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Nuclear orientation at very low temperatures Malakoff, Walter 1969

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Nuclear Orientation at Very Low  Temperatures  by Walter Malakoff Bachelor of Education (B.Ed) - Secondary, University of B r i t i s h Columbia, 1967  This thesis submitted i n p a r t i a l f u l f i l l m e n t of the requirements for the degree of Master of Science in the Department of Physics  We accept this thesis as conforming to the required standard  The University of B r i t i s h Columbia March,  1969  (i)  In presenting  this thesis in p a r t i a l f u l f i l l m e n t of the  require-  ments for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t freely available for reference  and study.  I further agree that  permission for extensive copying of this thesis for scholarly purposes may  be granted by the Head of my Department or by his  representatives.  It i s understood that copying or publication  of this thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission.  Department of Physics The University of B r i t i s h Columbia Vancouver 8, Canada  (ii)  ABSTRACT One  of the recently developed methods for  studying  nuclei depends on the orientation of the spin axes of the nuclei with respect  to some axis fixed in space.  Since there  is an association between the angular momentum properties  of a  nuclear system and d i r e c t i o n a l effects in the absorption emission of r a d i a t i o n by such a system, this nuclear  or  ordering  v.  is characterized by anisotropic effects in the interaction of the nuclei with r a d i a t i o n , whether p a r t i c l e or electromagnetic. This thesis encompasses the preliminary  work done in  assembling a system consisting of cryogenic equipment and electronics to measure the anisotropy  in radiation emitted from  radioactive n u c l e i oriented i n a ferromagnetic host l a t t i c e (iron) at very low temperatures (ro0.01°K) and changes in anisotropy  to observe the  with changes in temperature.  Chapter 1 contains a condensed account of the information  that can be obtained from oriented nuclei,, the  methods of producing oriented nuclei and the theory for extracting information Chapter 2 describes  from the observed  required  anisotropy.  the low temperature apparatus  includes a description of the low temperature cryostat,  the  Dewar vessels, the specimen assembly, the superconducting solenoid, and the p o l a r i z i n g solenoid.  and-  (iii)  Chapter 3 deals with.thermometry  at low temperatures,  the technique used for cooling a d i a b a t i c a l l y and the preparation of the C o  60  specimen used for thermometry.  Chapter 4 explains the function of each module of electronics used i n the experimental configuration. Nuclear orientation of C o  60  i s covered i n Chapter 5  and includes an analysis and discussion of r e s u l t s . Chapter 6 outlines the improvements to be made i n the design of a new low temperature system and includes a b r i e f summary of the future program of studies i n nuclear orientation at very low temperatures.  (Signature of Examiner)  (Signature of Examiner)  '  (iv) TABLE OF CONTENTS Page .  TITLE  (i)  ABSTRACT TABLE  (ii)  OF CONTENTS  (iv)  LIST OF TABLES  CHAPTER 1  (vi)  LIST OF FIGURES  (vii)  ACKNOWLEDGEMENTS  (viii)  NUCLEAR ORIENTATION AT LOW TEMPERATURES  1  Introduction Production of Oriented Nuclei at Low Temperatures (i) Brute Force Method (ii) Magnetic H.F.S. Polarization ( i i i ) Magnetic H.F.S. Alignment (iv) E l e c t r i c H.F.S. Alignment (V) Orientation i n Ferromagnetics and Antiferromagnetics Angular D i s t r i b u t i o n of y-Radiation from Nuclei Oriented i n Ferromagnets Information Obtained from the Study of Oriented Nuclei  1  12  CHAPTER 2 2.1 2.2 2.3 2.4 2.5  THE The The The The The  15 15 18 19 21 23  CHAPTER 3 3.1 3.2 3.3 3.4  CONTACT COOLING AND THERMOMETRY Introduction The Specimen used for Thermometry Preparation of the Specimen Magnetic Cooling  1.1 1.2  1.3 1.4  LOW TEMPERATURE APPARATUS Low Temperature Cryostat De\^ar Vessels Specimen Assembly Superconducting Solenoid P o l a r i z i n g Solenoid  .  3 3 4 5 5 6 7  25 25 27 30 32  Cv) CHAPTER 4  THE ELECTRONICS USED FOR DETECTING AND MEASURING THE ANTSOTROPY OF "RADIATION The Detectors (i) The Equatorial Detector (ii) The A x i a l Detector The Preamplifiers ' (i) The Fe't-Input Preamplifier (ii) The Transistorized Preamplifier used with the Photomultiplier The Linear Amplifiers Routing and Analysis of Signal Pulses (i) Timing Single Channel Analyzer (ii) The Delay Amplifier ( i i i ) The Baseline Restorer (iv) The Pulse Generator The Experimental Configuration for Co i n Fe Y  4.1 4.2  4.3 4.4  4.5  6 0  35 35 35 35 37 37 37 40 41 41 42 43 43 43  CHAPTER 5 5.1 5.2 5.3 5.4  NUCLEAR ORIENTATION OF C o Introduction The Experimental Procedure Analysis of the Results Discussion  49 49 51 55 57  CHAPTER 6  FUTURE PROGRAM  60  60  6.1  Improvements  6.2  Future Experimental Work  62  REFERENCES USED IN THIS THESIS  65  APPENDIX A l THE CORRECTION FOR THE SOLID ANGLE SUBTENDED BY THE COUNTERS APPENDIX A2 AMENDED INSTRUCTIONS FOR OPERATING VENTRON SUPERCONDUCTING SOLENOID  . 6 0  68 72  (vi)  LIST OF TABLES TABLE 5.1  Data:  24, July, 1968 C o  60  i n Fe  53  (vii) LIST OF FIGURES FIGURE 1.1  Simple Decay Scheme  10  FIGURE 2.1  Low Temperature Apparatus  16  FIGURE 2.2  Low Temperature Cryostat and Specimen Assembly  17  FIGURE 2.3  Current Sweep and Supply for Polarizing  Solenoid  22  FIGURE 2.4  Polarizing  Solenoid  24  FIGURE 3.1  Decay Scheme of C o  28  FIGURE 3.2  Co  FIGURE 3.3  Magnetic Saturation of Fe Sample  31  FIGURE 4.1  Fet Preamplifier for Solid State Detector Preamplifier and Power Supply f o r  33  NaI(Tl)-PM Detector  39  FIGURE 4.2  6 0  60  Thermometer  29  FIGURE 4.3  Experimental  FIGURE 4.4  Nal(T.l) Energy Spectra  FIGURE 4.5  Ge(Li) Energy Spectra  FIGURE 5.1  Anisotropy of C o i n Fe as a Function of Temperature e ( C o ) as a Function of Temperature 24, July, 1968  FIGURE 5.2  Configuration  ' '  44 46 47  6 0  50  60  FIGURE A l . l F i n i t e S o l i d Angle Subtended by Detector at Specimen  56  (viii)  ACKNOWLEDGEMENTS I would l i k e to thank the Physics Department of the University of B r i t i s h Columbia for extending to me the use of the Nuclear Magnetic of  Resonance Laboratory and the f a c i l i t i e s  the nuclear physics (Van De Graaff) group. I am most grateful to Dr. P. W. Martin for his  invaluable supervision, encouragement and f i n a n c i a l assistance (provided i n the form of a Graduate Assistantship). Thanks are also due to Dr. B. G. T u r r e l l for his assistance with the research problems and experimental work. Mr. L. Gorling i s being thanked for assembling and maintaining the low temperature  apparatus, and assisting i n  the experimental work. The Low Temperature Laboratory i s g r a t e f u l l y acknowledged for providing the l i q u i d helium required for the experimental work. I thank my wife for her encouragement.  CHAPTER 1 Nuclear Orientation at Low Temperatures 1.1  Introduction Thermal equilibrium nuclear orientation techniques at  low temperatures u t i l i z e the coupling of nuclear magnetic dipole moments with magnetic f i e l d s or e l e c t r i c a l moments with e l e c t r i c f i e l d gradients.  multipole  Orientation occurs \\fhen  the nuclear system i s coupled i n some way to an axis of quantization  (defined as the z-axis i n the following discussion).  In a f i e l d - f r e e space a system of i d e n t i c a l nuclei each of spin I (total angular momentum of the nucleus) a l l have the same energy since the 2 1 + 1 magnetic substates are degenerate. The application of a magnetic or inhomogeneous e l e c t r i c f i e l d to the magnetic or e l e c t r i c multipole moment of the nuclei causes each substate M to have a d i f f e r e n t energy E(M). population  of each substate i s governed by a Boltzmann  The function  of the form a  M  M  = A exp "  E ( M )  kT  When the temperature of the system i s such that the quantity B, defined by B = [E (M)  - E(M+'l)]/kT,  approaches unity the populations h i b i t s nuclear orientation.  become unequal and the system ex-  A population  to £<p*l i s generally required for observing  difference corresponding anisotropic effects  2  temperatures. The  magnetic f i e l d s or e l e c t r i c f i e l d s can be  e x t e r n a l l y , or may  applied  be obtained for example, by u t i l i z i n g  the  hyperfine magnetic f i e l d in a ferromagnetic host l a t t i c e or an internal For  e l e c t r i c f i e l d gradient within a c r y s t a l l i n e the general case Abragam and  expressed the  interaction  Pryce (1951) have  Hamiltonian as  follows:  / / = y [ g n Hz Sz + _(Hx Sx + Hy B  gj  D[Sz  2  Sy) ] +  - j S(S+1)] + A Sz Iz + B(Sx  + Q[iz  2  . i i(i D] +  solid.  Ix + Sy  Iy)  iM.  -  ->•  where y^ is the  Bohr magneton, H the  magnetic f i e l d , S the e f f e c t i v e i o n i c electrons) and  crystalline are  the  electron spin (spin of  A, B are hyperfine structure  coupling constants (for the magnetic moments and  interaction  D, Q are  the  g i i , gj_  perpendicular to  the  (z-axis) .  Ug term accounts for the  e l e c t r o n i c levels  in the  term represents the crystalline  (h.f.s.)  e l e c t r i c f i e l d s p l i t t i n g parameters and  magnetic f i e l d  the  between nuclear  u n f i l l e d electron s h e l l s ) .  ionic g-factors p a r a l l e l and  The  externally applied  s p l i t t i n g of  external magnetic f i e l d H.  s p l i t t i n g of the  electric field.  h . f . s . s p l i t t i n g resulting  The  The  electronic levels  A and  from the  the  in  B terms represent  interaction  D the the  between the  3  n u c l e a r magnetic moment and the u n f i l l e d e l e c t r o n s h e l l s . nuclear e l e c t r i c  quadrupole s p l i t t i n g  i n the  The  crystalline  electric  f i e l d g r a d i e n t i s g i v e n i n the Q-term.  The H*I term  accounts  f o r the d i r e c t i n t e r a c t i o n between the e x t e r n a l  field  and the n u c l e a r magnetic d i p o l e moment u . There are two types of o r d e r i n g i n an o r i e n t e d If  system.  the m a j o r i t y of n u c l e a r s p i n s p o i n t p a r a l l e l (or a n t i -  parallel)  to the  z-axis,  the e x p e c t a t i o n  value  <  I  > Z  -r 0 and  one has n u c l e a r p o l a r i z a t i o n d e f i n e d by  If,  however,  parallel <I  2  the spins  i n the  are p r e f e r e n t i a l l y p a r a l l e l and a n t i -  z - d i r e c t i o n , the e x p e c t a t i o n  value  of  > r 0 and one has n u c l e a r alignment d e f i n e d by P2  . <y> |  - \ Ki«i) (2I-D  Thus, one can have alignment without p o l a r i z a t i o n . The methods  f o r o b t a i n i n g o r i e n t e d n u c l e i are  discussed  below.  