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UBC Theses and Dissertations

The decay scheme of Fe⁵⁹ Hanson, Gordon Harold 1951

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an AS Gsp.» T H E D E C A Y S C H E M E O F F E ^ b y G o r d o n H a r o l d H a n s o n A t h e s i s s u b m i t t e d i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s fer t h e d e g r e e o f M A S T E R O F A R T S i n t h e D e p a r t m e n t o f P H Y S I C S W e a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e s t a n d a r d r e q u i r e d f r o m c a n d i d a t e s f o r t h e d e g r e e o f M A S T E R O F A R T S M e m b e r s o f t h e D e p a r t m e n t o f P h y s i c s T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a A p r i l , 1951 ABSTRACT T h e b e t a a n d g a m m a s p e c t r a o f h a v e b e e n e x a m i n e d i n a t h i n l e n s b e t a r a y s p e c t r o m e t e r . G a m m a r a y s w i t h e n e r g i e s o f l . l O H e v a n d 1.29 M e v w e r e d e t e c t e d u s i n g t h e p h o t o e l e c t r i c t e c h n i q u e w i t h a U r a n i u m r a d i a t o r . U s i n g a t h i n f o i l a s a s o u r c e , b e t a g r o u p s w i t h m a x i m u m e n e r g i e s o f 1.77 M e v a n d 0 . 4 J ? M e v w e r e f o u n d . T h e r e w a s n o e v i d e n c e o f a 0 . 2 6 M e v g r o u p a s r e p o r t e d b y o t h e r w o r k e r s . . . T e n t a t i v e d e c a y s c h e m e s a r e p r e s e n t e d . ! A C K N O W L E D G E M E N T S T h i s r e s e a r c h h a s b e e n c a r r i e d o u t u n d e r a G r a n t -i n - A i d m a d e t o D r . K . C . M a n n o f t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a P h y s i c s D e p a r t m e n t b y t h e N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a . T h e a u t h o r i s i n d e b t e d t o D r . M a n n f o r m u c h a d v i c e a n d a s s i s t a n c e r e n d e r e d t h r o u g h o u t t h e c o u r s e o f t h e w o r k . T h e a u t h o r i s g r a t e f u l t o t h e B r i t i s h C o l u m b i a T e l e p h o n e C o m p a n y f o r t h e a w a r d o f a s c h o l a r s h i p a n d t o t h e B r i t i s h C o l u m b i a I n d u s t r i a l R e s e a r c h C o u n c i l a n d t h e U n i v e r -s i t y o f B r i t i s h C o l u m b i a f o r t h e a w a r d o f . g r a n t s f o r s u m m e r w o r k . T A B L E O F C O N T E N T S P a g e I . G e n e r a l I n t r o d u c t i o n . 1 ( a ) E n e r g y D e t e r m i n a t i o n s ( b ) M e a s u r e m e n t s o f I n t e r n a l C o n v e r s i o n C o e f f i c i e n t s ( c ) C o i n c i d e n c e M e a s u r e m e n t s ( d ) M e a s u r e m e n t s o n I s o m e r i c S t a t e s o f N u c l e i i ( e ) A n g u l a r C o r r e l a t i o n M e a s u r e m e n t s I I . B e t a R a y S p e c t r o s c o p y ( a ) D e s c r i p t i o n o f D i f f e r e n t T y p e s ( b ) S p e c t r o m e t e r C h a r a c t e r i s t i c s ( c ) S o u r c e A r r a n g e m e n t s ( d ) C a l i b r a t i o n o f t h e S p e c t r o m e t e r ( e ) E x p e r i m e n t a l A p p a r a t u s - U s e d i n P r e s e n t W o r k I I I . T h e 2 6 F e ^ N u c l e u s . . . . . . . . . 15 ( a ) P r e v i o u s W o r k ( b ) S o u r c e s U s e d ( c ) B e t a a n d G a m m a R a y E n e r g i e s B i b l i o g r a p h y 22 LIST OP ILLUSTRATIONS F i g u r e F a c i n g P a g e 4 D i a g r a m o f T h i n L e n s S p e c t r o m e t e r 8 6 G a m m a R a y S p e c t r u m o f F e - ^ 9 16 .7. L o w E n e r g y B e t a G r o u p o f F e ^ 17 8 C o m p l e t e B e t a S p e c t r u m o f F e ^ ? 17 9 F e r m i P l o t o f L o w E n e r g y B e t a G r o u p . . . . . 17 10 F e r m i P l o t o f H i g h E n e r g y B e t a G r o u p . . . . . 17 G E N E R A L I N T R O D U C T I O N A t t h e p r e s e n t t i m e t h e p h y s i c s o f t h e n u c l e u s i s o n l y p a r t i a l l y u n d e r s t o o d . N o t h e o r y h a s y e t b e e n a d v a n c e d w h i c h c a n e x p l a i n a l l o f t h e e x p e r i m e n t a l r e s u l t s . I n m a n y c a s e s t h e e x p e r i m e n t a l d a t a i s i n a d e q u a t e . I t c a n n o t b e e x p e c t e d t h a t a c o n s i s t e n t t h e o r y o f t h e n u c l e u s , w i l l b e d e v e l o p e d w i t h o u t t h e a s s i s t a n c e o f m o r e c a r e f u l e x p e r i m e n t a l w o r k . O n e p f t h e m o s t f r u i t f u l m e a n s o f o b t a i n i n g n u c l e a r d a t a i s t h e s t u d y o f r a d i a t i o n s e m i t t e d b y n a t u r a l o r 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 . D e s p i t e t h e g a p s i n o u r k n o w l e d g e , s o m e p r o p