r i m e n t a l field values .3  Magnet pole face E x p e r i m e n t a l f i e l d values  JL_1  JL  0. 0  0  2  4  6  JL 8  10  Distance f r o m pole face (inches) FIGURE II-2. FRINGING F I E L D OF MAIN M A G N E T  12  113  contour f o r the magnet s u r f a c e which would give p e r f e c t focus at the c o l l e c t o r (see F i g u r e II-3).  A least squares c i r c l e (in the sense of  minimizing T ~ ^ t -  ) was computed through the s u r f a c e  At, J  points to d e t e r m i n e the best r a d i u s a s s o c i a t e d with i o n beams of v a r y i n g angular d i v e r g e n c e  ot' ( F i g u r e II-4). m six  F o r values of the  angular d i v e r g e n c e exceeding 2 deg i t i s apparent that 7.0 inches i s the i d e a l c u r v a t u r e f o r the magnet s h i m s .  T h i s m a y be c o m p a r e d  w i t h the value of 6.0 inches where the effect of f r i n g i n g f i e l d s i s neglected (see F i g u r e II-4). The magnet s h i m s were designed to have a c y l i n d r i c a l s u r f a c e of 7.0 i n c h r a d i u s .  They were a c c u r a t e l y m a c h i n e d f r o m  grade 1010 hot r o l l e d carbon s t e e l plate obtained f r o m D r . A u l d of the T R I U M F group at U. B. C.  P r o v i s i o n was made f o r h o l d i n g  the s h i m s r i g i d l y i n p o s i t i o n on the m a i n magnet pole f a c e s , although adjustment of t h e i r p o s i t i o n along the pole face was p o s s i b l e A f t e r mounting the s h i m s the magnet was p o s i t i o n e d as shown i n F i g u r e II-1.  P r e c i s e a l l i g n m e n t was then a c h i e v e d e x p e r i -  m e n t a l l y by a d j u s t i n g the s h i m p o s i t i o n and the v e r t i c a l p o s i t i o n of the magnet to give o p t i m u m peak shape.  F i g u r e II-5 shows the  peak quality obtained f o r b e a m s of angular d i v e r g e n c e 1.9 and 3.0 deg r e s p e c t i v e l y . Although the r e s o l u t i o n was e x c e l l e n t (400-450), the h i g h e r _ angular d i v e r g e n c e , coupled w i t h w i d e r s o u r c e s l i t s , gave rounded  R e q u i r e d reduction in path length (in the magnetic field) to give perfect focus at the c o l l e c t o r A A t (inches)  F I G U R E II-3. C U R V A T U R E O F S E C O N D O R D E R F O C U S S I N G SHIMS  Ideal curvature of s h i m s a s s u m i n g (i) no c o r r e c t i o n f o r the effect of f r i n g i n g f i e l d s outside the m a i n magnet (represented by dots) or (ii) a c o r r e c t i o n a p p l i e d to account for the f r i n g i n g f i e l d s (represented by c r o s s e s ) . Note the different s c a l e s used f o r the two axes. When A t and A z are plotted on the same scale the least squares c i r c l e s f i t t i n g the two sets of points have r a d i i of 6.1 and 7.0 inches r e s p e c t i v e l y (using data f o r = 0, . 