1.2  P r o d u c t i o n of o r i e n t e d n u c l e i at low (i)  temperatures  Brute f o r c e method: A l a r g e magnetic f i e l d , H , a p p l i e d to an ensemble  o f n u c l e i causes adjacent n u c l e a r magnetic by a Zeeman energy d i f f e r e n c e  uH  —.  s t a t e s to  The i n t e r a c t i o n  separate  depends  4  only on the existence of a f i n i t e nuclear magnetic moment v, and the observable orientation on the temperature of the system such that B = E(M) - E(M+.l) v_H ^ kT IkT (1.1).? The low temperatures required are usually obtained by =  1  a  g  m  e  n  t  i  o  n  e  d  a  b  o  v  e  a d i a b a t i c a l l y demagnetizing suitable crystals such as Ce  2  Mg3 ( N 0 ) 3  1 2  '24 H 0. 2  Superconducting solenoids providing  strong, steady, l o c a l i z e d f i e l d s enable this method to be used without the d i f f i c u l t i e s previously encountered.  These  involved having to apply simultaneously a strong f i e l d while having to demagnetize a suitable paramagnetic s a l t to obtain the low temperature.  The brute force method i s useful f o r  studying metals but i s not p r a c t i c a l for insulators due to p r o h i b i t i v e l y long nuclear relaxation times. Since E(M)  E(-M), the direct interaction leads to  nuclear p o l a r i z a t i o n , i . e . non-vanishing terms of P j . (ii)  Magnetic h.f.s. p o l a r i z a t i o n Gorter (1948) and Rose (1949) independently  proposed methods of p o l a r i z i n g nuclei by u t i l i z i n g the hyperfine magnetic f i e l d produced by the u n f i l l e d electron s h e l l in paramagnetic ions. of 10  5  - 10 gauss. 7  This magnetic f i e l d i s of the order At low temperatures the nuclei orient  themselves i n these f i e l d s and when a small external f i e l d i s ~ applied to the electronic magnetic moments the nuclear •orientation of the system becomes ordered.  The A, B and g  terms account for this interaction resulting i n nuclear  5 polarization.  Cooling i s obtained by a d i a b a t i c a l l y de-  magnetizing a separate paramagnetic s a l t or'by using the substance being investigated as i t s own cooling agent. the  In  l a t t e r case the lowest temperature i s determined by the  f i e l d on the system and hence presents a problem since a small external f i e l d must remain applied. netic moment i s 9^ 10 the  3  But since the ionic mag-  times as large as the nuclear moment,  external f i e l d can be applied without appreciable  increase i n T. ( i i i ) Magnetic h.f.s. alignment In some paramagnetic crystals the c r y s t a l l i n e f i e l d interacts with the o r b i t a l angular momenta of the electrons and thus produces one or more preferred axes f o r the  ionic magnetic moments.  The electronic spins are also  s i m i l a r l y affected due to spin orbit coupling.  Bleaney (1951)  suggested that, at s u f f i c i e n t l y low temperatures, nuclear orientation could be observed since the nuclear magnetic moments would interact with the ionic magnetic moments and a l i g n themselves i n these f i e l d s .  The Hamiltonian f o r an ion  in such a system would include the D term, with z being the d i r e c t i o n of the c r y s t a l axes of symmetry. (iv)  E l e c t r i c h.f.s. alignment Pound (1949) proposed the orientation of nuclei  u t i l i z i n g the l o c a l e l e c t r i c f i e l d s , the directions of which are  fixed with respect to the crystallographic axes.  This  6  method r e l i e s on the interaction of the nuclear e l e c t r i c quadrupole moment and the gradient of the e l e c t r i c (/vlO  1  5  esu/cm  3  field  required to give an energy separation of the  required magnitude).  This e l e c t r i c f i e l d can be produced by  an asymmetrical d i s t r i b u t i o n of the electron cloud immediately around the nucleus (strongly deformed nuclei are required) such as i n the rare-earth and actinide elements. (v)  Orientation i n ferromagnetics and anti-ferromagnetics: At  temperatures below the Curie Point (500 - 700°C  for most iron alloys) a small external magnetic f i e l d  (^1000  gauss) may polarize the magnetic domains of ferromagnetic materials and produce u n i d i r e c t i o n a l magnetization at saturation.  In some rare earth metals, the c r y s t a l l i n e  anisotropy or the anisotropic exchange  interaction causes a  preferred d i r e c t i o n of domain magnetization. In both of these cases the electronic spins align themselves along or perpend i c u l a r to the domain magnetization and the nuclear magnetic moments then experience magnetic f i e l d s which r e s u l t i n the alignment of the n u c l e i .  This h.f.s. interaction and  s u f f i c i e n t l y low temperatures of the order of 0.01°K - 0.1°K could be used to obtain nuclear orientation. Nuclear orientation i n antiferromagnetic crystals depends on the alignment of the electronic magnetic moments along a preferred axis i n the crystals (at temperatures below the  Neel temperature) , and that the nucleus of the magnetic ion  7  .experiences a r e l a t i v e l y large magnetic f i e l d which does not average to zero because of the rapid f l i p p i n g of the spins from one d i r e c t i o n to the a n t i p a r a l l e l d i r e c t i o n , a phenomenon which results from a degeneracy of the antiferromagnetic The  state.  orientation mechanism in ferromagnets and a n t i -  ferromagnets i s complicated by ferromagnetic and a n t i f e r r o magnetic exchange, and by c r y s t a l l i n e anisotropics since i t is possible to have more than one axis of alignment.  Investigations  by Hanna et al (1960) also showed that the magnetic f i e l d at the s i t e of the nucleus i s a n t i p a r a l l e l to the bulk magnetization of the ionic moments and supported the theory that s-electrons  entered into the h.f.s. i n t e r a c t i o n .  Samoilov (1960), Kogan (1961) and Stone and T u r r e l l (1962) investigated the f i e l d s induced in diamagnetic atoms dissolved into ferromagnets and found magnetic f i e l d s up to a few m i l l i o n gauss.  The use of ferromagnetic hosts  seems to be a viable method for orienting nuclei and seems only to be r e s t r i c t e d by whether or not the p a r t i c u l a r element to be investigated can be dissolved in i r o n , or  any  other suitable ferromagnet such as cobalt or n i c k e l .  1.3  Angular d i s t r i b u t i o n of y-radiation from nuclei oriented i n ferromagliets This section provides a b r i e f summary of the theory  of the angular d i s t r i b u t i o n of y-radiation emitted by  8  oriented nuclei in ferromagnets. are  More comprehensive  discussions  found i n the a r t i c l e s by Ambler (1960), -Roberts and  Dabbs (1961) , Huiskamp and Tolhoek (1961), Blin-Stoyle, Halban and Grace (1961), Blin-Stoyle and Grace (1957), Steenland and Tolhoek (1957) , de Groot (1952) , Morita (1961) , and Ambler (1951 and 1963) . As mentioned  in section (1.2), in ferromagnets the  electron spins are polarized by strong exchange interactions. The Hamiltonian of Abragam and Pryce (1951) for the ferromagnetic case takes the form:  1  Vn  1  = -y rA S + I  where g  n  .1  gn y n  5  i s the nuclear g-factor, u  i s the nuclear magneton  and a l l other terms are as defined i n section (1.1). the  H e  f f is  hyperfine f i e l d at the nucleus and consists of the  external f i e l d and the f i e l d due to the polarized electrons, where i n general:  -M_  »  H  ^  ^n n y  .When the ferromagnet i s saturated, H ^^ g  at every nucleus i s  along the same axis since a l l the magnetic domains are p a r a l l e l . At  low temperatures the Boltzmann d i s t r i b u t i o n (see section  1.1)  9  among the magnetic s u b s t a t e s p r o v i d e s bulk p o l a r i z a t i o n o f the nuclei. The oriented  angular d i s t r i b u t i o n o f y - r a d i a t i o n  nuclei  a system o f  i s g i v e n by:  =  W(6)  from  B  U  F  P  (cos  8)  V  even where 9 i s the angle to the z - a x i s . parent n u c l e i Satchler  The o r i e n t a t i o n  o f the  i s g i v e n by the B^ f a c t o r s , d e f i n e d by Gray and  (1955) as f u n c t i o n s o f the Boltzmann d i s t r i b u t i o n  (see s e c t i o n orientation  1.1). The U produced  v  are parameters d e s c r i b i n g  the d i s -  by the B- and y- t r a n s i t i o n s p r e c e d i n g the  y decay whose a n i s o t r o p y i s being measured.  They are f u n c t i o n s  of the angular momenta, L , c a r r i e d o f f i n the t r a n s i t i o n (see 0  figure  1.1). The F  y  parameters d e s c r i b e the observed y-  t r a n s i t i o n and are f u n c t i o n s of the angular momenta o f the transitions  (as L i i n f i g u r e  1.1). These l a t t e r parameters  have been t a b u l a t e d by Ferentz and Rozenzweig related  (1953) , and the  Racah c o e f f i c i e n t s , by Simon e t a l (1954).  are the Legendre polynomials  The P^  o f order v . The maximum value o f  v i s g i v e n by: v < s m a l l e s t o f 2 I , 2 I i , 2L max 0  1  where I Q i s the t o t a l angular momentum of the parent n u c l e i , 1^ the t o t a l angular momentum of the nucleus preceding the observed  t r a n s i t i o n , and L the angular momentum c a r r i e d away by  the unobserved t r a n s i t i o n . to even v means p h y s i c a l l y  The r e s t r i c t i o n o f the polynomial that  the i n t e n s i t y of the  10  I.  (3  fe  L  I:  FIGURE SIMPLE DECAY S C H E M E  11 r a d i a t i o n from the nucleus pointing i n one d i r e c t i o n i s exactly the same when the nucleus points i n the opposite d i r e c t i o n , i . e . only the "alignment" terms contribute. In the p a r t i c u l a r case where a mixed 8 - t r a n s i t i o n occurs, u" replaces 1  v  ,  v  =  i n the above equation and i s given  I I j ) * * (Io I I j ' )  U (Io  2  v  g  1 + A  V  where j . and j ' P  P  p  2  are the angular momenta of the leptons and  A i s the r a t i o of the amplitudes of the i '  decay to the j  decay.  0  p  P  S i m i l a r l y , i f the observed y - t r a n s i t i o n consist of  magnetic dipole (M ) and e l e c t r i c quadrupole  (E ) transitions  x  2  the F c o e f f i c i e n t becomes: v F ( I ^ z LL) + 2 a F CI 1 1 2 F * '= • • 1 a v  L L +  l ) t a F ( I I L + l L + l) 2  v  1  2  2  +  where a i s the admixture r a t i o of the transitions (see Biedenharn and Rose (1953)). One also observes y-rays which are preceded by more than one observed t r a n s i t i o n (as i n C o ) . 6 0  U  V  = U ^ V  •U ^ V  The U  v  are then written:  ... U ^ , where n i s the number of V  t r a n s i t i o n s preceding the observed t r a n s i t i o n . The W(9)  equation i s v a l i d only when the population of  the intermediate nuclear state i s not affected by interaction with the l a t t i c e v i b r a t i o n s , r e c o i l of the nucleus from i t s o r i g i n a l l a t t i c e p o s i t i o n , or changes i n magnetic or e l e c t r i c  12 f i e l d gradients following a 3-decay, but depends only on the i n i t i a l equilibrium d i s t r i b u t i o n and the character of the preceding emission process  i n the cascade.  