e r t i e s o f n u c l e i ! a r e well k n o w n . I t h a s b e e n d e f i n i t e l y e s t a b l i s h e d t h a t d i s c r e t e e n e r g y l e v e l s e x i s t w i t h i n t h e n u c l e u s a n d t h a t b y s u i t a b l e m e a n s i t m a y b e e x c i t e d t o a n y o f t h e s e . T h e i n d i v i d u a l n e u t r o n s a n d p r o t o n s w h i c h c o n s t i t u t e t h e n u c l e u s e a c h h a v e a c e r t a i n s p i n a n d p a r i t y a s s o c i a t e d w i t h t h e i r w a v e f u n c t i o n s a n d i t i s k n o w n t h a t t h e s e q u a n t i t i e s c o m b i n e t o g i v e a r e s u l t a n t s p i n a n d p a r i t y f o r t h e n u c l e u s a s a w h o l e . I t h a s b e e n f o u n d t h a t c e r t a i n c o n f i g u r a t i o n s o f n e u t r o n s a n d p r o t o n s w i l l f o r m a s t a b l e n u c l e u s w h i l e o t h e r c o n f i g u r a t i o n s a r e u n -s t a b l e . U n s t a b l e n u c l e i : ! m a y d e c a y t o a s t a b l e f o r m b y e m i t t i n g f o r e x a m p l e a p o s i t i v e o r n e g a t i v e e l e c t r o n o r b y c a p t u r i n g a n o r b i t a l e l e c t r o n , ' t h e e x a c t m e t h o d d e p e n d i n g o n t h e e n e r g y a v a i l a b l e a n d o n s e l e c t i o n r u l e s . T h e r e s i d u a l n u c l e u s i s f r e q u e n t l y i n a n e x c i t e d s t a t e a n d s u b s e q u e n t l y d r o p s t o t h e 2 ground state by the emission of one or more gamma rays. The investigation of these particles and quanta often makes possible the calculation of the energies, spins and parities of the various states in a nucleus and the determination of the correct mode of decay. As there are s t i l l many isotopes about which complete information i s not available, i t is important for more experimental work to be done. (a) Energy Determinations Referring to figure 1 , i t is apparent that the measure-ment of the maximum energy of the beta particles allows the determination of the energy difference between the i n i t i a l state of the original nucleus and a f i n a l state of the daughter nucleus, while the measurement of the energies of the gamma rays, i f any, F i g . I. allows the determination of the energy difference between the states in the daughter nucleus. If beta particles of more than one maximum energy, are emitted, the difference corresponds to an energy difference in the daughter nucleus.. With this information i t is usually possible to draw a tentative decay scheme i l l u s -trating the relative position of the energy levels providing the spectrum is not too complex. 3 The continuous distribution of energy in a beta spectrum makes i t impossible to determine the end point ex-actly from the graph of number of particles versus energy as is shown in figure 2(a). A different type of plot, shown in figure 2(b), based on Fermi's theory of beta decay, in which Fig. 2. the graph intersects the energy axis at a definite angle avoids this difficulty.. The theory predicts that the Fermi plot will have a shape which ,is. .to some extent dependent on the spin and parity changes associated with the transition, being for instance a straight line in "allowed transitions". This together with a knowledge of the half-life of the isotope and of the total energy involved generally gives a good indication of the spin change. (b) Measurements, of Internal Conversion Coefficients A nucleus in an excited state may decay to the ground state by giving the excitation energy to one of its orbital electrons. This process, known as internal conversion, results 4 i n a l i n e spectrum of electrons being emitted from the source, the energy of the electrons corresponding to the difference between the excit a t i o n energy of the nucleus and the binding energy of the K or L o r b i t a l electron emitted. These conver-sion l i n e s are thus superimposed on the continuous beta spectrum as shown i n figure J>, The p r o b a b i l i t y f o r this process to occur depends on the spin and parity changes—that i s on the multipolarity of the gamma rad i a t i o n involved i n the a l t e r -native method of decay, and on the energy of the t r a n s i t i o n . The r a t i o of the number of Fig. 3 conversion electrons to the number of gamma rays.for a p a r t i c u l a r t r a n s i t i o n i s c a l l e d the int e r n a l conversion c o e f f i c i e n t . Theor-e t i c a l values for these, c o e f f i c i e n t s have recently been, calculated by M.E. Rose 1 with the aid of new computing machines. I t i s possible therefore t o measure these c o e f f i c i e n t s and by comparing with the th e o r e t i c a l r e