2, t .4, ... ± 4.0°). +  115  Optimum radius of c u r v a t u r e (inches)  7.0  t -  -Jj  C o r r e c t i o n a p p l i e d for fringing field  / X  6.5  /  /  No c o r r e c t i o n applied 6.0  max M a x i m u m angular divergence  of ion b e a m (degrees)  F I G U R E II-4. O P T I M U M C U R V A T U R E O F SHIMS A S A F U N C T I O N OF B E A M  DIVERGENCE  The s c a t t e r of the data points is due to t r u n c a t i o n e r r o r s i n the c a l c u l a t i o n s and the use of r e l a t i v e l y few t r a j e c t o r i e s . E a c h point r e p r e s e n t s the r a d i u s of c u r v a t u r e of the l e a s t s q u a r e s c i r c l e fitting through a l l values of A t and d.z c o r r e s p o n d i n g to t r a j e c t o r i e s with i n i t i a l angles of i n c l i n a t i o n to the lens a x i s of <=0, +0.2, 0 . 4 , +0.6, . . . + max +  206 208  n  207  n  FIGURE  II-5. P E A K S H A P E F O R TWO THE ANGULAR  VALUES OF  DIVERGENCE  208 208  206 207  n 206  ri  li  i—J  L  207  A  u (b)  =• 3.0 des  CK  max P a r t of the Pb s p e c t r u m i s shown for each value o f ^ ^ , The second o r d e r f o c u s s i n g s h i m s were used to obtain the above s p e c t r a .  117  peak tops and a s l i g h t a s s y m m e t r y to the peaks.  The l o w e r  d i v e r g e n c e ( c o r r e s p o n d i n g to two 0.010 i n c h s o u r c e c o l l i m a t i n g s l i t s s e p a r a t e d by 0.31 inch) was adopted i n the p r e s e n t r e s e a r c h . •This i s s t i l l a f a c t o r of two g r e a t e r than the divergence used with only f i r s t o r d e r f o c u s s i n g .  The effect of angular divergence on  b e a m w i d t h at the c o l l e c t o r i s shown f o r f i r s t and second o r d e r f o c u s s i n g i n F i g u r e II -6.  118  \  F I G U R E II-6. B E A M SPREADING AT THE C O L L E C T O R FOR FIRST AND SECOND ORDER FOCUSSING B e a m s p r e a d i n g at the c o l l e c t o r due to b e a m angular d i v e r g e n c e at ion source  0  . 1  2  3  B e a m d i v e r g e n c e at exit s l i t of i o n s o u r c e  4  max  (degrees)  The effect of angular divergence ( ^ ) at the ion source e x i t s l i t on the b e a m width at the c o l l e c t o r i s shown f o r both f i r s t and second o r d e r f o c u s s i n g . The s c a t t e r of data points f o r the case of second o r d e r f o c u s s i n g r e f l e c t s the s m a l l n u m b e r of t r a j e c t o r i e s used i n the c a l c u l a t i o n s ( = 0 , + 0.2,' + 0.4,' ....+o^ max'). m  J  x  a  x  119  A P P E N D I X III M E A S U R E D ISOTOPIC RATIOS FOR A L L GADOLINIUM  ANALYSES  The e x p e r i m e n t a l v a l u e s of the i s o t o p i c r a t i o s obtained f o r each set of scans of the G d i n T a b l e s I I I - l to III-8. rG^d  1 6 0  (or G d O ) s p e c t r u m  +  +  The i s o t o p i c r a t i o s G d  1 5 5  are listed  /Gd  l 6  AG ^ jd! 5 8 /Gd , ,160 , • a lnl been n o r m a li i-z eA and have d to ^Gi d 1  / r  °, Gd 5  u  .9361 to e l i m i n a t e the effect of m a s s d i s c r i m i n a t i o n .  