In practice this  means that the time between successive emissions must be short ( i . e . ^ l O "  9  sec).  The anisotropy of the angular d i s t r i b u t i o n of yradiation i s given by:  E =  ^^^(1/2)^^°^  w  n  e  r  e  W(TT/2) are the observed y-ray i n t e n s i t i e s normalized  anc  *  to the  intensity of a "warm" or unpolarized system as observed p a r a l l e l and perpendicular to the z-axis.  1.4  Information  obtained from the study of oriented nuclei  Depending on the nucleus being oriented one can obtain information either about the orientation mechanism or the nuclear process  involved.  Atomic information may be  obtained i f the nuclear part i s known, especially the nuclear spins involved and the nature of the 6- and y- transitions which occur.  On the other hand nuclear data such as spins,  p a r i t i e s , and magnetic moments can be obtained i f the hyperfine f i e l d i s known. If one measures the angular d i s t r i b u t i o n of the yradiation (see Tolhoek and Cox (1952)) one can obtain the multipole character of the t r a n s i t i o n since the W ( 9 ) previously defined (section 1.3) contain the  terms (with  v = even, since only the alignment terms contribute) which  13  depend on the m u l t i p o l a r i t y of the t r a n s i t i o n .  A considerable  orientation i s required to obtain the orientation parameters having higher values of v. Measurement of the linear p o l a r i z a t i o n (see Bishop et a l (1952)) would permit one to distinguish between the e l e c t r i c or magnetic character of the radiation since the r a t i o of the intensity of radiation perpendicular and p a r a l l e l to the z-axis is d i f f e r e n t f o r e l e c t r i c and magnetic t r a n s i t i o n s , being larger for e l e c t r i c than magnetic. Measurement of c i r c u l a r p o l a r i z a t i o n would give the orientation parameters with odd v (see Wheatley et a l (1955)) . Therefore i f the orientation parameters are obtained from an experiment they can be used to: (i)  obtain knowledge about the mechanism of  orientation i f this i s not already known; (ii)  obtain the temperature from the parameters i f the  hyperfine i n t e r a c t i o n i s known, so that the angular d i s t r i bution of the y-radiation can be used as a thermometer; ( i i i ) obtain the magnetic moment of the parent nucleus i f the mechanism of orientation as well as the nuclear process is known.  One can determine, from the measured values of B , H  f the value df 8 and from 8 , the value of u , i f —e f— IkT ;  v  i s known.  The sign of the magnetic moment could be determined by measuring the c i r c u l a r p o l a r i z a t i o n of the radiation.  Con-  versely, i f ii i s known, H ~ can be determined d i r e c t l y from 6; f  14 (iv)  obtain the values of nuclear spins and p a r i t i e s  from the multipole order and the e l e c t r i c or magnetic character of  the y - t r a n s i t i o n s .  I f the i n i t i a l and the f i n a l angular  momenta i n a Y - t r a n s i t i o n are known one can determine the i n i t i a l t o t a l angular momentum preceding the 3-transition since the temperature  dependence of the angular d i s t r i b u t i o n  also depends on the i n i t i a l total" angular momentum (see Cox and Tolhoek (v)  (1953)), and obtain information about the preceding B-  t r a n s i t i o n i f the spins and p a r i t i e s involved i n the Y t r a n s i t i o n are known. (De Groot and Tolhoek  -  Referring to the theory of B-decay (1950)) i f one knows the ratio of the  nuclear matrix elements involved, one can determine the r e l a t i v e magnitude of the Gamow-Teller and Fermi terms i n the Hamiltonian f o r B - i n t e r a c t i o n .  Conversely, i f the  r e l a t i v e magnitude of Gamow-Teller and Fermi terms was known, one could calculate the r a t i o of the nuclear matrix In  elements.  addition, Y-radiation from oriented nuclei has an  i n t r i n s i c value since sources with oriented nuclei might be used as sources of l i n e a r l y or c i r c u l a r l y polarized YV r a d i a t i o n , which could conceivably be applied in other experiments  (Cox and Tolhoek  (1953)).  CHAPTER 2 The Low Temperature  Apparatus  The apparatus used i n the experiment described i n this thesis was designed by Dr. B. G. T u r r e l l and was a modified version of the one described by T u r r e l l (1963). shows the schematic arrangement Dewar vessel 2.1  Figure 2.1  of the system including the  assembly.  The low temperature cryostat A diagram of the low temperature cryostat, with the  specimen assembly, i s shown i n Figure 2.2, which i s less than one-half  actual s i z e .  The cryostat assembly was suspended  from the Dewar vessel top cap (see Figure 2.1) by the stainless s t e e l pumping tubes.  A brass can soldered to the top cap of  the cryostat assembly formed the outer jacket which could be evacuated to thermally  i s o l a t e the inner jacket.  The volume  between the two caps immediately below the top cap of the outer jacket and suspended from i t constituted the 1°K cryostat.  This space could be evacuated and l i q u i d Helium  (4.2°K)  could be admitted from the inner Dewar vessel v i a the needle valve c o n t r o l l e d by a long rod extending above the Dewar vessel top cap. A brass can was soldered to the 1°K cryostat to form ' the inner jacket which contained was used to f a c i l i t a t e mounting apart.  the specimen.  Wood's metal  the apparatus and taking i t  The whole assembly was suspended by the stainless steel  pumping tubes and contained  i n a glass Dewar vessel (the  16 E£3  CfevV4# V^Stru T O ?  cflP  P U M P I H G LIMES  INNER DEWAR VESSEL O U T S R D E W A R V=SS£t-  NEEDLE  VALVE  INfLET  I'K C R Y O S T A T M A ^ G A N G U S ArvifMONtUfvl S U L P H A T E PILL ( D . I ' K )  P E R S I S T E N T  C U R R E N T  S W I T C H  CHROME POTASSIUM MUM PlLC ( 0 . 0 ) 2 S U P H K C o ; IDUCTIM&  SOLENOID  JACKET COPPER HsiAT LIK'K \HV1ER  QUTCR  C©°-Fe  JACKET  SAMPLE  LOW T E M P E R A T U R E A P P A R A T U S *****  17 PUtvlPlNG  LINE5 . ^STPI\^\.ESS S"\'££"L)  COPPER RADlATlO^ BAFFLES Rf\D\^T\Ohi T R A P  M E E D L E VALVE  U  OUTER  -  JACKET  MAMGANOUS SOLPvAATt  -PILL  SOLE"K]G\D -  iii' I  CHRoMtS POTASS\OM A L U M F I L L \AJrrn COPPER WeAtUKlK  to i  -  TOF^OL  Fp£iV\etf_  iMMai- J A C K E T I *  LOW TEMPERATURE CRYOSTAT AND SPECIMEN ASSEMBLY  FIGURE-  2.2  18 l i q u i d helium Dewar vessel) which, i n turn, was suspended i n another glass Dewar vessel (the l i q u i d nitrogen Dewar v e s s e l ) . Figure 2.1 indicates the arrangement and i d e n t i f i c a t i o n of these parts.  A l l spaces of the inner and outer jacket were  connected to a glass vacuum system allowing each to be independently  evacuated or f i l l e d with helium exchange gas.  The system was precooled with l i q u i d nitrogen i n the outer glass Dewar vessel using approximately 1 centimeter of dry nitrogen exchange gas i n the interspace of the inner glass Dewar vessel.  After the system had precooled, the interspace  of this Dewar vessel was pumped out and l i q u i d helium was transferred into the inner vessel to surround and cover the outer jacket about 2 inches above the i n l e t to the needle valve. When the cryostat had cooled to l i q u i d helium temperature (4.2°K) with helium exchange gas i n a l l spaces, the cryostat was pumped and l i q u i d helium admitted to i t through the,inlet into the pumping tube controlled by the needle valve.  The  outer jacket was then evacuated, and the helium i n the cryostat pumped to 1°K.  2.2  The Dewar vessels Both of the glass Dewar vessels were made i n the  Physics Department by Mr. J . Lees, glassblower. vessel was about 13 centimeters centimeters  The inner  i n diameter (O.D.) and 92  i n length and had a glass valve which f a c i l i t a t e d  f i l l i n g the interspace with dry nitrogen exchange gas for the  19  precooliiig process, and evacuating that space before a l i q u i d helium transfer.  Since helium gas was capable of moving  through the walls of the Dewar vessel, the valve also provided a means f o r p e r i o d i c a l l y flushing the interspace. vessel was suspended  This Dewar  on adjustable brass rods from the Dewar top  cap and i s o l a t e d from the atmosphere with a rubber washer between the upper l i p of the vessel and the top cap. The outer Dewar vessel was 17.5 centimeters i n diameter (O.D.) and 84 centimeters i n length, and had a sealed-off evacuated interspace.  It was also suspended  from adjustable  brass rods fastened to the top cap assembly. Copper r a d i a t i o n baffles were soldered to the stainless s t e e l pumping tubes between the outer jacket and the Dewar top cap to reduce the l i q u i d helium b o i l - o f f rate i n the inner glass Dewar. The whole apparatus, i . e . the Dewar vessels and the cryostat, the glass vacuum system, and the various other attachments, was fixed on an aluminum frame suspended on rubber shock-absorbers s i t t i n g on the concrete floor of the laboratory.  This isolated the apparatus from the vibrations  of the building which would have caused appreciable heat leaks into the specimen  2.3  The specimen  assembly when at very low temperatures.  assembly  The specimen assembly  (see Figure 2.2) i n the Co -Fe 60  20 experiment employed a cooling p i l l of chrome potassium alum contained i n a tufnol cylinder 10 centimeters long and 2 centimeters i n diameter (I.D.)'  Contact with the specimen  plates was made v i a a copper heat link which consisted of approximately 5000 strands of enamel-coated  copper wire  (number 38 A.W.G. (B§S)) and 25 centimeters in length. The sample end of the heat link was cleaned of enamel with Stip-X and the wires were soft-soldered into a 3-faced form about 1.5 centimeters long.  Each face was about 1 centimeter square.  The specimen plate was soft-soldered onto one of these faces and oriented to face the germanium detector (see Chapter 4). The upper end of the heat link extended the f u l l length of the t u f n o l former and was impregnated with chrome potassium alum "jam". The chrome potassium alum "jam" \\'as made by grinding 60 grams of the s a l t to a very fine powder, and then adding to  i t a 50/50 mixture of glycerol and saturated chrome  potassium alum solution. temperatures. to  This cools to a glass at low  It was hoped that specimen temperatures down  .012°K (the Neel point of the chrome potassium alum) could  •be obtained by demagnetizing this p i l l . The chrome potassium alum p i l l was connected by a l a t t i c e of s i x 1 millimeter stainless steel tubes to the manganous ammonium sulphate p i l l which was cooled to about 0.1°K  i n the residual f i e l d of the superconducting solenoid.  