s u l t s determine what spin and pa r i t y changes may occur. This method has become almost standard p r a c t i s e . (c) Coincidence Measurements Frequently there are two or more beta groups and many gamma rays associated with the decay of one Isotope. I t then becomes d i f f i c u l t to draw a unique decay scheme as i t i s not known 5 w h i c h gamma rays' a r e a s s o c i a t e d w i t h t h e v a r i o u s b e t a g r o u p s . S i n c e t h e l i f e t i m e o f an e x c i t e d s t a t e i s u s u a l l y e x t r e m e l y s h o r t , c o i n c i d e n c e c o u n t i n g t e c h n i q u e s a r e u s e d to d e t e r m i n e w h e t h e r o r not t h e r e a r e any gammas f o l l o w i n g b e t a s o f a c e r t a i n e n e r g y . C o i n c i d e n c e s may be s o u g h t between b e t a s and gammas, b e t a s and c o n v e r s i o n e l e c t r o n s o r between gammas and gammas. The beta-gamma and b e t a - c o n v e r s i o n e l e c t r o n c o i n c i d e n c e r a t e s o b t a i n e d a t d i f f e r e n t b e t a e n e r g i e s c o r r e s p o n d i n g t o t h e d i f f e r e n t b e t a g r o u p s h e l p t o d e t e r m i n e w h e t h e r gamma, r a y s f o l l o w t h e b e t a d i s e n t e g r a t i o n s . The gamma-gamma c o i n c i d e n c e r a t e s d e t e r m i n e w h e t h e r two gammas a r e i n c a s c a d e o r w h e t h e r t h e y e a c h r e s u l t f r o m t r a n s i t i o n s d i r e c t l y t o t h e g r o u n d s t a t e . When t h e r e a r e s e v e r a l gamma r a y s , t h e v a r i o u s c o i n c i d e n c e r a t e s t o -g e t h e r w i t h t h e knowledge o f t h e r e l a t i v e i n t e n s i t i e s o f b o t h gammas and b e t a s o f t e n makes p o s s i b l e t h e d r a w i n g o f a r e l i a b l e d e c a y scheme where o t h e r w i s e t h i s w o u l d have been i m p o s s i b l e . (d) Measurements on I s o m e r i c S t a t e s o f N u c l e i i O c c a s i o n a l l y a n u c l e u s may r e m a i n i n a n e x c i t e d s t a t e f o r an a p p r e c i a b l e t i m e . D e c a y t o t h e g r o u n d s t a t e proceeds' w i t h a d e f i n i t e h a l f - l i f e as i n a l p h a and b e t a e m i s s i o n . When t h i s h a l f - l i f e i s g r e a t e r t h a n a b o u t 10"9 s e c i t i s p o s s i b l e t o measure i t by e m p l o y i n g d e l a y e d c o i n c i d e n c e t e c h n i q u e s . The c h a n n e l o f t h e c o i n c i d e n c e m i x e r w h i c h h a n d l e s t h e b e t a p u l s e s i s d e l a y e d f o r v a r y i n g t i m e s . As t h i s d e l a y i s i n c r e a s e d , t h e number o f c o i n c i d e n c e s d e c r e a s e s e x p o n e n t i a l l y c o r r e s p o n d i n g t o t h e d e c a y c u r v e o f t h e e x c i t e d l e v e l . S i n c e t h e l i f e t i m e o f a n 6 e x c i t e d s t a t e i s d e p e n d e n t i n p a r t o n t h e s p i n c h a n g e i n v o l v e d i n t h e t r a n s i t i o n t o a l o w e r l e v e l , i t i s p o s s i b l e t o o b t a i n i n t h i s w a y a n i n d i c a t i o n o f t h e s p i n o f t h e e x c i t e d s t a t e . ( e ) A n g u l a r C o r r e l a t i o n M e a s u r e m e n t s U n d e r c e r t a i n c o n d i t i o n s o f s p i n a n d p a r i t y c h a n g e , t h e r e i s a c o r r e l a t i o n b e t w e e n t h e a n g l e s a t w h i c h b e t a s a n d g a m m a s o r s u c c e s s i v e g a m m a s a r e e m i t t e d . W i t h i n t h e l a s t f e w y e a r s a g r e a t d e a l o f w o r k h a s b e e n d o n e i n t h i s f i e l d a n d m a n y i s o t o p e s h a v e b e e n e x a m i n e d . W h e n t h e r e i s . a n a n g u l a r c o r r e l -a t i o n , c o m p a r i s o n w i t h t h e o r y s o m e t i m e s a l l o w s t h e a s s i g n m e n t o f s p i n c h a n g e s t o t h e t r a n s i t i o n i n q u e s t i o n . V / i t h m o s t i s o t o p e s a c o m b i n a t i o n o f t h e a b o v e t e c h -n i q u e s i s n e c e s s a r y t o g e t a l l t h e d e s i r e d d a t a . M o s t n u c l e a r e n e r g y , s t a t e s a r e a s y e t u n k n o w n e i t h e r a s t o e n e r g y , s e q u e n c e o r a s t o s p i n a n d p a r i t y a s s i g n m e n t s a s a r e s u l t o f t h e d i f f i c u l t y o f i n t e r p r e t a t i o n o f e x p e r i m e n t a l r e s u l t s . A g a i n , s o m e i s o t o p e s h a v e s u c h s h o r t h a l f - l i v e s t h a t t h e m e a s u r e m e n t s t h e m s e l v e s a r e e x t r e m e l y d i f f i c u l t t o m a k e ; s o m e a r e d i f f i c u l t t o p r e p a r e b e -c a u s e o f t h e l o w c r o s s s e c t i o n o f t h e p a r e n t m a t e r i a l f o r n e u t r o n o r p r o t o n b o m b a r d m e n t w h i l e o t h e r s o c c u r a s g a s e s o r l i q u i d s a n d a r e n o t s u s c e p t i b l e t o t h e t e c h n i q u e s c o m m o n l y u s e d w i t h s o l i d s . F u r t h e r i n v e s t i g a t i o n i n t h i s f i e l d i s n e c e s s a r y a n d n o d o u b t n e w t e c h n i q u e s w i l l h a v e t o b e d e v e l o p e d b e f o r e t h e b o d y o f i n f o r -m a t i o n i s c o m p l e t e . 