A  6  / /Gd ^ ^ 1  1 5 7  6  /  0  =  correction  was made f o r the i s o t o p i c c o m p o s i t i o n of oxygen i n a l l a n a l y s e s w h i c h u s e d the G d O  spectrum  +  ( T a b l e s 111-3, III-4 and III-8).  The p r o c e d u r e f o r m a k i n g this c o r r e c t i o n i s given i n S e c t i o n 5. 5. The e s t i m a t e d e r r o r s i n T a b l e s III-1 to III-8 c o r r e s p o n d to two s t a n d a r d deviations  of the mean.  F o r each set of scans the  i s o t o p i c r a t i o s and the p a r a m e t e r s A and B w e r e computed by the method d i s c u s s e d i n Sections 5. 3 and 5. 4. its  The weighted m e a n and  s t a n d a r d d e v i a t i o n w e r e computed f o r the data i n e a c h c o l u m n of  T a b l e s III-1 to III-8 f r o m the equations  i=l,n  <H  i = l,n  CH' (-  inl.n  *Y  (n - 1) i=l,n  - ^  2-  )  \  (III-l)  1 2 0  In these equations,  and ^J". r e p r e s e n t the mean value of a  p a r t i c u l a r i s o t o p i c r a t i o (or p a r a m e t e r ) , and the e s t i m a t e d standar d e v i a t i o n of the mean, f o r set i ..  T h e s e computations were  p e r f o r m e d u s i n g one m o r e s i g n i f i c a n t digit than has been shown i n the tables.  The e n t r i e s i n the bottom r o w of each of the T a b l e s  I I I - l to III-8 a r e the values of JA_ and 2 (III-l). and 6-2.  given by equations  T h e s e quantities a r e the data which appear i n T a b l e s 6-1  TABLE I I I - l .  Analysis  Set  1  Gd-Jl  Gd-j4  #  Gd-J5  1  TRIPLE FILAMENT  Gd No. o f Gd scans  6  7  1 5 5  * Gd  1 5 7  1 6 0  Gd  1 6 0  * Gd^ Gd  8  *  1 6 0  A  B  .6?6l +.0008  .7161 ±.0009  ±.0026  1.1363  1.3615 ±.0012  1.6916 ±.0038  +.0004  .6767  .7162 ±.0003  1.1367 ±.0017  1.3606 +.0009  1.6922 +.0034  2  6  .6764 ±.0007  .7166 +.0008  1.1358 +.0014  1.3611 ±.0013  1.6893 +.0019  3  6  .6777 ±.0021  .7154 ±.0015  1.1348 ±.0012  1.3586 ±.0042  1.6898 ±.0026  1  6  .6?67 ±.0013  .7161 ±.0010  1.1355 ±.0017  1.3606 ±.0026  1.6898 ±.0032  2  6  .6769 ±.0002  .7161 ±.0003  1.1364 ±.0005  1.3602 ±.0003  1.6917 ±.0008  3  6  , .6767 _.ooo6  .7165 ±.0005  1.1365 +.0012  1.3606 +.0013  1.6911 ±.0023  4  6  Average  *  ANALYSES ON SAMPLE G d - J  Fractionation  1.6902 ±.0010  .67686 .71620 ±.00015 ±.00016  I.13620 I.36030 I . 6 9 O 8 3 ±.00044 ±.00027 ±.00067  correction corrected  t o Gd?-^^/Gd}-^=  .9361  A n a l y s e s G d - J 2 and G d - J 3 were r e j e c t e d b e c a u s e o f B a F i n t e r f e r e n c e w i t h t h e Gd+ s p e c t r u m . T h i s was c a u s e d by r a i s i n g the s i d e f i l a m e n t temperature too q u i c k l y without a l l o w i n g s u f f i c i e n t t i m e f o r Ba t o d e c a y a t l o w e r t e m p e r a tures. #  The  +  p a r a m e t e r s A and B a r e d e f i n e d  i n Table  6-1.  1 2 2  TABLE I I I - 2 .  