21 2.4  The superconducting  solenoid  The superconducting s o l e n o i d used was a custom made device manufactured by Ventron  designed  f o r our a p p a r a t u s .  (Magnion I n c . ,  B u r l i n g t o n , Mass. 01803).  It  consisted  solenoid  of n i o b i u m - t i t a n i u m a l l o y  diameter  2.03  a t h i n brass  inches, former.  amperes.  ampere at  It  was  of a model CF 40-200-45039 inches  l o n g , of  inner  inches,  The a c t i v e winding \vas 7.0  wound on  inches  long and  of 48.6K gauss at a c u r r e n t o f  The magnetic c o n v e r s i o n r a t i o was 794 gauss per  51 amperes.  s i s t e d of a p e r s i s t e n t solenoid  8.5  cooling  144 Middlesex T u r n p i k e ,  and outer diameter 3.815  produced a nominal maximum f i e l d 61.2  f o r magnetic  The remainder of the c o i l winding conc u r r e n t switch which enabled  to be operated i n the p e r s i s t e n t  power r e q u i r e d ) .  mode (no  the external  The s o l e n o i d was powered and c o n t r o l l e d by a  V e n t r o n CF 100 Power Supply designed  specifically  for  the  CF 40-200-45039 s o l e n o i d and was capable o f producing 100 amperes w i t h v i r t u a l l y no r i p p l e .  The supply i n c l u d e d a  s e l e c t a b l e - s w e e p c u r r e n t c o n t r o l and l i m i t e r , two 250 ampere heater switch,  current supplies  f o r the p e r s i s t e n t  and an automatic p r o t e c t i v e  consisting  of a quench-sensing  d e c o u p l i n g mechanism.  Modified  power supply and s o l e n o i d  device  current  f o r the  solenoid,  c i r c u i t and power supply instructions  for operating  the  are found i n Appendix A 2 .  The r e s i d u a l f i e l d remaining a f t e r was  milli-  swept out manually u s i n g the r e v e r s i b l e  a demagnetization current control  • > ©  S-S2.V.D.C.  ZZJl  POS 2N371S.  CURRENT CONiTROL  0-5 AMP |/7\ 6 IOK;  SLACK" CURRENT  7  MT X VAC *  -'A/5  o  c—  3  H  £  ~  LIMIT • . 1 - 5 AMP  { V oor}  &3V.A.C. 20,00Oj*f 15V  FIGURE 2.3  C U R R E N T S W E E P A N D S U P P L Y F O R - P O L A R I Z I N G "SOLENOID  NJ NJ  23 of F i g u r e 2.3 and a 12-volt automobile b a t t e r y which a l s o doubled  2.5  i n f u n c t i o n to power the p o l a r i z i n g s o l e n o i d .  The p o l a r i z i n g s o l e n o i d The  polarizing  niobium-zirconium  s o l e n o i d was a superconducting  a l l o y wire  (25% zirconium)  having  device of nylon  i n s u l a t i o n wound i n a h e l i x around i t . I t was wound on a brass former, as shown i n F i g u r e 2.4, with 900 turns on s e c t i o n 1, 1800 turns  on s e c t i o n 2, and 1440 turns on s e c t i o n 3.  tabs were spot-welded to the superconducting l a p o f about 1 i n c h t o ensure good c o n t a c t h e a t i n g o f the s u p e r c o n d u c t o r ) .  Platinum  wire with an over(to avoid Joule  Number 20 (A.W.G.) copper wire  leads were s o f t - s o l d e r e d to t h e p l a t i n u m  t a b s , along which  power was s u p p l i e d t o the s o l e n o i d u s i n g the c u r r e n t c o n t r o l of F i g u r e 2.3 and a 12-volt heavy duty automobile b a t t e r y . was estimated  It  (see Figure 3.3) that about '3 amperes of c u r r e n t  were r e q u i r e d i n t h i s s o l e n o i d t o m a g n e t i c a l l y s a t u r a t e the i o n sample.  A c u r r e n t o f 4 amperes was s u p p l i e d t o ensure that  the sample was  saturated.  24  CHAPTER 3 Contact Cooling and Thermometry 3.1  Introduction Non-paramagnetic  substances cannot be self-cooled by  adiabatic demagnetization, hence a suitable paramagnetic s a l t must be employed  as a cooling agent.  Generally, direct  contact between the specimen and the cooling agent i s not practicable (see Chapter 1 section 1 . 2 ( i i ) ) , and as a result they are usually coupled by a copper heat link (see Figure 2.2). Since eddy-current heating should be reduced to a minimum, this l i n k i s usually i n the form of a number of copper s t r i p s or  i n the form of a large number of thin insulated copper wires.  According to Mendoza (1948) , f o r a copper heat link i n a pressed p i l l  of powdered s a l t , the heat flow i s given by: Q = 100 A  ( T j - T ) ergs/sec. 3  3  2  where A i s the area of contact, and Tj and T of  the s a l t and copper respectively.  advantageous  2  (3.1)  the temperatures  I t i s obviously  to have an area of contact as large as possible.  A large cooling p i l l would have the advantage of providing a large heat sink. Chrome potassium alum i s a very useful cooling agent as i t can be demagnetized down to i t s Neel point (0.012°K) and has "a large s p e c i f i c heat between 0.1 and 0.01°K.  26 Some d i f f i c u l t y i s  encountered i n measuring  temperatures below 0 . 0 5 ° K  in a contact-cooled  the magnetic s u s c e p t i b i l i t y determine i t s  S i n c e the heat  if  there  is  a heat  leak i n a s p e c i f i c  difference  to  instance  leak to the  can  specimen.  experiment i s not known,  of zero heat  i n t i m a t e c o n t a c t would e x i s t could e x i s t  a large tempera-  i n temperature cannot be r e a d i l y  temperature approach that of the  50%  so that  between the specimen and c o o l i n g s a l t  especially  Only i n the  Although  temperature, h e a t - t r a n s f e r becomes i n c r e a s i n g l y  ture difference  this  system.  of the alum s i n k c o u l d be used  more d i f f i c u l t at low temperatures,  exist,  specimen  estimated.  leak would the  specimen  alum ( 0 . 0 1 2 ° K ) .  No such  i n p r a c t i c e , and e r r o r s up to  i n the temperature d i f f e r e n c e  between the  s u s c e p t i b i l i t y measurement and the specimen temperature.  It  would be d e s i r a b l e to have a thermometer which measured the specimen temperature d i r e c t l y . In n u c l e a r o r i e n t a t i o n experiments where the y - r a y a n i s o t r o p y i s measured, f o r a g i v e n decay, (see  Chapter 1 s e c t i o n  t r o p y depends therefore, increases.  1.3)  on 6 =  . . t_r  — , so that IkT  Thus not only i s  as a c c u r a t e l y as p o s s i b l e , attained,  and the v a r i a t i o n i n the  on the v a r i a t i o n i n B ^ .  depends  are  the more accurate i s  the  aniso-  The a n i s o t r o p y , it  i t necessary  but a l s o ,  constant  increases to measure  the lower the  result.  as 6 temperature  temperature  27 The specimen used i n our experiment was a plate of C o  6 0  iron a l l o y soft-soldered to the end of the copper heat link as described i n Chapter 2.  3.2  The specimen used for thermometry The above considerations suggest the advantages i n using  a radioactive isotope with a known decay scheme as a thermometer.  This was  i n fact the reason C o  60  was suggested by  Grace et a l (1955) and proposed and used for measurements in iron by Stone.and T u r r e l l (1962) .  If the isotope were incorp-  orated i n the specimen and the interaction of the nucleus i n this environment well-understood, i . e . y ^eff known, a measurement of the y-anisotropy would determine i t s temperature, and hence, the  temperature of the specimen being studied. Co  (1)  6 0  was selected for the following reasons:  The decay scheme i s \\rell established, and i s shown i n  Figure 3.1 obtained from Landolt-Boinstein (1961).  The nuclear  magnetic moment has been measured spectroscopically and i s 3.754 n.m. (2)  (Lindgren (1962)).  The hyperfine f i e l d on the nuclei of cobalt atoms i n  an iron l a t t i c e has been extensively studied, and a value of H £ £ = 2.88 x 10 e  1% to 171. (3)  5  gauss i s v a l i d for cobalt concentrations o£ '  (Matthias et a l (1966)).  The y-radiation anisotropy observed i s large, so that  high accuracy can be attained i n the temperature measurement.  28  .+  CO  6 0  (53Y) ..5I3MEV)  (3 (WSIvtEV)  tf(l.!73MEV)  •tf 0 , 3 3 2 M E V )  0  i  Nl  6 0  DECAY S C H E M E O E C O  ( S T A B L E )  29  30 (4)  The isotope has a long h a l f - l i f e  (5.3 years).  Using the tables of Simon et a l and Ferentz and Rosenzweig, f o r the C o  60  decay, we obtain U  U^ F i , = -0.243 f o r both y-decays. for 1 = 5  2  F  2  = -0.421 and  The values of B  2  and Bi,  were obtained from a tabulation against 6 =  by Blin-Stoyle and Grace.  H  eff IkT  The axial and equatorial i n t e n s i t i e s  are given by: W(o)  = 1 + U F B 2  2  2  W(TT/2) = 1 - 1 / 2  + U Fi B 4  U F B 2  2  t  2  u  + 3/8 U I ^ B I ,  These functions are tabulated against 1/T i n Figure 3.2 and provide the temperature scale.  3.3  Preparation of the specimen To determine the strength of the magnetic f i e l d required  to saturate the 0.2 millimeter x 1 centimeter x 1 centimeter iron sample with the p o l a r i z i n g solenoid, two c o i l s , each of 50 turns and 1 centimeter  i n diameter and 2 centimeters  long,  were connected i n opposition and i n series with the sensitive range of a Scalamp galvanometer (W. G. Pye § Co. Ltd., Cambridge, England) and a suitable damping resistance.  The  c o i l s were then placed i n a high power solenoid and the sample plate was moved from one c o i l to the other. ment, pick-up  With this  arrange-  i n the c o i l s was avoided, and the signal obtained  was proportional only to the magnetic s u s c e p t i b i l i t y of the sample.  The specimen was found to saturate in f i e l d s of about  31  o  V  -0-  ^ -z. Q *  fu LU _j LL LU  -  M  ~  or 3 LU LU  2  <*> 0  o  %  rf  L.GORLWG  0.-2 0,4 O.67 .0.9  l.'o  ,1.2.  1-5  UQ  MAGNETIC F I E L D (KGAUS.S)  FIG.3.3  M A G N E T I C S A T U R A T I O N OF Fe S A M P L E  32 1.6K  gauss  (see Figure 3.3).  This saturation test was  performed  at room temperature, but no s i g n i f i c a n t change i s expected at low temperatures. The sample was cleaned and a C o C l 6 0  2  solution, with  v i r t u a l l y no c a r r i e r cobalt ( C o ) , was placed on the surface 5 9  of the plate so that about 2uC of C o  60  were present.  Since  iron i s more e l e c t r o p o s i t i v e than cobalt, iron atoms go into solution replacing the cobalt of the metallic chloride, leaving the cobalt to spontaneously plate onto the surface of the iron plate.  The solution on the plate was then evaporated with a  heat lamp, and the plate furnaced at 950°C in a stream of dry hydrogen f o r 24 hours.  Under these conditions, the C o  diffused well into the sample.  This was  atoms  checked by etching the  specimen plate a f t e r furnacing and measuring measurable  60  the a c t i v i t y .  decrease i n a c t i v i t y was observed.  The specimen  No was  then soft-soldered to the copper heat l i n k .  