7 I I BETA RAY SPECTROSCOPY A powerful instrument for studying radiations from radioactive isotopes i s the magnetic focussing beta ray spect-rometer. This instrument i s based on the fact that an electron moving i n a magnetic f i e l d whose d i r e c t i o n i s perpendicular to the motion of the electron w i l l move i n a c i r c u l a r path whose radius i s proportional to the momentum of the electron. A p p l i -cation of the Lorentz force law to an electron moving i n a mag-netic f i e l d y i e l d s the equation Hev = m y2 , where H i s the magnetic f i e l d strength i n gauss, p i s the radius of the c i r c l e described by the electron and m, e and v are the mass, charge and ve l o c i t y respec-t i v e l y of the electron. Therefore Ho = m v which i s proportional to the momentum of the ' e electron. Since t h i s i s the property used i n beta ray spectroscopy, i t follows that the instrument measures momentum rather than energy. Among the several forms of the instrument are the TT or semi c i r c u l a r focussing type, the t h i n lens and the solenoidal types. These types d i f f e r i n the form of the focussing employed, and i n the source and counter arrangements. In a l l of them how-ever, the electrons pass from the source through a f i e l d which can be varied,and into a counter. The entire path i s i n a vacuum. 8 (a) Description of D i f f e r e n t Types The TT type spectrometer provides a uniform magnetic f i e l d , the source and detector being so arranged that the par-t i c l e trajectory l i e s i n a plane to which the f i e l d i s normal. Both geiger counters and photographic plates have been widely used as detectors. When a geiger counter i s used the f i e l d must be varied i n small steps to focus p a r t i c l e s of d i f f e r e n t momenta at the counter window. When photographic plates are used only one s e t t i n g of the f i e l d i s required and the b a f f l e s are removed. The 7T type of instrument i s p a r t i c u l a r l y suitable for low energy work since the t o t a l path length may be made' small thereby reducing scattering. In addition i t i s possible to use counters with very t h i n windows by placing the counters inside the vacuum system and using low pressure f i l l i n g mix-tures. The t h i n lens and solenoidal spectrometers are similar i n construction and d i f f e r only i n the p o s i t i o n of the b a f f l e s and of the magnetic f i e l d c o i l s . The t h i n lens spectrometer i s shown i n d e t a i l i n figure 4 . The source 1? and counter J are placed at opposite ends of a long tube. The p a r t i c l e s pass down the tube and are focussed at the counter by the magnetic f i e l d which i s p a r a l l e l to the length of the tube. A system of b a f f l e s i s arranged as shown( C,D,E,G,H ). These define an annulus down which with the proper magnetic f i e l d , electrons of a given energy w i l l pass. The magnetic f i e l d used with the t h i n lens spect-rometer i s produced by " t h i n " c o i l s placed at the center of the * : i • A. : 1 i r p " -B T o P L J M P r : 3 . 4. 9 tube and focussing takes place only i n t h i s region of the path. The theory and design of t h i s type of spectrometer has been de-2 scribed by M . Deutsch, L.G. E l l i o t and R.D. Evans. A modification of t h i s instrument,the double focussing spectrometer has two c o i l s placed symmetrically about the center of the tube. In the solenoi&al spectrometer, the f i e l d c o i l s are wound around the tube f o r i t s entire length thus producing a f i e l d that i s a x i a l l y homogeneous. (b) Spectrometer Charac t e r i s t i c s The important c h a r a c t e r i s t i c s of any spectrometer are i t s transmission and. i t s r e s o l u t i o n . Transmission i s the f r a c t i o n of a l l p a r t i c l e s i n a given momentum i n t e r v a l which reach the counter. Resolution i s the a b i l i t y to d i s t i n g u i s h between par-t i c l e s of d i f f e r e n t momenta, normally defined as ^ P/P where P i s the momentum. The transmission factor must be large enough to avoid having to use very strong sources or unreasonably long periods of counting. In most work i t i s desirable to have the resolution as high as possible i n order that gamma ray energies can be accurately determined. I t should be noted that the conditions of high trans-mission and high resolution are mutually exclusive i n any one instrument so that optimum conditions represent a compromise between these two factors. The transmission and resolution of the solenoidal types are generally higher than for the thin lens but t h i s i s accomplished at the cost of greater complexity and higher current requirements. 