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The decay scheme of Se⁷⁵ Schneider, Harvey Roy 1957

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THE  DECAY SCHEME OF SE ^ 7  by HARVEY ROY SCHNEIDER  A THESIS SUBMITTED IN PARTIAL FULFIIMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER  OF ARTS  in the Department of Physics  We accept this thesis as conforming to the standard required from candidates for the degree of MASTER OF ARTS  THE  UNIVERSITY  OF BRITISH COLUMBIA 1 9  57  ABSTRACT  Excited levels of As?^ were investigated by studying the decay of  Scintillation detection techniques were employed  to measure, the relative gamma ray intensities, gainma-gainraa coincidences, and conversion electron-gamma coincidences.  A decay  scheme with five excited As*^ levels at 200, 265, 280, 30J? and U02 kev. i s proposed. Multipolarities are assigned to the transitions between these levels on the basis of conversion coefficients, determined from previously measured conversion intensities.  In presenting  t h i s thesis i n p a r t i a l f u l f i l m e n t of  the requirements f o r 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 f r e e l y available f o r reference  and study.  I further  agree that permission f o r extensive copying of t h i s thesis for scholarly purposes may  be granted by the Head  of my Department or by h i s representative.  I t i s under-  stood that copying or publication of t h i s thesis f o r f i n a n c i a l gain s h a l l not be allowed without my permission.  Department The University of B r i t i s h Columbia, Vancouver Canada, Date  written  ACKNOWLEDGMENT  The research described i n this thesis has been made possible through a Grant-in-Aid-of-Research to Dr. K.C. Mann, by the National Research Council. The author wishes to express.his sincere thanks to Dr. K.C. Mann for his guidance and assistance throughout this work.  TABLE  Abstract  OF  CONTENTS  .  i  Acknowledgments •  I. II.  i i  Introduction  1  E x p e r i m e n t a l Techniques  17  A . S c i n t i l l a t i o n Spectrometer  17  B . Gamma-Gamma C o i n c i d e n c e S p e c t r o m e t e r C . Beta-Gamma C o i n c i d e n c e Spectrometer  .  .  .  .  21  . . . . . .  2k  D. Angular C o r r e l a t i o n  27 75  III.  E x p e r i m e n t a l R e s u l t s On The Decay Of S e ^ 1  .  .  .  .  1. O u t l i n e o f P r e v i o u s Work 2. R e s u l t s o f t h e P r e s e n t I n v e s t i g a t i o n  28 28  .  .  .  .  29  3. D i s c u s s i o n o f t h e Decay Scheme o f A g ^ .  .  .  .  36  U. C o n c l u s i o n  .  .  .  .  Uo  Appendix I  Ul  Appendix I I  bh  Bibliography  h%  TABLE OF ILLUSTRATIONS  Following Page  Figure 1.  Relationship Between A and Z for Odd A Isobars . .  2  2.  A Typical Primary Beta Spectrum  2  3.  A Typical Decpy Scheme  10  lu  Basic Angular Correlation System  10  5.  Block Diagram of a Scintillation Spectrometer  6.  A Typical Gamma Ray Spectrum  7.  Scintillation Detector  18  8.  Detector Electronics  18  9.  Gamma Ray Collimation  18  . . 17 • . 17  10.  Gamma- Gamma Coincidence Spectrometer  21  11.  Pulse Height Analyzer Output  21  12.  Beta-Gamma Ray Coincidence Spectrometer  13.  End Plate Modification  lU.  Gamma Ray Spectrum of Se ^  29  15.  Gamma-Gamma Coincidence Spectrum  31  16.  Beta-Gamma Coincidence Spectrum  3&  17.  Decay Scheme of Se?5  36.  . . . . 2"? 2$  7  1.  INTRODUCTION.  I t has been w e l l established that the constituents of the atomic nucleus, c a l l e d nucleons, are o f two types, protons and neutrons. These p a r t i c l e s have very nearly the same mass, one atomic mass unit, but d i f f e r i n regard t o electro s t a t i c charge and magnetic moment; the proton being p o s i t i v e l y charged and having a magnetic moment of 2.79 nuclear magnetons, while the neutron i s uncharged and has a magnetic moment of -1.91 nuclear magnetons. The number of protons i n a nucleus (the atomic number) i s denoted by Z and the number of neutrons by N. A shorthand method f o r i n d i cating the composition of a nucleus i s as follows:  TflJhere X i s the chemical symbol corresponding to Zand A i s the atomic mass number, which i s equal t o N + Z. Quantum mechanical arguments applied t o the neutron-proton system comprising the nucleus, show that such a system can exist only i n discrete energy states.  VJhen the nucleus possesses minimum energy i t i s s a i d t o be  i n i t s ground state; states.  a l l other possible energy states are c a l l e d excited  In addition to energy, each state i s characterized by an angular  momentum quantum number (spin) and a p a r i t y .  The spin gives the t o t a l  angular momentum of the nucleus i n u n i t s of "R , while the p a r i t y defines the symmetry properties of the wave function which describes the state.  - 2 -  The parity may be either even (+) or odd (-) depending upon whether the wave function i s symmetric or anti-symmetric with respect to a reflection of the coordinate system. Certain nuclei are energetically unstable against decay to other nuclei. The condition for instability depends upon the masses involved and may be expressed as follows:  Suppose that "a" represents a nucleus  of Z protons and N neutrons, let M(a) represent the mass of this nucleus "a".  Now consider the N+ Z particles being redistributed among particles  "b" and "c", where M(b) and M(c) represent the masses of these particles measured separately. Then i f , M(a) > M(b) + M(c), particle "a" i s energetically unstable against spontaneous decay to "b" and "c". The difference i n the masses of the two nucleon arrangement usually appears as kinetic energy of the particles "b" and "c". I f one plots the atomic mass versus Z for a typical group of isobaric nuclei (i.e. nuclei with same A but d i f f erent Z) with odd A say, a curve such as that i n figure 1 i s obtained. In this case nucleus 3 i s stable while nuclei 1, 2, k and 5 are energetically unstable against decay to the lower masses. This instability usually results i n the spontaneous change of charge of the nucleus through beta decay. That i s , nucleus 1 decays to nucleus 2 by emitting a negative electron (negatron) and 2 decays 3 by a similar process. We describe this process as being caused by the spontaneous decay of one of the nuclear neutrons into a proton and a negatron. This event  I I  1-2  Z-l  Z  Z4I  1+2  ATOMIC NUMBER Relationship between A and Z  for  odd A isobars Figure I  MOMENTUM  A typical primary  beta  Figure  2  spectrum  - 3  -  then changes the atomic number of the nucleus from Z to Z 1, but of course the mass A remains unchanged. Thus a l l beta decay process take place between isobars. For the decay of nuclei 5 to 1+ to 3, two processes are possible; either the nucleus decays by emitting a positive electron (positron) or i t decays by capturing an orbital electron. This process i s defined as the reverse of negatron emission; a nuclear proton converts to a neutron by emitting a positron or absorbing a negative orbital electron. Thus there results a change of atomic number from Z to Z-l i n either case, and as before A remains unchanged. Each of these beta decay processes require a certain amount of energy before they can occur. This minimum energy requirement, expressed i n terms of atomic masses of the parent and daughter atoms i s given i n Table 1. Table 1 Parent M M  at  (Z-l, N+l)  a t (Z+1,N-1)  M  at  (Z+l, N-l)  Daughter >  M  >  ^  >  M  at  (Z,N)  for ^ decay.  (M)  ™e fore* decay.  + 2  at  (Z,N) for orbital electron capture.  Where m i s the mass of the electron. e  The momentum distribution of particle radiations occuring i n beta decay have been analyzed for a large number of radioactive nuclides using  - h -  a variety of spectrometers and nuclear spectro scopic techniques. In every case i t i s found that the beta particles are emitted with a continuous momentum (and hence also energy) distribution from zero up to some maximum. An example of the type of beta spectrum one obtains i s shown i n figure 2. The maximum energy with which a beta particle i s emitted i s equal to the energy difference "between the parent and daughter nuclei as given i n Table 1. When the beta particle i s emitted with an energy less than the maximum however, there appears to be a violation of the law of conservation of energy, since no other particle i s observed which, together with the beta emission, would account for the total disintegration energy. Another difficulty arises from angular momentum considerations. Protons, neutrons and electrons have intrinsic spins of -jg- 1fi and obey Fermi statist i c s . Thus even A nuclei must have integral values of total spin while odd A nuclei must have odd half-integral total spins. Since beta decay takes place between isobars, any spin change between parent and daughter nucleus must be integral or zero. Now i f the decay radiation consists of only one electron, then the total spin change must always be odd halfintegral. Thus the law of conservation of angular momentum appears not to hold. In order to reestablish the validity of the conservation laws, Pauli i n 1927 postulated the simultaneous emission of a second particle, the neutrino, i n beta decay processes. To agree with observation this  -5-  p a r t i c l e must have zero charge and a very small (probably zero) mass. Then the conservation laws are s a t i s f i e d i f the neutrino has a spin S \  and shares the d i s i n t e g r a t i o n energy with the electron i n beta decay.  In the case of decay by o r b i t a l electron capture, the neutrino c a r r i e s away the entire d i s i n t e g r a t i o n energy, since there are no primary  charged  p a r t i c l e s emitted. The properties of the neutrino admit very l i t t l e i n t e r a c t i o n with matter. For t h i s reason, attempts at d i r e c t detection of the p a r t i c l e have not been successful, although c e r t a i n measurements on the r e c o i l momentum of the daughter nucleus and the emitted electron are consistent with the postulate of the neutrino.  