3.4  Magnetic  cooling  The Ventron superconducting solenoid described in Chapter 2 was used for magnetic cooling.  It was  r i g i d l y mounted  from the top cap of the apparatus. After the specimen assembly had been cooled to 1°K, the f i e l d was  sloi^ly applied with a few microns of helium exchange  gas in the inner jacket.  This was supplemented  by gas pre-  viously absorbed on the lower s a l t p i l l container, and evolved  33 when i t was heated during magnetization. The temperature of the  specimen assembly was deduced from the pressure i n the  inner jacket, and the cryostat temperature judged from the head in an o i l manometer connected to i t .  After the s a l t  pill  cooled to i t s o r i g i n a l temperature the remaining exchange gas was pumped away and the f i e l d then reduced slowly.  Any gas  present before the demagnetization would, at 0.1°K be adsorbed on the s a l t p i l l , for at this temperature the vapour pressure of helium i s very small.  It was advantageous  to adsorb any gas  present at a r e l a t i v e l y high temperature where the increase i n entropy for a given amount of heating i s small. r e a d i l y seen by considering the second law of &Q = TdS).  (This can be  thermodynamics,  After equilibrium was reached around 0.1°K the f i e l d  was  reduced to a minimum.  was  connected and the residual f i e l d i n the magnet was reduced  to a value (^10  The si^eep power supply of Figure 2.3  gauss) which we anticipated would enable the  s a l t p i l l to approach 0.012°K.  It was essential that the sweep  rate be slow otherwise eddy-current heating of the sample occurred. After the demagnetization, the p o l a r i z i n g f i e l d applied slowly to saturate the specimen plate.  The  was  y-ray  i n t e n s i t i e s along and perpendicular to the axis of the magnetic field  (a-axis) were then observed, and counts N ( 0 )  c o  ^ and  N(*/2) cold obtained (see Figure 4.3 for the position of counters r e l a t i v e to the specimen). Normalization counts w  the  e  N(0) warm and  r  e  Nv( T T / 2 )  warm  were obtained at the end of a "run"  34  when the specimen had warmed above 0.1°K.  A convenient  measure of the y-ray anisotropy i s given by: £  . WQr/2) - WCo)  f  w  h  e  r  e  W(ir/2)  W = ^cold N  .  The W(o)  and W(TT/2) are given t h e o r e t i c a l l y by  'Varmequation (1.9). NOTE:  The temperature of the cooling p i l l of chrome potassium  alum could have been derived from i t s magnetic  susceptibility  determined with a mutual inductance c o i l located over the outer jacket and positioned at the centre of the chrome potassium alum p i l l  container.  However, a short c i r c u i t developed some-  where between the primary and secondary windings•and any attempt to  use t h i s technique was temporarily abandoned.  CHAPTER 4 The Electronics Used for Detecting and Measuring the Anisotropy of y-radiation 4.1  The  detectors  A strong magnetic f i e l d from the solenoid used for adiabatic demagnetization, as well as from the p o l a r i z i n g solenoid, discouraged the use of a sodium  iodide-photo-  m u l t i p l i e r detector close to the sample.. A f i e l d of the order of 45K  gauss defocussed the tube even i f shielded with  mu-metal and soft i r o n . detector was  Since a l i t h i u m - d r i f t e d germanium  conveniently  magnetic f i e l d s i t was  available and was  not affected by  used in the equatorial plane.  A  suitably shielded sodium iodide-photomultiplier detector used i n the a x i a l d i r e c t i o n (a-axis) since i t was  not  was  located  as close to the magnetic f i e l d as the equatorial detector. (i)  The  equatorial  detector  The  equatorial detector was  a trapezoidal lithium-  d r i f t e d germanium device manufactured for Dr. G. G r i f f i t h s by Nuclear Diodes (P.O.  Box  135,  Prarie View, I l l i n o i s  The model number of the detector was  60069).  LGC-4.1S and the model  number of the complementary cryostat (Union Carbide) was CFR-10-3P.  This detector had an active area of 11.2  square  centimeters with a d r i f t e d depth of 11 millimeters and 28.5  millimeters in length.  The  was  i n t r i n s i c volume faced a  thin aluminum window located 12 millimeters from the face. The window was  0.5  millimeters in thickness.  The  detector  36 had a r e l a t i v e peak e f f i c i e n c y at 1.332  Mev  of 3.9%  (compared  to 2"X 2" Nal) and, with the above window, a resolution of 5.0 kev (FWHM) at 1,332 9:1.  It was  Mev.  The peak to Compton r a t i o  was  reverse biased at 1250 volts using a model 210  Ortec Detector Control Unit and a 250 v o l t battery in series with this control u n i t . was  The leakage current of the detector  c e r t i f i e d to be 0.4 nano-amperes and i t s capacitance to  be 20 picofarads at 1250 v o l t s .  The detector face was located  11 centimeters from the sample. (ii)  The a x i a l detector The axial detector consisted of a Thalium  activated sodium iodide-photomultiplier detector assembled by Harshaw Chemical Company (1945 East 97th Street, Cleveland, Ohio 44106) . A sealed sodium iodide c r y s t a l of diameter 5.6 centimeters and length 7.0 centimeters was  optically  coupled with s i l i c o n e grease to an RCA model 6342A photom u l t i p l i e r tube. number Co383) .  The Harshaw model number was A resolution of 5.61  858  at 1 .332 Mev  measured with the tube operating at a positive 1105  (serial was volts.  The voltage between the focus grid and signal ground (or cathode) was ground was  112 volts and between the f i r s t dynode and signal  237 v o l t s .  a model 412B  Fluke High Voltage D.C.  detector window was the sample.  The photomultiplier was operated with Power Supply.  The  located 26 centimeters from the center of  37 4.2  The p r e a m p l i f i e r s (i)  The FET input p r e a m p l i f i e r : . The p r e a m p l i f i e r (see  lithium-drifted  circuit  Middletown, Conn. 06457).  i n c o r p o r a t e d a modified p o l e / z e r o  than 0.1% f o r outputs  had an e x t e r n a l l y - s e l e c t a b l e  gain  The  cancellation  capable of r a p i d recovery from o v e r l o a d s .  l i n e a r to b e t t e r  bias  the  p r e a m p l i f i e r manufactured by Canberra  (50 S i l v e r S t r e e t ,  preamplifier  used with  germanium d e t e c t o r was s i m i l a r to the model  1408A c h a r g e - s e n s i t i v e Industries  F i g u r e 4.1.)  I t i^as  from 0 to ±5 v o l t s a n d .  (X2 or X10).  The d e t e c t o r  i s o l a t i o n was 2000 VDC and the output impedance was  51 ohms.  A cable  14 f e e t  i n l e n g t h and compatible with  model 1410 Canberra I n d u s t r i e s  L i n e a r A m p l i f i e r p r o v i d e d the  r e q u i r e d ±24VDC and ±12VDC power. the n o i s e c o n t r i b u t i o n was 2.0 60 nanoseconds (ii)  (using  With the  time of  constant).  The t r a n s i s t o r i z e d p r e a m p l i f i e r used with the p h o t o m u l t i p l i e r ^ contains  9.5 v o l t power s u p p l y .  unity voltage  g a i n but s u f f i c i e n t  53 ohm c o a x i a l cable panel.  externally-mounted venient  detector  Kev (FWHM) w i t h a r i s e  the  c i r c u i t diagram f o r  p r e a m p l i f i e r used with the a x i a l d e t e c t o r  electronics  20 p f  a 2 microsecond RC shaping  F i g u r e 4.2  associated  the  14 f e e l  as  the  This preamplifier provided power g a i n to d r i v e a  i n l e n g t h t e r m i n a t i n g on the  The output impedance was 47 ohms. two-position  An  (DPDT) switch p r o v i d e d con-  output s i g n a l p o l a r i t y s e l e c t i o n .  w i t h compactness  as w e l l  the  as an important f a c t o r  The u n i t was  and t h i s  built  preamplifier  • F E T P R E A M P L I F I E R FOR' S O L D - S T A T E  DETECTORS  -2*V  ->,T0  SECOND PRE  A M ?  TEST  I N P U T (C  SECOND  >To  IN3I4 m  PRE  IN750  IN400Z H7VAC4  ~1  L/VV\~:J  <  -N  FIGURE  4.2 •  A A A r  7VAG  1  1K400Z  200^?  95V  25V  PREAMPLIFIER A N D P O W E R SUPPLY FOR NAICTL)-PM  DETECTORS  AMP  •4.0 was mounted d i r e c t l y on the p h o t o m u l t i p l i e r with a dual male BNC c o a x i a l connector  4.3  (UG-491A/U).  The l i n e a r a m p l i f i e r s The model 1410 L i n e a r A m p l i f i e r s manufactured by  Canberra I n d u s t r i e s  (see  section  4.2  above) were used with  the d e t e c t o r systems d e s c r i b e d above.  These a m p l i f i e r s allow  a v a r i e t y of shaping modes f o r high r e s o l u t i o n n u c l e a r spectroscopy  and c o u n t i n g .  The 1410 c o u l d be  conveniently  switched on the f r o n t panel i n t o s i n g l e  or double d i f f e r e n t i -  a t i n g delay l i n e , or RC-shaping modes.  In the RC pulse  shaping mode, RC time constants 0.1  to 7 m i c r o s e c o n d s .  seconds delay l i n e s  were switch s e l e c t a b l e  In the delay l i n e mode, 1.2  (one prompt and one delayed)  simultaneously a v a i l a b l e . single  micro-  were s t a n d a r d .  Two u n i p o l a r (one prompt and one delayed) bipolar  from  and two  outputs were  This allowed the use of  the  d i f f e r e n t i a t e d s i g n a l f o r energy r e s o l u t i o n while  double d i f f e r e n t i a t e d s i g n a l could be used f o r zero timing.purposes.  The delayed s i g n a l s  m u l t i c h a n n e l pulse h e i g h t a n a l y s i s were completed  (see  section  crossover  were then a v a i l a b l e f o r  after  4.5 b e l o w ) .  the  logic  decisions  An i n t e r n a l switch  c o u l d remove the i n t e g r a t i o n from the b i p o l a r outputs order to o b t a i n the sharpest  the  in  zero c r o s s i n g timing while  r e t a i n i n g i n t e g r a t i o n on the u n i p o l a r s i g n a l s  f o r optimum  41 energy analysis with the kicksorter.  The r i s e time of each  unit was less than 90 nanoseconds (with the integration switch OFF and in the double delay l i n e mode). was  c e r t i f i e d to be less than 1 nanosecond  Crossover walk  over a dynamic  range of'0.5 to 8 volts (with amplifier i n the double delay l i n e mode and the integration switch at any position from 0.1 to 0.7 microseconds).  The noise output was c e r t i f i e d to be  less than 8 microvolts (rms) measured with single RC shaping time of 1 microsecond at f u l l gain. The integral nonlinearity i s less than 0.11 from 0.3 v o l t s to 10 volts output.  The amplifier gains were found to  be very stable with time and were specified to have gain s t a b i l i t y of better than 0.005%/°C from 20°C to 50°C. The linear amplifier used with the l i t h i u m - d r i f t e d germanium detector employed an RC time constant of 2 microseconds.  The linear amplifier used with the sodium  iodide-  photomultiplier assembly was operated i n the double delay l i n e mode (DDL).  