10 (o). Source Arrangements The requirements as to size and shape of the source d i f f e r somewhat according to the type of rad i a t i o n being studied. It i s always necessary however that the source diameter be small. Large sources aff e c t the resolution adversely. A beta ray source must be t h i n and must be mounted i n such a way that no heavy material i s behind i t . I f these conditions are not met, loss of energy and scattering occur i n the source i t s e l f and some electrons are backscattered through nearly 1 8 0 ° from the backing material. This w i l l r e s u l t i n a d i s t o r t i o n of the shape of the spectrum with a resultant d i s t o r t i o n i n the Fermi p l o t . The greatest trouble occurs at low energies because the scattering cross section i n -creases as the energy decreases. Thus low energy determinations are d i f f i c u l t to make and the detection and measurement of low energy beta and gamma rays require special precautions against t h i s e f f e c t . Suitable materials for the mounting of beta sources are thin sheets of mica, films of material such as collodiom and th i n f o i l s of l i g h t metals. I t may sometimes happen that the source becomes charged as a r e s u l t of emitting electrons and the resultant e l e c t r i c f i e l d d i s t o r t s the spectrum. It is then necessary that the source backing be made conducting. The whole f r a g i l e arrangement i s usually mounted'inside a heavier hollow capsule which can stand the a i r pressure when the spectrometer is evacuated. When the source i s a positron emitter, i t i s necessary to d i s tinguish between the positi v e and negative electrons. In the t h i n lens spectrometer this may be done by i n s e r t i n g a s p i r a l 11 b a f f l e i n t h e c e n t e r o f t h e s p e c t r o m e t e r t u b e . E i t h e r p o s i t r o n s o r e l e c t r o n s a r e t h e n i ' o c u s s e d , d e p e n d i n g o n t h e d i r e c t i o n o f t h e c u r r e n t i n t h e f i e l d c o i l s . A t y p i c a l s o u r c e a r r a n g e m e n t f o r d e t e c t i o n o f b e t a r a d i a t i o n i s s h o w n i n f i g u r e j ? t a ) . m a k i n g u s e o f t h e p h o t o e l e c t r i c e f f e c t . . A r a d i a t o r o f a m a t e r i a l w i t h h i g h 2T s u c h a s u r a n i u m o r l e a d i s p l a c e d a t t h e p o s i t i o n n o r m a l l y o c c u p i e d b y t h e b e t a s o u r c e . T h e s a m e c o n d i t i o n s a s t o d i a m e t e r a n d t h i c k n e s s a s a p p l i e d t o b e t a s o u r c e s a p p l y h e r e e x c e p t t h a t t h e r e i s n o o b j e c t i o n t o h e a v y b a c k i n g m a t e r i a l s i n c e m o s t o f t h e p h o t o e l e c t r o n s a r e e j e c t e d i n t h e f o r w a r d d i r e c t i o n . G e n e r a l l y a n a b s o r b e r o f l i g h t m e t a l i s p l a c e d b e t w e e n t h e s o u r c e a n d r a d i a t o r t o a b s o r b a l l t h e p r i m a r y b e t a r a d i a t i o n . A t y p i c a l g a m m a r a y s o u r c e i s s h o w n i n f i g u r e 5 ( b ) . P h o t o e l e c t r o n s e j e c t e d f r o m t h e i n n e r e l e c t r o n s h e l l s o f t h e r a d i a t o r t h e n p a s s d o w n t h e s p e c t r o m e t e r a n d a r e f o c u s s e d a t . t h e c o u n t e r . T h e e n e r g y o f t h e g a m m a r a y i s g i v e n b y E ^ = ^ S p . E + ^ K,L> w h e r e E p . g i s t h e e n e r g y (a) (b) Fig. 5. T h e d e t e c t i o n o f g a m m a r a d i a t i o n i s a c c o m p l i s h e d b y 12 of the photoeleotron and Is the binding energy of the K or L electrons i n the radiator material. Usually two peaks corres-ponding to photoelectrons ejected from the K and L s h e l l s are ob-served. The cross-section for the photoelectric, e f f e c t f a l l s o f f rapi d l y for increasing gamma energy but most nuclear gamma rays are i n the energy region where the cross section i s s u f f i c i e n t l y high f o r measurements to be made. (d) C a l i b r a t i o n of the Spectrometer The magnetic f i e l d i s proportional to the current i n the