TRIPLE FILAMENT ANALYSIS Gd-j6  Gd  Set  Gd-J6  No. o f Gd scans  1 5 5  * Gd Gd  1 6 0  i 6  * Gd  °  Gd  1 5 8  *  1 6 0  A  B  +.0009  +.0004  1.1361  1.3573 +.0021  1,0023  +.0013  .7165 ±.0008  1.1358 ±.0012  1.3603 ±.0025  1.6894 ±.0018  6  .6770 +.0009  .7165 1.1349 +.0006 '+.0006  1.3598 +.0018  1.6873 +.0024  4  6  .6758 ±.0011  ±.0010  .7157  1.1356 I.3623 + .000? " ±.0021  1.6910  5  6  .7160  1.1358  1.3596 +.0008  1.6907  6  6  7  1  6  2  5  3  .6782 +.0011 .6?68  .6772 +.0004  +.0002  +.0003  1.6884  +.0021  +.0008  +.0004  .7159  1.1354 +.0004  1.3600 ±.0009  1.6899 +.0010  6  .6769 +.0003  .7158 +.0002  1.1355 +.0006  1.3601 +.0006  ±.0013  8  6  .6766 +.0005  .7159 ±.0004  1.1359 ±.0006  1.3608 ±.0010  ±.0011  9  6  .6771  .7156 +.0006  1.1354 1.3598 ±.0019 ±.0020  1.6906 ±7 0029  .7155 +.0005  1.1355 +.0005  10  .6770  .7172  +.0004  10  ±.0010  6  .6763 +.0004  Average  *  1 5 7  Fractionation  1.6904  1.6910  1.3614 1.6910 +.0007 ±.0011  .67685 .71593 1.13566 I.36029 1.69038 ±.00026 ±.00014 ±.00021 ±.00054 ±.00054 correction normalized  The p a r a m e t e r s A a n d B a r e d e f i n e d Only s t a t i s t i c a l e r r o r s -two standard d e v i a t i o n s  t o Gd 5^/Gd ^°=  i n Table  1  1  6-1.  a r e shown. They c o r r e s p o n d t o o f t h e mean.  .9361  123  TABLE I I I - 3 .  SINGLE FILAMENT ANALYSES ON  155  No. o f \ ~ . Set scans Gd  *  G d  Analysis  Gd-JSl  Gd-JS2  l f e 0  1  5  / Gd  *  d  t  1 6 0  158 * \ ° Gd  Gd-J  G  A  l f e Q  B  .6770  .715^  1.1343  1.3598  1.6888  ±.0008  ±.0010  ±.0018  ±.0017  ±.0030  1  6  .6771 ±.0015  .7158 ±.0006,  1.1361 +.0006  1.3597 ±.0029  1.6915 ±.0013  2  6  .6767 ±.0010  .7159 ±.0012  1.1359 ±.0008  1.3604 ±.0020  1.6909 ±.0028  .6?693 . 7 1 5 7 5 1 . 1 3 5 8 6 I.36OOI 1 . 6 9 1 0 + . 0 0 0 2 2 ±.00029 ±.00063 ±.00042 ± . 0 0 1 3  Average  *  157  G n  SAMPLE  Fractionation  c o r r e c t i o n normalized to Gd^^/Gd^^ = 0  .9361  A l l a n a l y s e s were p e r f o r m e d u s i n g t h e G d O s p e c t r u m . The r a t i o s have b e e n c o r r e c t e d f o r t h e i s o t o p i c c o m p o s i t i o n o f oxygen. +  Rhenium h a v i n g a s p e c i f i e d p u r i t y o f 99.9% r e f i n e d r h e n i u m was n o t u s e d f o r t h e s e two The  parameters  A and  B are defined  was u s e d . analyses.  i n Table  6-1.  Zone-  124  TABLE I I I - 4 . • SINGLE FILAMENT ANALYSIS G d - J Z l  Gd*55 * G d " 1  No. o f Gd Set scans  Analysis Gd-JZl  °  Gd  1 6 0  M * 1  Gd  8  *  1 6 6  A  B  1  6  .6774 ±.0005  .7162 ±.0005  1.1351 ±.0015  1.3592 ±.0011  1.6885 ±.0033  2  6  .677'+ ±.0008  .7162 ±.0008  1.1356 ±.0012  1.3592 ±.0016  1.6897 ±.0022  3  6  .6767 ±.0003  • .7153 1.1356 ±.0007 ±.0010  1.360? ±.0006  1.6917 ±.0024  4  6  .6768 ±,0012  .7156 ±.0004  1.1360 ±.0008  1.3606 1.6920 ±.0024 •'±.0018  5  5  .6774 ±.0004  .7160 ±.0003  1.1357  1.6904 ±.0013  6  6  .6769 ±.0011  .7162 ±.0007  1.1355 ±.0006  1.3593 ±.0007 1.3603 ±.0022  7  3  .6772 ±.0010  .7161 ±.0005  1.1361 ±.0018  1.3597 ±.0021  1.6909 ±.0047  8  5  .6?70 ' .7163 I.I363 ±.0013 ±.0009 ±.0011  1.3601 ±.0026  1.6911 ±.0013  9  6  .6766 ±.0016  1.1364 ±.0033  1.3609 ±.0031  1.6942 ±.0079  .7150 ±.0010  ±.0004  1.6895 ±.0020  .67702 .71591 1.13570 1.35999 1.69070 ±.00025 ±.00023 ± . 