4.4  Routing and analysis of signal pulses (i)  Timing single channel analyzers: Canberra Industries model 1435 Timing SCA's were  used as single channel pulse height (energy) analyzers. The Timing SCA's combined, i n one module, the two functions of single channel pulse height analysis and  42 pulse crossover or leading edge (timing) discrimination. In the Window analyzer mode, the pulse height analyzer portion of the module generated  a logic pulse whenever a unipolar or  bipolar input pulse f e l l within the energy range defined by the Baseline and Window Width controls.  In the Discriminator  analyzer mode, only the Baseline r e s t r i c t i o n applied.  When-  ever the energy r e s t r i c t i o n s established by the controls were met, the timing portion of the model 1435 generated  a timing  signal when a b i p o l a r input signal crossed the zero voltage baseline, or at the leading edge of a unipolar input s i g n a l . Leading edge timing could also be used on bipolar input signals by selecting the Unipolar operating mode. Two prompt, simultaneous  routing pulses, one negative  and one p o s i t i v e , were available as outputs. output pulse for use with time-to-pulse-height  A fast negative converters was  also available and could be delayed from 0-1000 nsec.  The  negative routing pulse was directed to either the Canberra Industries models 1474 or 1473 scaler or to the Ortec model 430 scaler to be counted.  The positive pulse was used, i n con-  junction with the Nuclear Data Model 101 k i c k s o r t e r , primarily for setting up and adjusting the windows of the timing single channel analyzers. (ii)  The delay amplifier: A model 427 Ortec Delay Amplifier was used to  compensate f o r the time difference between the signal pulse  43 and the coincidence logic pulse a r r i v i n g at the k i c k s o r t e r . The delay a m p l i f i e r was  only e s s e n t i a l for setting up and  adjusting,the windows of the timing single channel analyzers. ( i i i ) The baseline restorer: A model 438 Ortec Baseline Restorer was  employed  as an i n v e r t e r to provide a signal compatible with the kicksorter input  requirements.  (iv)  The pulse generators: Datapulse model 101 generators were triggered  by the timing single channel analyzers and provided a signal of p o s i t i v e p o l a r i t y and amplitude  required by the coincidence  c i r c u i t of the k i c k s o r t e r . Fine delay times were obtained simultaneously with the above operation using the Delay mode of the generators.  •4.5  The experimental  configuration for C o  The experimental  60  in Fe  configuration used for measuring  anisotropic e f f e c t s i n the y-radiation from C o  60  in an iron  (Fe) host l a t t i c e i s found in the block diagram of Figure The y - r a d i a t i o n was  4.3.  detected in the a x i a l d i r e c t i o n  by the sodium iodide s c i n t i l l a t i o n detector described in section 4.1  above.  A t y p i c a l " s i n g l e s " spectrum was Figure 4.4(a). connected  obtained as in  The trigger input of the pulse generator  to the p o s i t i v e output of the timing single  was  channel  F I G U R E 4.3  SCALER  SCA  ?NYT  v TO DATAPULSE  TO DELAY 'AMPLIFIER  D£V>'AR  ' A I  . ASSEMBLY; LINEAR AMPLIFIER SAMPLE  SCALER  SCA  I  EQUATOR! At  H  A TO DATAPULSE-  i DETECTOR  1  .173 M E V  .a  DELAY AMPLIFIER.  AXIAL  1.173+1.332 M E V  SCALER  SCA  BASELINE RESTORER  TO KICKSORTER ANALYSE  NAl(TL) DETECTOR AND PHOTOM'ULTIPLIER  ! -! DATAPULSE  A A  PREAMP  A A Ll NEAR AMPLIFIER  SCA  J £ L  - v T O KICKSORTER  r TO SCOPE TRIGGER  r  SCALER  EXPERIMENTAL. CONFIGURATION  1.173 M E V  COINCIDENCE  45'  analyzer which was  adjusted to put out a l o g i c pulse only i f  the input signal ranged from 1 Mev 1.173  Mev photopeak.  to the upper l i m i t of the  The pulse generator output was  adjusted  to be greater than 3 v o l t s , as required by the coincidence c i r c u i t af the k i c k s o r t e r , and s l i g h t l y wider than the pulse presented  to the kicksorter analyzer section.  The k i c k s o r t e r ,  in the coincidence mode, remained inhibited to pulse height analysis u n t i l a coincident pulse "opened" the analyzer to the pulse presented at the input (corresponding to the energy l i m i t s determined by the single channel analyzer). single channel analyzer was  The other timing  set to energies ranging from 1 Mev  to the upper l i m i t of the 1.332  Mev photopeak of C o  60  t y p i c a l gated spectrum appeared as in Figure 4.4(b). of the above cases the negative outputs  and a In both  from the single  channel  analyzers went to scalers which recorded a l l the "singles" accepted by the present range of the single channel analyzers. The timing single channel analyzers e f f e c t i v e l y integrated an energy spectrum between preset upper and lower l i m i t s and the scalers counted and displayed the t o t a l number of events  in those l i m i t s , in the f i r s t case the photopeak of  the 1.173  Mev  events  y-radiation and in the second case, the number of  from 1 Mev  to the upper limit, of the 1.332  The equatorial y-radiation was d r i f t e d germanium detector. was  Mev  photopeak.  detected by the lithium-  The remainder of the c i r c u i t r y  e s s e n t i a l l y i d e n t i c a l to the arrangement used with the  46  o  .10  20  O  SO  40  50  (.cf) SlMGLES  FIGURE  NAI E N E R G Y S P E C T R A  60  47  jo  )S  N->  >  ^2  -2  .2  o  30  2 0  40  60  so  (a) SINGLES  X  ^) 2  Ul > Iii IL O  (Y HI CD Z.  0  ,  30  10  CHANNEL  Lo  40,^  -J  NUMBER  (b) GATED FIGURE  4.5  GEOLOENERGY' SPECTRA  \ , - * - , ,t  I  »  ,t  fl f  i _ *  48 sodium iodide detector except that the l i n e a r amplifier used an RC time constant of 2 microseconds instead of the double delay line mode.  A t y p i c a l "singles" energy spectrum and a  gated spectrum from 1 Mev to the upper l i m i t of the 1.332  Mev  photopeak are found i n Figures 4.5(a) and 4.5(b), respectively.  CHAPTER 5 Nuclear O r i e n t a t i o n 5.1  of C o  6 0  Introduction V e r y low temperatures of the o r d e r of 10~ °K are 2  necessary to orient n u c l e i . by  Such temperatures are o b t a i n e d  a d i a b a t i c a l l y d e m a g n e t i z i n g a s u i t a b l e paramagnetic  I t i s very d i f f i c u l t ,  salt.  however, t o measure the v e r y low  t e m p e r a t u r e s a t w h i c h n u c l e i are o r i e n t e d w i t h p r e c i s i o n  since  t h e , m a g n e t i c s u s c e p t i b i l i t y o f the c o o l i n g s a l t does not necessarily  i n d i c a t e the temperature of the specimen  (see  C h a p t e r 3) s i n c e l a r g e t h e r m a l b a r r i e r s between the s a l t t h e heat l i n k e x i s t at v e r y low temperatures and a l s o a c t i v e h e a t i n g can cause t h e r m a l g r a d i e n t s l i n k and the specimen.  and  radio-  between the heat  A c o n v e n i e n t means by which  low  t e m p e r a t u r e s can be measured i s t o use o r i e n t e d n u c l e i whose mechanism of o r i e n t a t i o n i s w e l l u n d e r s t o o d and whose n u c l e i are  known t o e x h i b i t a l a r g e temperature dependence.  incorporated consistent  Co  6 0  i n Fe i s one such thermometer h a v i n g a l a r g e i n y - r a d i a t i o n at v e r y low t e m p e r a t u r e s .  anisotropy  The h y p e r f i n e  field  (  H e  f f ) of C o  6 0  dissolved  i n Fe, i t s  n u c l e a r magnetic moment ( p ) , and i t s decay scheme are w e l l known.  Co  of  or 49% a t a temperature of 0.012°K (see F i g u r e 5.1).  0.49  Thus C o  6 0  6 0  also e x h i b i t s a large anisotropy  o f the o r d e r  appears t o be a good thermometer and t o t h i s  end  has been used s u c c e s s f u l l y by G o r t e r e t a l (1953) Bleaney e t a l (1954) and Poppema e t a l (1955).  50  51  Co  60  was  employed in our apparatus to test how  the system functioned.  well  The measured anisotropy should  indi-  cate the lowest temperatures attainable by our system and i t might also reveal problems that were not anticipated i n the preliminary design of the low temperature  5.2  The experimental  apparatus.  procedure  The preparation of the C o described in Chapter 3.  60  - Fe specimen has been  Precooling, cooling to 1°K, and  the  procedure for adiabatic cooling has been described in d e t a i l in Chapters 2 and 3, respectively. The electronics used to detect the y-radiation has been described in Chapter 4. Considerable d i f f i c u l t y had been encountered in obtaining a suitable demagnetizing f i e l d inefficient  ('^45K gauss) due to the  operation of the persistent current switch in the  superconducting  solenoid.  current switch was  operated  Whenever the heater of the persistent for more than 20 minutes an amount  of heat (from the switch) s u f f i c i e n t to cause the superconductor to go normal at one of i t s input terminals, caused the quenchsensing c i r c u i t of the CFC-100 Power Supply to deactivate the current supply.  The e f f e c t of removing the current source  was.  similar to charging the solenoid too r a p i d l y , i . e . an L ^-~ <  voltage appeared at the solenoid terminals, a quench \\ras i n d i cated, and l i q u i d helium was boiled off due to Joule heating and eddy current heating.  Hence i t was  e s s e n t i a l that the  52 superconductor minutes.  The  be charged  to a d e s i r e d l e v e l i n l e s s than  c u r r e n t , however, c o u l d not be  r a p i d l y than suggested by the manufacturers  i n c r e a s e d more  i n the o p e r a t i n g i n s t r u c t i o n s p u b l i s h e d  of the s o l e n o i d (Ventron I n c . ) .  a c c e p t a b l e upper l i m i t  of 36 amperes was  then  conductor, when the upper l i m i t was  An  estimated  a c c o r d i n g t o the recommended c h a r g i n g r a t e , and  the  a c h i e v e d , was  super-  rapidly  i n t o the p e r s i s t e n t mode ( i . e . heater c u r r e n t reduced and then the power supply c u r r e n t reduced a quench. J u l y 24,  No quenching 1968  f i e l d was  was  at 0 = 0° and  ( a x i s of the p o l a r i z i n g The J u l y 24,  I t was  Groups 1, 2, and s o l e n o i d was  field  a temperature  (•WK)  and was  groups 1, 2 and  on  obtained.  A field  started with  the q u a n t i z a t i o n a x i s  field). obtained during the "run" of  taken d i r e c t l y from the log book.  3 were taken immediately  turned ON,  the p o l a r i z i n g to  90° from  data i n Table 5.1 was  1968.  to zero  a p p l i e d a f t e r the r e s i d u a l  had been swept out and the counting procedure the counters  put  t o zero) to avoid  occurred with t h i s method and  a s u c c e s s f u l demagnetization  4-ampere p o l a r i z i n g  20  i n that o r d e r .  a f t e r the p o l a r i z i n g  Group 4 was  taken with  ON but a f t e r the specimen had warmed up  where no a n i s o t r o p i c e f f e c t s were observable used 3.  