f i e l d c o i l s since no i r o n i s used i n most instruments. In order to c a l i b r a t e the spectrometer, the current required to energy focus electrons of one definite/must be accurately known. The electrons ejected from a radiator by the O.jjll Mev a n n i h i l a t i o n r a d i a t i o n coming from a positron emitter are suitable for t h i s purpose. Generally i t is possible to measure gamma ray energies and beta ray end points to better than one percent and to achieve a resolution of from f i v e percent to as good as two percent i n momentum. (e) Experimental Apparatus Used i n Present Work The spectrometer used i n the work reported i n t h i s thesis i s of the t h i n lens type. The resolution i s approximately 3.j?f° i n momentum and the transmission i s about 0.4%. The magnetic f i e l d i s produced by four concentric c o i l s of wire, any number of which may be connected i n s e r i e s . When a l l four c o i l s are used, electrons of up to 3 Mev may be conveniently focussed at the 13 counter with the current source available. As the r e s o l u t i o n i s somewhat better when only the outer c o i l s are used i t i s generally desirable to use as few as the energy of the p a r t i c l e s being investigated w i l l permit. The spectrometer is aligned p a r a l l e l to the horizontal component of the earth's f i e l d , while the v e r t i c a l component i s cancelled out by a pair of large Helmholz c o i l s situated above and below the spectrometer table. A b e l l type geiger counter was used as a detector. A flange on the end of the counter f i t t e d into a c i r c u l a r groove i n the spectrometer and thus provided a vacuum tight f i t . The window was 6 mm i n diameter and was covered by a mica sheet of thickness 2 mg/cm^ which transmitted electrons of energies above 30 Kev. A "one-shot" multivibrator was used as a quenching c i r c u i t . I t provided a quenching pulse .of 300 v o l t s about AQOy/sec long. I t i s necessary that the current i n the f i e l d c o i l s be c a r e f u l l y controlled over a considerable range and that i t can be e a s i l y changed i n steps of any desired s i z e . In this laboratory a l l the. current was passed through a bank of t h i r t y eight 6AS7-G's operated i n p a r a l l e l . A small standard resistance was i n series with the tubes. The voltage across the standard resistance was compared with that from a Rubicon potentiometer which i n turn was calibr a t e d against a standard c e l l . These two voltages were chopped at 60°/sec by a Brown converter and the resultant square wave was amplified and r e c t i f i e d . The output was fed back to the grids of the 6AS7-G's i n the correct phase to correct any difference i n the two voltages being compared. This c i r c u i t controlled variations..of up to lOc/sec. For higher frequencies, an A.C. amplifier was used to feed back a correcting voltage to the grids of the 6 A S 7 - G ' s . This bank of tubes w i l l pass up to 1G amps. When more current than t h i s was required, some of the current was bypassed through appropriate r e s i s t o r s i n p a r a l l e l with the tubes. 15 I I I THE 26 F E ^ NUCLEUS (a) Previous Work Previous research on Fe''? has been done by J.J". • Livingood and G.T. Seaborg,^ and by M. Deutsch and collabor-4 5 ators. " Livingood and Seaborg used absorption techniques to measure gamma and beta end point energies. They reported a prominent beta group of energy 0.4 Mev and* a much les s i n -tense group of approximately 0 .9 Mev. Gamma rad i a t i o n of about 1 Mev was also reported. Deutsch's group used a t h i n lens spectrometer s i m i l a r to the one us.ed i n t h i s work. They found two gamma rays of energies 1.10 Mev and' 1.50 Mev and of approximately equal i n t e n s i t y . A Fermi plot of t h e i r beta spectrum showed two groups with end points of 0.26 Mev and 0.46 Mev. These were also of approximately equal i n t e n s i t y . In addition they reported a high energy t a i l of low int e n s i t y on t h e i r beta spectrum extending to about 1.1 Mev. They attributed this t a i l to Compton electrons produced i n the source backing and showed that i t s r e l a t i v e i n t e n s i t y depended on the thickness of the backing. Coincidence measurements indicated that the 1.50 Mev gamma was i n coincidence with the 0.26 Mev beta and that the 1.10 Mev gamma was i n coincidence with the 0.46 Mev beta. I 16 (b) Sources Used 59 The source used i n thi s research was Fe-" prepared by an (n, X) reaction from F e ^ i n the Chalk River p i l e of the National Research Council. The ra d i a t i o n time was fourteen days. The i r o n was i n the form of a wire about 1 mm i n diameter and 16 cm long weighing one gram. The s p e c i f i c a c t i v i t y was 0 . 