0 0 0 1 8 ±.00050 ±.00069  Average  *  1 6  •  Fractionation  c o r r e c t i o n normalized  t o G d ^ ^ / G d ^ - - . 9361 1  0  The G d O s p e c t r u m was u s e d t h r o u g h o u t . The r a t i o s have been c o r r e c t e d f o r the i s o t o p i c c o m p o s i t i o n o f oxygen. +  High p u r i t y  zone-refined  r h e n i u m was u s e d  The p a r a m e t e r s A and B a r e d e f i n e d  for this  i n Table  6-1.  analysis,  125  TABLE  III-5.  TRIPLE FILAMENT ANALYSES ON  N o  Analysis  .  Gd ** « 1  o  f  Set scans  Gd  1 6 0  G  d  «7  Gd  SAMPLE.Gd-US  * 158 *  1 6 0  G d  Gd  A  1 6 0  B  1  6  .6764 1.0009  .7153 -.0015  1.13^7 +.0011  I.3609 1.0018  I.6898 +.0035  2  8  .6768 +.0004  .7163 +.0003  1.1357 +.0006  I.3603 +.000?  1.6897 +.0011  3  10  .6776 ±.0015  .7160 +.0009  1.1369 +.0017  1.3589  +.OO31  1.6931 +.0028  Gd-US5  1  6  .6768 +.0003  .7159 +.0006  1.1363 +.0004  1.3602 +.0006  1.6919 +.0014  Gd-US6  1  6  .6?6l +.0018  .7169 +.0008  1.1360 +.0020  +.OO36  +.OO33  Gd-US7  1  8  .6770 ±.000?  .7176 +.000?  1.1358 +.0004  I.3601 +.0014  1.6868 ±.0019  Gd-us4  Average  •• F r a c t i o n a t i o n  I.36I8  1.6890  .67682 , ? l 6 3 9 1.13595 1.36026 I . 6 9 0 0 +.00015 ±.ooo48-+.00034 ±.00027 ±.0016  correction normalized  The p a r a m e t e r s A and B a r e d e f i n e d  to G d ^ / G d 1  6  i n Table 6 - 1 .  l 6  °= .9361  126  TABLE  III-6.  ANALYSIS Eu-AB USING TRIPLE  15^ *  Analysis Eu-AB  „ Gd No. o f S e t s c a n s - Gd  X 3 )  1^7 * 160  Gd '  Gd  A_  .7160 1.0005  1.1357 1.0013  1.354o 1.0034  .7162 1.0010  1.1362 1.0019  1.3486 1.0020  .7160 1.0013  I.1345 1.000?  1.3510 I.0031  .7160 1.0005  1.1356 1.0005  1.3501 1.0026  .7161 1.000?  1.1361 1.0015  1.3525 1.0015  4  .6808^ ..7169 1.0020 1.0007  1.1352 1.0021  1.3525 +.0040  6  .6815 ±.000?  1.1353 ±.0005  1.3510 ±.0014  1  6  ,6800 1.001?  #  2  6  .6828 1.0010  #  3  6  .6816 1.0016  4  6  .6820 1.0013  5  6  .6808 1.0008  6 7  .6815 ±.0006  Average.  #  #  #  #  .7162 ±.0006  B #  #  #  #  #  #  #  I.6903 1.0025 1.6911 1.0027 1.6877 1.0041 1.6901 1.0010 1.6911 1.0028 I.6872 1.0052 I.6892 ±.0013  . 7 l 6 l 4 1.13533 1.3512* 1.68987 ±.00024 ±.00035 ±.0012 ±.00065  c o r r e c t i o n normalized to Gd^^/Gd^^.9361  *  Fractionation  #  T h e s e r a t i o s show e v i d e n c e o f L a O 155. No c o r r e c t i o n was p o s s i b l e .  The p a r a m e t e r s  1^8 * 160  9A~1_  ±:>  Gd  FILAMENTS  A and B a r e d e f i n e d  +  c o n t a m i n a t i o n a t mass  i n Table  6-1.  127  TABLE I I I - 7 .  ANALYSIS  Gd-AB USING TRIPLE  155 Gd No. o f Set scans Gd X D : >  Analysis  *  Gd  1 6 0  Gd-AB  Gd  1 5 7  1 6 0  * Gd Gd  1 5 8  FILAMENTS  *  1 6 0  A  B  1  6  .6968 .7160 +.0078 ±.0005  1.1355 ±.0023  1.322 ±.015  I.690 ±..005  2  6  .7161 .6867 +.0015 ±.0017  1.1335 ±.0046  l.3^-l  1.685 + .00?  