to normalize  the counts  obtained i n  TABLE 5.1 24, JULY, 1968; POLARIZING FIELD CURRENT: TRIAL  COMMENTS ( A l l Counts Per 2 Min.)  4 AMPERES; DEMAGNETIZING CURRENT: .36 AMPERES  EQUATORIAL COUNTER: Ge(Li) 1.175 PLUS 1.332 Mev 1.173 Mev  AXIAL•COUNTER : Nal(Tl) 1.173 Mev 1.173 PLUS 1.33 2 Mev  Counts Before Mag.  28196 ± 167.7  7356 ± 86  8754 + 93.6 .18158 ± 134.8  (1)  Average of 6 TwoMinute Runs  28597 ±  28.2  7530 ± 14 .4 8280 ± 15.2  17343  +  22.2  (2)  6 Minute Run  2 8698 ±  56.5  7523'+ 28 .9 7993 ± 29.8  17124  +  43.7  (3)  6 Minute Run  28582 ±  56.4  7439 ± 28 .8 8278 ± 30.3  17376  +  43.9  Warm Count (avg. of One 12-Min. and Three 6-Min. Counts  27619 ±  33.3  7228 ± 17 .1 8873 ± 18 .9 18580  +  27.2  CD  W  N(Cold) N(Warm).  1 .0354 ±  0.0001  1.0417  0.9331  0.9334  +  0.0002  (2)  w  N(Cold) N(Warm)  1.0390 -± 0.0002  1.0408  0.9008  0.9216  +  0.0003  C3)  w  N(Cold) N(Warm)  1 .0348 ±  1.0291  0.9329  0.9351  +  0.0002  ANISOTROPY  0.0001  1.332 PLUS 1.173 Mev  1. 173 Mev  CD  £  0.098 ± 0.001  0 .104  APPROX. TIME AFTER DEMAGNETIZATION 14 MIN  (2)  e.  0.113 ± 0.002  0 .135  22 :MIN  (3)  z'  0.096 ± 0.001  0 .093  28 ]MIN  54  The  following  corrections  i n the raw data were  cons i d e r e d : (i)  There was an i s o t r o p i c background  surroundings i n the observed i n t e n s i t i e s time i n t e r v a l s used, t h i s (ii) ground the  of a n i s o t r o p i c  For the  included  1.332 Mev y - r a d i a t i o n .  a back-  However  since  o b t a i n e d i n the 1.173 Mev p l u s  c o r r e c t i o n was not e s s e n t i a l .  1.332 Mev counts were used to determine  s i n c e they p r o v i d e d l a r g e r s t a t i s t i c s same a n i s o t r o p y .  i n t h i s experiment plus  .  1.173 Mev y - r a d i a t i o n was used only to check the con-  Mev window, t h i s  the  6 0  c o r r e c t i o n was o f the order of 1.2%.  The 1.173 Mev y-ray i n t e n s i t y  s i s t e n c y o f the r e s u l t s  plus  of C o  from the  1.332  The 1.173 Mev  the temperature  and both y's e x h i b i t e d  There were no higher-energy t r a n s i t i o n s  to contribute s i g n i f i c a n t l y  to the 1.173 Mev  1.332 Mev counts. (iii)  A c o r r e c t i o n due to the f i n i t e  tended at the specimen  solid  angle sub-  by the y-ray counters (see Appendix A l )  was r e q u i r e d but c o u l d not be made s i n c e the exact shape o f the  l i t h i u m - d r i f t e d germanium d e t e c t o r was unknown.  improvements w i l l be made that w i l l correction  Subsequent  enable us to make t h i s  (see Chapter.6).  (iv)  Since e was being measured no c o r r e c t i o n  from the decay o f the C o  6 0  arising .  source was required' as. W(o) and  W(TT/2) were a t t e n u a t e d i n the same p r o p o r t i o n .  55 The  errors quoted i n the values of the anisotropy of  the 1.173 Mev plus 1.332 Mev y-radiation the s t a t i s t i c a l uncertainties  were obtained from  i n the t o t a l counts contributing  to the experimental points.  5.3  Analysis The  of the results  decay scheme of C o  60  was shown i n Figure  nuclear magnetic moment and the spin of the C o  60  3.1. The  ground state  have been measured as 3.754n.m. and 1 = 5 , respectively, as indicated i n Chapter 3.  The p a r i t i e s of the various  levels i n  the decay are a l l positive so that the i n i t i a l decay i s an allowed electron-capture The  transition.  normalized a x i a l and equatorial  i n t e n s i t i e s are  given by: W(o)  = 1 + U F B 2  2  + Ui^B,,  2  • W(ir/2) = 1 - i U F B 2  2  2  + | U^B,,  the anisotropy being defined by = W(TT/2) - W(o)  E  WO/2)  For both of the y-transitions of C o  60  and the preceding  unobserved 3 - t r a n s i t i o n , as indicated i n Chapter 3, U F 2  \) V h  h  2  = -0.421 = -0.243  56  57  The C o 11H 3 =  orientation parameters B  60  eff .  The variations of B  2  and B  2  u  are functions of  and B^ are given by B l i n -  IkT Stoyle and Grace (1959).  Thus we are able to calculate the  v a r i a t i o n of the y-anisotropy, e, with 1/T since y, H and K are known quantities.  Figure 5.1 summarizes, graphically,  the values of e plotted as a function of  1/T.  The three experimental points obtained were plotted on Figure 5.2.  It i s apparent that a low temperature  of the  order of 0.037°K was obtained in this preliminary experiment. The temperature and  indicated by the order of points (1)  (2) suggest that the specimen had been warmed on the  a p p l i c a t i o n of the p o l a r i z i n g f i e l d or that thermal equilibrium has not been achieved bet\\reen the cooling s a l t and the specimen, or both, before the counting had commenced. T h e o r e t i c a l l y , Figure 5.1 suggests that at a specimen temperature  of the Neel temperature  (0.012°K) of the cooling  s a l t one should expect an anisotropy of the order of about 0.49  5.4  or 4-9% .  Discussion The results obtained suggest that we can achieve  temperatures  approaching  those required for orienting nuclei  (^0.01°K), but some modifications in the low apparatus  temperature  are essential before any o r i g i n a l experimental  58  work can be attempted.  The following l i s t of improvements are  essential: (i)  To reduce the l i q u i d helium b o i l - o f f rate due  to heat leaks along the cryostat and magnet supports, along the  e l e c t r i c a l wiring, and i n p a r t i c u l a r , along the large  copper leads to the superconducting solenoid. (ii)  To reduce the vibrations being transmitted to  the  specimen during an experiment.  the  pumps and other associated equipment mechanically coupled  to  (These originated from  the frame or d i r e c t l y to the low temperature apparatus). ( i i i ) To redesign the magnetic s u s c e p t i b i l i t y system  so that r e l i a b l e estimates of the temperature of the cooling p i l l can be made. (iv)  To reduce the effects on the temperature of the  cooling p i l l caused by the residual f i e l d remaining i n the Ventron superconducting solenoid (even after sweeping i t to some minimum level) , and (v) to  To have the persistent current switch modified  enable us to achieve higher demagnetizing f i e l d s , and  hence, lower temperatures. The f i r s t four problems are discussed i n Chapter 6 where solutions are also suggested. Ventron Inc. has v o l u n t a r i l y agreed to i n s t a l l a recently-developed persistent current switch which i s supposed  59 to operate more e f f i c i e n t l y is  than the one c u r r e n t l y i n use.  a n t i c i p a t e d that t h i s w i l l  alleviate  the problems  ered i n attempting to achieve a high demagnetizing  It  encount-  field  ( ^45K gauss) . It  is  a n t i c i p a t e d , with the above improvements,  that  i t w i l l not be d i f f i c u l t to achieve a temperature approaching 0.012°K, a f t e r which some o r i g i n a l experimental s t u d i e s be attempted.  will  Chapter 6 i n d i c a t e d the general d i r e c t i o n of  the s t u d i e s i n n u c l e a r o r i e n t a t i o n at very low temperatures.  CHAPTER 6 Future Program 6.1  Improvements P r i o r t o any new experimental work s e v e r a l  improvements  i n the apparatus w i l l be e s s e n t i a l : (i)  A l a r g e heat leak e x i s t s i n the p r e s e n t  system  along the c r y o s t a t s u p p o r t s , along the e l e c t r i c a l w i r i n g , and p r i m a r i l y a l o n g the large copper cap o f the Dewar assembly  leads extending from the top  to the t e r m i n a l board that was  u s u a l l y immersed i n l i q u i d helium during the course o f an experiment. stainless  The support system being designed f o r a new a l l -  s t e e l Dewar assembly  w i l l incorporate a l i q u i d  n i t r o g e n " p o t " between the top cap and the t e r m i n a l board or gas from a 20°K H conserve l i q u i d (ii)  2  gas c i r c u l a t o r to shunt the heat leaks and  helium.  C o n s i d e r a b l e h e a t i n g o f the sample  from v i b r a t i o n s  t r a n s m i t t e d to the specimen  the course o f an experiment.  assembly  during  The h e a t i n g e f f e c t s were not  t o l e r a b l e s i n c e very low temperatures  could not be a c h i e v e d ,  and i f a c h i e v e d , could not be maintained. originated  resulted  The v i b r a t i o n s  from the r o t a r y backing pump, from the helium  pumping l i n e which was connected to other systems as w e l l as to p i s t o n - t y p e pump, and from the manual o p e r a t i o n s performed d u r i n g an experiment the pumping l i n e s  on the v a r i o u s v a l v e s and stop-cocks on  and on the p r e s s u r e - m o n i t o r i n g c o n t r o l s .  61 These e f f e c t s w i l l be minimized by mounting the low temperature system on a r i g i d chassis (which i t s e l f w i l l be bolted to a concrete f l o o r ) and by separating and i s o l a t i n g from the system the various controls and pumps.  The pumps w i l l be isolated by  using more f l e x i b l e connectors than previously employed and f i x i n g these r i g i d l y to the f l o o r , a concrete wall or a heavy p i l l a r between the pumps and the apparatus. ( i i i ) The s u s c e p t i b i l i t y c o i l - mutual inductance system w i l l be redesigned to function r e l i a b l y at very low temperatures. In p a r t i c u l a r , care w i l l be taken to avoid ground loops and pickup from associated e l e c t r i c a l and electronic equipment i n the v i c i n i t y of the c o i l , the leads, and the mutual inductances used i n the bridge network. (iv)  The effects of the residual f i e l d remaining i n  the superconductor after a demagnetization w i l l be reduced by wrapping Netic mu-metal (Magnetic Shields D i v i s i o n , Perfection Mica Co., 1322 North Elston Avenue, Chicago, I l l i n o i s , 60622) around the outer jacket of the cryostat just inside the bore of the superconductor and extending in either d i r e c t i o n over the paramagnetic p i l l to shield i t completely'.  The mu-metal  would r e a d i l y saturate in high magnetic f i e l d s but would shield out low f i e l d s of several hundred gauss.  Increased  cooling e f f i c i e n c y would result but the presence of the mu-metal would necessarily require that the s u s c e p t i b i l i t y c o i l be dispensed with.  This would not pose a problem since  the y-radiation anisotropy of C o  60  (or some other suitable  62 radioactive isotope such as Mn-54) i n Fe would provide a d i r e c t and r e l i a b l e measure of temperature. (v)  In the future lithium-drifted germanium  detectors w i l l be incorporated into the low temperature apparatus rather than used externally.  