5 mc per gram. About 13 cm of t h i s wire was wound i n a l i g h t c o i l of diameter 1 cm for use as a gamma source. A uranium radiator of thickness 100 mg/cm2 mounted on a brass disc of s u f f i c i e n t thick-ness to absorb the primary betas served as a source of photo-electrons* Two beta-.sources were prepared by hammering out a small portion of the wire into a f o i l . The edges were trimmed so as to provide sources of a diameter of 6 mm. The thicker of these two sources was about 2.5 mg/cm^ thick while the thinner one was 13 mg/cm^ thick. (c) Beta and Gamma Ray Energies- The gamma ray spectrum i s shown i n figure 6. Two gammas of energies 1 . 1 0 4 i .007 Mev and 1 . 2 9*i .007.Mev e x i s t . Subtraction of the Compton background shows TL f photoeleotron peaks at l.lOMev and 1.29 Mev. A.very small peak occurs at 0.57 Mev (0.295 on Pot.). Whether a gamma ray of t h i s energy exists i s open to question. Certainly i t i s at the l i m i t of detection of the instrument at these counting rates. The two prominent gammas are i n agreement with Deutsch's work. '"} i 1 . The complete beta spectrum i s shown i n figure 8 ; . X I t was obtained i n two parts. The high energy portion was obtained using a l l four spectrometer c o i l s , while the intense low energy group shown dotted, required only two c o i l s . This l a t t e r group i s shown i n f u l l i n figure %. The Fermi plots of the two groups are shown i n figures 9 and 10. In figure 9 the f u l l l i n e rep-resents the Fermi plot of the low energy group only, the other having been subtracted while the dotted l i n e s are the plots of the two parts of the composite spectrum. The end point energies are 1.77 Mev and 0.45 Mev. I t is to be noted that there i s no evidence of int e r n a l conversion of either gamma ray. It i s clear from figure 9 that there i s no evidence for a beta group with a maximum energy of 0'.26 Mev as reported by Deutsch. I f such a group were present to as l i t t l e as per-haps 10% of that of the main group there would be an abrupt change i n the slope of the Fermi plot at 0.26 Mev. I t i s not possible that source absorption and scattering just cancelled out this change since the plot of the spectrum obtained with the thinner source i s a similar straight l i n e (not shown). Exact cancellation would not be l i k e l y with two sources of di f f e r e n t thickness. The eff e c t of source thickness to be ex-pected i n th i s connection may be roughly estimated by comparing thi s beta d i s t r i b u t i o n with that of the 0.32 Mev positron group from Zn^3, This spectrum was taken i n th i s laboratory i n 1949. The source was. a::zinc f o i l of thickness 25 mg/cm^. The Fermi plot showed no source d i s t o r t i o n above 0.13 Mev. The Fe source used i n thi s work was much thinner yet shows no evidence of a beta group of maximum energy 0.26 Mev. 18 The group with maximum energy of 1.77 Mev i s believed to be due to a small amount of phosphorus impurity i n the iron . p32 has a beta spectrum with a maximum energy of approximately 1.75 Mev. Only a very small amount of impurity i s required since the s p e c i f i c a c t i v i t y of phosphorus a f t e r fourteen days i r r a d i a t i o n i s 1200 times that of i r o n . 6 The half l i f e of P^ 2 i s only fourteen days compared with f o r t y - s i x for thus making i t possible to follow the intensity of the group and to determine i t s o r i g i n . Measurements taken nine days apart showed a decrease of almost t h i r t y percent i n intensity of the high energy group compared with a drop of about ten percent for the lower energy group. The 1.77 Mev beta group may therefore almost c e r t a i n l y be assigned to P^2. No other impurity could have the observed decay rate and in t e n s i t y . Two alte r n a t i v e decay schemes fo r IPe-5? may be postulated on the basis of the above r e s u l t s . The f i r s t one shown i n figure (a) would mean that coincidences could be obtained between 1.29 1.10 [ 0.19 (a) (b) F i g . II. the 1.10 Mev and the 1.29 Mev gamma rays. However E.K. Darby and G. Williams? working i n th i s department have found that there are no such coincidences. This scheme must therefore be rejected. The alt e r n a t i v e , shown i n figure 11(b) requires that a 0.19 Mev gamma be i n coincidence with that of 1.10 Mev. No peak corresponding to a gamma ray of 0.19 Mev was obtained i n the gamma spectrum shown i n figure 6. However, since the binding energy of the K s h e l l of uranium i s 114 Kev ,any re-s u l t