3  6  #  #  .6808  #  ±.0055  .7184  ±.0051  #  ±.003  1.1354 1.352 ±.0083 ±.011  #  .6867 .71607 1.1351 ±.0033 ±£00032 ±.0011  Average  #  #  correction normalized  1.34l ±.007  1.684  ±.009  I.6878 ±.0036  #  *  Fractionation  t o Gd ^^/Gd ^°=.9361  #  These r a t i o s show e v i d e n c e o f L a O c o n t a m i n a t i o n a t massj 155. No c o r r e c t i o n was p o s s i b l e .  1  1  +  :  The p a r a m e t e r s  A and B a r e d e f i n e d  i n Table  6-1.  128  TABLE III-8.  ANALYSIS GE-AB USING A ZONE-REFINED RHENIUM SINGLE FILAMENT  Gd ^ No. o f A n a l y s i s Set scans G d 1  * Gd  1 5 7  * Gd  Gd  1 6 0  Gd  1 6 0  GE-AB  Average  *  1 5 8  *  1 6 0  •A  B  1  6  .6769 ±.0013  .7169 ±.0011  I.I367 ±.0016  1.3601 ±.0026  1.6904  2  6  .6760 ±.0005  .7161 ±.0007  1.1357 ±.0005  1.3619 ±.0011  1.6900 ±.0019  3  6 '  .6766 +.0006  .7160 ±.0004  1.1351 ±.0005  1.3608 ±.0011  1.6890 ±.0011  4  6  ±.0004  .7160 1.1355 +.0002 ±.0005  1.3606 ±.0009  1.6900 + . 00.08  5  6  .6767 ±.0012  ±.0004  1.1363 ±.0007  1.3606 ±.0024  1.6918 ±.0017  6  6  .6766 ±.0010  .7158 ±.0005  1.1352 ±.0010  1.3609 ±.0021  1.6897 ±.0012  7  6  .6?66 ±.0009  .7157 ±.0009  1.1355  1.3608  1.6906 ±.0043  .6767  .67652  .7160  ±.0024  ±.0018  ±.0045  .71598 1.13557 1.36092 1.68991  ±.00024 ±.00012 ±.00031 ±.ooo44 ±.00058  F r a c t i o n a t i o n c o r r e c t i o n normalized t o Gd?-^^/Gd^^°~ .$361  A l l r a t i o s were obtained u s i n g the GdO spectrum. The r a t i o s have been c o r r e c t e d f o r the i s o t o p i c composition of oxygen. +  The parameters A and B are d e f i n e d i n Table 6-1.  129  A P P E N D I X IV SLOPE OF THEORETICAL CORRELATION  LINES FOR  NEUTRON  CAPTURE 157 160 * L e t X and Y be the v a l u e s of the G d /Gd 158 160 * and Gd /Gd i s o t o p i c r a t i o s when n o r m a l i z e d to 156, 160 Gd /Gd .9361. Then  ,156  160 j - - ^T6u- -  X=  1  Y-G^P  Gdl60  75(  f 9361)  Gd 156,^ \  1^160/  / i G d  (l--50(gToO--936l)/(g^o)j  (IV-1)  where the i s o t o p i c r a t i o s i n equations (IV-1) a r e the m e a s u r e d r a t i o s , without n o r m a l i z a t i o n . C o n s i d e r a t i o n of the t h e r m a l neutron capture p r o c e s s e s leads to the f o l l o w i n g d i f f e r e n t i a l equations  d(Gd  1 5 7  /Gd  1 6 0  )  d(Gd  1 5 8  /Gd  1 6 0  )  d(Gd  1 5 6  /Gd  1 6 0  )  ^55  d(Gd  1 5 8  /Gd  1 6 0  )  ^157  / Gd x  1 5 5  '  Gd '' i J  (cont. over)  130  d(Gd  1 5 5  /Gd  1 6 0  )  d(Gd  1 5 8  /Gd  l 6 D  )  ^ 5 55 //GGdd „ v  i 1J 5J 5  \  (IV-2)  „ ,157 ' ^157 ' G d 1 5 7  F r o m equations (IV-1) and (IV-2) i t can be shown that  I-  dY dX  S{ Qu{ (IV-3) F o r s m a l l changes i n i s o t o p i c c o m p o s i t i o n we maya s s u m e that G d ^ ^ / G d ^ ^  =  .9361, w h i c h g r e a t l y s i m p l i f i e s equation  (IV-3), and leads to a slope f o r the c o r r e l a t i o n line f o r t h e r m a l n e u t r o n capture (see F i g u r e s 6-1 and 6-3) of d Y / d X =  - .763 near  t e r r e s t r i a l Gd (using data f r o m T a b l e s 1-1 and 6-1). 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