This would reduce  the attenuation of the y-radiation, improve the s o l i d angle of the detector (assuming that the detectors are large enough and more closely located to the sample than previously) , and also to improve the resolution of the y-transitions being measured since the e f f e c t on the l i n e width due to scattering would be l e s s .  This system of detectors would" be p a r t i c u l a r l y  useful i n instances where the y - t r a n s i t i o n from the thermometer closely correspond, i n energy, to the y - t r a n s i t i o n s being observed, or i n the case where two or more d i f f e r e n t y-transitions are close together i n energy.  6.2  Future experimental work Other thermometers  that show large anisotropies at  low temperatures w i l l be calibrated to make the system versatile.  It would, f o r example, be d i f f i c u l t to measure  the anisotropic effects i n F e  5 9  the 1.10 Mev y - t r a n s i t i o n of F e  i n Fe using C o 5 9  60  i n Fe since  and the 1.173 Mev of C o  60  * '  f a l l outside the resolution c a p a b i l i t i e s of the sodium iodide detector.  This problem might not be as d i f f i c u l t i f lithium-  d r i f t e d germanium detectors were incorporated into the low temperature system.  One could also use a suitable computer  63 program to separate or " s t r i p " the F e composite spectrum of F e  5 9  5 9  and Co -.  spectrum from the  This involves a  60  computation which uses the r a t i o ( s ) of contributions from d i f f e r e n t t r a n s i t i o n s and gives the number of events corresponding to each of these transitions i n the t o t a l number of events observed at a given energy. are  Both of the above methods  r e l a t i v e l y complex for long-lived radioactive isotopes,  however they become very complex for short-lived radioactive isotopes since time then complicates the computations.  A  simplier method would be to incorporate a Mn * thermometer, 51  as the 0.835 Mev y t r a n s i t i o n of Mn  i s well-separated i n  54  energy from the 1.10 Mev y - t r a n s i t i o n of F e  5 9  and only  corrections f o r the contributions of higher energy transitions need to be made.  Several other radioactive isotopes, like  Mn , which have r e l a t i v e l y long l i f e t i m e s , have well-under5Jt  stood t r a n s i t i o n s and show appreciable  temperature-dependent  anisotropic e f f e c t s at low temperatures, w i l l be calibrated and used f o r thermometry,  the selection of any p a r t i c u l a r one  depending on the sample nuclei being studied. The study of oriented nuclei at low temperatures w i l l be extended to other radioactive isotopies that can be alloyed with iron,  Eventually nickel and cobalt alloys w i l l be  investigated.  Nuclear magnetic resonance experiments w i l l be  incorporated into the system to measure some of the "unknowns", in p a r t i c u l a r , the e f f e c t i v e f i e l d , H  f f  .  Hopefully, a  64  s i g n i f i c a n t contribution w i l l be made to the understanding of orientation mechanisms and nuclear processes involved in $- and Y"- t r a n s i t i o n s .  65  R E F E R E N C E S USED  A b r a g a m , A . , a n d P r y c e , M. H.  205 (1951) 135.  IN THIS  THESIS  L., P r o c . Roy. Soc. S e r . A  Ambler, E., P r o g r e s s i n C r o g e n i c s H e y w o o d L o n d o n ! 2_ (1960) 23~3~;  ( e d . b y K.  Mendelssohn;  Ambler, E., N u c l e a r O r i e n t a t i o n , - U n i v e r s i t y T e c h . R e p . (1961) 248.  of Maryland,  Ambler, E., Methods i n E x p e r i m e n t a l P h y s i c s A c a d e m i c P r e s s , N.Y.) (1963) 162.  ( e d . b y L.  Bleaney,  B., P r o c . P h y s .  S o c , A  Marton;  6_4_ (1951) 315.  B i s h o p , G. R. , D a n i e l s , J . M., G o l d s c h m i d t , G., H a l b a n , I I . , K u r t i , N. , a n d R o b i n s o n , F. N . H., P h y s . R e v . 88^ (1952) 1432 . Biedenharn,  L. C ,  Blin-Stoyle, N u c l . Phys.  3 (1953) 63.  (1953) 729.  and Rosej  M. E . , R e v . mod. P h y s . 25_  R. J . , G r a c e , M. A . , a n d H a l b a n , H., P r o g r .  B l e a n e y , B . , D a n i e l s , J . M., G r a c e , M. A . , H a l b a n , K u r t i , N . , R o b i n s o n , F. N. H., a n d S i m o n , F. E . , P r o c . R o y . S o c . A 221_ (1954) 170. Blin-Stoyle,  (1957) 555.  Cox,  R. J . , a n d G r a c e , M. A . , H o r d b u c h D e r P h y s i k 42_  J . A. M. , a n d T o l h o e k , H. A . , P h y s i c a  19_ (1953) 673.  De G r o o t , S. R. , a n d T o l h o e k , H. A . , P h y s i c s De G r o o t , S. R., P h y s i c s F e r e n t z , M.,  H.,  1J3 (1952) 1201.  and Rozenzweig,  ANL (1953) 5324.  N., T a b l e  of F  Coefficients,  1_4 (1948) 504.  Gorter,  C. J . , P h y s i c s  Gorter, Physics  C. J . , P o p p e m a , 0.  1_7 (1953) 1030.  1_6 (1950) 456.  J . , S t e e n l a n d , M. J . , a n d B e u n , J . A . ,  G r a c e , M. A . , J o h n s o n , C. E . , K u r t i , N . , S c u r l o c k , R. G., a n d T a y l o r , R. T., C o n f e r e n c e d e P h y s i q u e d e s b a s s e s t e m p e r a t u r e s , P a r i s , 2-8 S e p t . 1955 ( P a p e r n o s . 150, 159).  66  Gray, T. P., and Satchler, G. R. , Proc. Phys. S o c , A 68 (1955) 349. Hanna, S. S., Heberle, J . , Perlow, G. J . , Preston, R. and Vincent, D. H., Phys. Rev. Letters 4 (I960) 513.  S.,  Huiskamp, W. J . , and Tolhoek, H. A., Progress i n Low Temperature Physics 3 (ed. C. J . Gorter; North-Holland Publishing Co., Amsterdam, 1961) 333. Kogan, V., Kulkov, V. D., N i k i t i n , L. P., Reinov, N. M., Sokolov, I. A., and Stelmah, M. F., Soviet Physics JETP 12_ (1961) 34; 13 (1961) 78. Landolt-Bornstein, Springer-Verlag (1961) 2-124. Lindgren, in Perturbed Angular Correlations (edited by Karlsson, E., Matthias, E., and Siegbahn, K . ; NorthHolland Publishing Co., Amsterdam, 1964) 385. Mendoza, E., Ceremonies Langevin-Perrin College de France, 5 (1948) . Morita, M. , Lecturers Thor. Phys. Colorado 4_ (1961) 358. Matthias, E., and H o l l i d a y , R. J . , Phys. Rev. Letters 1_7 (1966) 897. Pound, R. V., Phys. Rev.,  76^ (1949)  1410.  Poppema, 0. J . , Steenland, M. J . , Beun, J . A., and Gorter, C Physics 21 (1955) 233. Rose, M. E. , Phys. Rev. 75_ (1949) 213. Roberts, L. D., (1961) 175.  and Dabbs, J . W. T., Ann. Rev. Nucl. Sc. 11  Simon, A., Van Der S l u i s , J . H., and Biedenharn, L. C , Numerical Tables of Racak C o e f f i c i e n t s , ORNL (1954) 1679. Steenland, M. J . , and Tolhoek, H. A., Progress in Low Temperature Physics 2 (ed. by C. J . Gorter; North-Holland Publishing Co., Amsterdam, 1955) 292. Somoilov, B. N., S k l y a r e v s k i i , V. V., and Stepanov, E. P., Soviet Physics JETP 11_ (1960) 261; 9 (1959) 44BL, 972 , 1383 Stone, N. J . , and T u r r e l l , B. G., Physics Letters 1_ (1962) 3  67  Tolhoek, H. A.,, and Cox, J.. A. M. , P h y s i c s 18 (1952) 357 . T u r r e l l , B. G., Ph.D. T h e s i s , Oxford  (1963).  Wheat l e y , J . C , Huiskamp, W. J . , Diddens, A. N., S t e e n l a n d , M. J . , and Tolhoek, H. A., Physics 21 (1955) 841 ..  68 APPENDIX Al  The Correction for the Solid Angle Subtended by the Counters' The angular d i s t r i b u t i o n of the y-ray i n t e n s i t i e s from a system of oriented nuclei i s given by: W(e)  where A  2  = U F B 2  2  = 1 +, A P 2  2  (6) + A,,Pi, (e)  (Al.l)  , • Aj, = Ui^F^B^ , and P(e) are the Legendre  2  polynomials. Let us consider the case of the " a x i a l " intensity counted by a c r y s t a l subtending angle W Q at the specimen as i n Figure A l . l . The Legendre polynomials are given by: P  2  = |(3 c o s  6 - 1)  2  Pn = -(35 cos 8  4  6 - 30 cos  (A1.2(a)) 2  6 + 3)  (A1.2(b))  and the s o l i d angle i s given in terms of the half-angle 6 by w = 2TT(1 - cos 6)  from which  du = 2u s i n 0 d 6 The count registered i n the i n t e r v a l e to 6 + de i s given by equation ( A l . l ) , and the a x i a l registered by the counter, which subtends  a half-angle a, i s  ,69  SPECIMEN  DETECTOR  FINITE SOLD ANGLE SUBTENDED BY DETECTOR AT SPECIMEN RGURE M-I  70 W(o) ' =  / ° [1 .+. A P ( o ) • + A^P^Ce)]. do) o 2  2  •to  0  . / o  /  (A1.3(a))  d  M  •  [1 + A P ( e ) + A ^ P ^ C e ) ] s i n e de . a / s i n e de o 2  0  2  (A1.3(b))  :  I + ~  j  Li  (3 c o s e - l ) 2  "  s i n e de ' '  1  0  / o  —  +  s i n e de  / (35 cos^e - 30 cos e + 3) s i n e de o / s i n 9 de (A1.3(c)) o 2  I  8  —  -  from where W(o)' = 1 + —cos a(l+cos a ) A + icos a(l+cos a ) ( 7 c o s a - 3 ) A 2 8 2  2  = 1 + g A 2  2  +  gl+  A  where 2  = — cos a (1 + cos a) , and 2  g  4  = — cos a (1 + cos a) ( 7 cos 8  (A1.3(d)) (A1.3(e))  4  g  4  2  a - 3)  71 T h i s can be compared v/ith the true a x i a l count, obtained byputting  6=0  i n equation ( A l . l ) , i . e . W(o)  In  fact  = 1 + A  +  2  Ah  these s o l i d angle c o r r e c t i o n f a c t o r s are v a l i d  any observed angle so that i n g e n e r a l W(e)'  = 1 + g A P 2  2  2  (e) + g ^ P , ,  (6).  over  72 APPENDIX A2 Amended Instructions For Operating' .Veritron. Superconducting soTenoid and~CFC 100 Power Supply 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.  19. 20. 21.  Check that the diode i s connected across the input terminals (above Dewar assembly). Check that output from power supply i s connected + to white cable, - VE to black cable. Check quench sensing leads are connected - white to terminal 3, black to terminal 4. Check that heater leads are connected to terminals 7 and 8. Turn current control to minimum p o s i t i o n . Set desired current l i m i t . Check that sweep rate knob i s disengaged. Press stop button on sweep selector. Press P.C. o f f button. Turn on AC power and wait a minimum of 3 minutes (warm up period). Press DC on button. Manually rotate current control pot u n t i l 5 amps i s indicated on the meter. Check that heater current control #1 i s maximum counterclockwise and turn heater 1 on. Turn up heater current u n t i l a voltage jump i s seen (persistent switch i s now normal). Return current control to minimum. Engage desired sweep rate and press up_ button to increase current l i n e a r l y . Depress stop button when desired current level i s reached ( i f below selected current l i m i t ) . To place magnet into persistent mode wait u n t i l voltage has decreased to a minimum and turn the heater current pot to zero. Carefully lower the power supply current to zero manually or \^ith the downsweep f a c i l i t y . To return to the sweep mode, c a r e f u l l y turn up the supply current to i t s previous level and turn up the heater current to i t s o r i g i n a l l e v e l . To de-energize gradually reduce the current to zero (manually or with sweep f a c i l i t y ) . When voltage reaches a minimum depress the P.C. o f f button. » '• If a quench i s indicated press quench reset button and return current control pot to zero. Do not press P.C. off button before returning current control pot to zero.  

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