i n g photoelectrons from t n i s t r a n s i t i o n would have an energy of only about 75 Kev. Any peak would c e r t a i n l y be d i s -torted to the point where detection was unlikely, since the cut-off energy due to scattering and counter window absorption occurs at about 60 Kev with our spectrometer arrangement. I t i s well to note too that a beta group leading d i r e c t l y to the ground state of Co-^ would have an energy of about 1.75 Mev and would therefore be masked by the P^ 2 Impurity. It would therefore appear on the basis of the evidenc of these measurements, that the decay scheme of figure 11(b) best f i t s the data. I t i s d i f f i c u l t to understand the non-appearance of the 0.26 Mev beta group as found by Deutsch and co-workers, who on the basis of t h e i r evidence postulate the decay scheme shown i n fi g u r e 12. These workers too, found no evidence of a 0.2 Mev.gamma ray which t h e i r decay scheme seems suggest. They concluded from the absence of a di r e c t trans-i t i o n between ground states that the I . 5 6 Mev beta-transition i s highly forbidden and therefore that the spin difference be-tween the ground states of Ee^9 and Co^9 i s large. They found 20 59 5 9 no coincidences between gamma-rays, Fe C o i n agreement with the work of Darby p~ 0.26 •20 and Williams of t h i s department. The two gamma-rays were found to be of approximately equal i n t e n s i t y . The 1.30 r e l a t i v e i n t e n s i t i e s of the 1 .2? and 1.10 1.10 Mev t r a n s i t i o n s as obtained i n th i s laboratory are estimated to be Fig. 12. about 1 : 1 . 2 . The scheme shown i n figure 1 1 ( b ) i s consistent with Deutsch's findings except for the 0.26.Mev beta-group. I t is possible that a high energy beta t r a n s i t i o n takes place but i t w i l l at best, be very weak i n comparison to the 0 .45 Mev group. In view of the r e l a t i v e l y high energy of the t r a n s i t i o n i t i s concluded that a beta-transition between ground states i s pro-bably forbidden, the degree of forbiddeness being uncertain. Some tentative conclusions may be drawn by an a p p l i -8 9 cation of the- nuclear s h e l l models of Nordheim or of Mayer 7 to the "Fe59 and Go39 n u c l e i . According to the f i r s t author the 33rd neutron of ^ l e ^ should have a configuration while the 27th proton of 27 co^^ should have a g ^ term. Measurements on the spin of Co-^ confirm the l a t t e r assignment with a measured value of 7 / g . On t h i s basis then, the spin difference i s 2 and the p a r i t y change i s NO (even even)which makes the trans-i t i o n second forbidden according to the Gamow-Teller s e c t i o n r u l e s . The Mayer nuclear s h e l l model predicts configurations 21 for the ground states of Fe^9 and Co39 to be f ^ and f ^ res-pectively which would mean a spin change of 1 and parity change NO (odd odd) and hence an allowed t r a n s i t i o n . The evidence i s such that t h i s i s u n l i k e l y . I t i s concluded then that the spin of the ground state of Fe^9 i s probably 3/2 and i t s pa r i t y i s even. Since the Fermi plot of the 0.46 Mev beta-group i s a straight l i n e , this t r a n s i t i o n may well be allowed which would mean that possible spin values for the high energy excited state of C o ^ could be l / 2 , 3/2 or 5/2 with parity even. Since a choice of 1/2 for this state would mean a spin change of 3 for the 1.29 Mev t r a n s i t i o n corresponding to e l e c t r i c octupole r a d i a t i o n 5 such a t r a n s i t i o n might well be metastable ( i . e . -long h a l f - l i f e ) and there i s no evidence for t h i s . Therefore 3/2 or 5/2 i s probably a better choice. Since there was no measurable i n t e r n a l conversion of either gamma-ray, estimates of t h e i r m u l t i p o l a r i t i e s cannot be made. It must be emphasized however that a l l three spin values are possible, and that with-out more information on the missing 0.19 Mev gamma l i t t l e more can be said. 22 BIBLIOGRAPHY 1. M.S. Rose, G.H. Goertzel, B.I. Spinrad, J. Harr and P. Strong. Phys. Rev. 1 6 , 1883, 1949. 2. M. Deutsch, L.G. E l l i o t t and R.D. Evans. R.S.I. 15. 178, 1944. 3 . J".J. Livingood and G.T. Seaborg. Phys. Rev. j>4, 51, 1938. 4. M. Deutsch, A. Roberts and L.G. E l l i o t . Phys. Rev. 61, 389, 1942. 5. M. Deutsch, J\R. Downing, L.G. E l l i o t , J.W. Irvine J r . and A. Roberts. Phys. Rev. 62_, 3 , 1942. 6. " P i l e Produced Isotopes" N.R.C. Atomic Energy Project. 7. E.K. Darby and G. Williams, Private Communication. 8. L.W. Nordheim, Phys. Rev. 2J?, 1894, 1949. 9. M.G. Mayer, Phys. Rev. 78., 16, 1950. 

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