Fermi Theory of Beta Decay ( l ) Momentum D i s t r i b u t i o n . Among the early evidence supporting the neutrino hypothesis was success of Fermi's theory of beta decay^  the  According t o t h i s theory the  continuous beta spectrum results from a s t a t i s t i c a l sharing of the d i s i n tegration energy by the electron and neutrino. This means that both the electron and neutrino energies (and hence also momenta) are s t a t i s t i c a l l y distributed, subject however t o the condition: Emax • E i- E p  v  (l)  Where, Emax i s the d i s i n t e g r a t i o n energy, E  p  and-E^ are the neutrino and  beta p a r t i c l e energies r e s p e c t i v e l y . The d i s t r i b u t i o n of the momenta d*  - 6 -  the emitted beta particles i s predicted by this theory, to be: P:(p) dp * k : | H i p (Emax -E) F(z,p) dp 2  (2)  2  fl  VJhere, P(p)dp i s the probability that a beta particle w i l l be emitted with momentum between p and p + dp;  k i s a constant; |-H^j_{. i s a tran-  sition matrix describing the probability that a nucleus w i l l undergo beta decay; and F(z,p) i s the Fermi function, a complex function that corrects the momentum distribution for the effect of the nuclear coulomb f i e l d on the beta particle. The Fermi function has been evaluated and tabulated  for Z • 1 to 100 and a wide range of p.  Decays involving no change i n orbital angular momentum (  o  f  the nucleus are called allowed, while decays for which a\f 0 are called forbidden transitions. The matrix element iH^ j_l i s independent of the :  beta ray energy for allowed decays, but this i s not i n general, true for 3  forbidden decays. Consequently the shape of beta spectra for forbidden transitions are usually different from those for allowed transitions. (8) Selection Rules. The selection rules to be used to determine nuclear spin and parity changes resulting from beta decay depend upon whether the neutrino and electron are emitted with parallel or anti parallel spins. The assumption of parallel spins results i n Gamow-Teller selection rules, whereas anti parallel spins result i n Fermi selection rules: see  - 7-  Table 2: Table 2 Transition Classification  Fermi AI _TT  Alloifed  0  1st Forbidden  No  0£L Yes (not 0-»0)  Gamow-Teller _I AIT 0, *- 1 No (not 0"»0) 0, i l Yes (not 0->0) V  2nd Forbidden  il,*2  No  (not 0 ~ l )  22)  - 2 *3  No  (not 0*-*2)  (3) Comparative Half l i f e . By integrating equation (2) we get the total probability of decay per unit time, * = k l H l J F (z,p) 2 (Emax -E) dp 2  p i  P  (3)  X i s related to the half l i f e of the decay by, \ " g l Q  (U)  2  Ti 2  Therefore from equations (3) and (U) we get,  T| Where f(z, E-^) i s the value of the integral i n equation (3). Finally rewriting equation (5) gives f Ti 2  °c  1  i-Hfii 2  (6)  - 8 -  f T i i s known as the comparative half l i f e of the transition. Its value i s of assistance i n determining whether a transition i s allowed or to what degree i t i s forbidden. Since '.iHf^l decreases very rapidly with increasing forbiddeness, 2  i t i s necessary to consider only the order of f T i to indicate the class of the transition. Hence i t i s customary to use log^ofTi. values. If both the energy and the half l i f e of the decay are known, the log-^QfTi value may i n principle be calculated. The computations are difficult and fort3  unately nomographs based on such calculations are available . Approximate ranges of logiofT|- values for the f i r s t three classes of beta transitions are given i n Table J>\ Table 3 Classification  Approximate logiQfTi value.  Allowed  3-6  1st Forbidden  6-10  2nd Forbidden  10  On the basis of log^^Ti- values alone one cannot unambiguously determine the classification of a beta transition, since there i s some overlap of the values from one class to another. (U) K - Capture. The decay probability given i n equation (3) applies only to decays involving the amultaneous emission of a beta particle and a neutrino. When the nucleus decays by orbital electron capture, however, certain modi-  - 9-  fications are necessary; for example, since the neutrino i s the onlyparticle emitted the factors expressing the statistical sharing of energy between the beta particle and neutrino do not appear. Also, to obtain equation (2) plane wave approximations for the electron and neutrino were used. For orbital electron capture processes the wave function of the capture electron - usually one of the k shell electrons - i s used. With these modifications the total decay probability for k-capture may be written as: ^  :|.H .l f (z, Emax) 2  =  fi  k  (7)  In this case Emax i s the maximum energy of the emitted neutrino, and consequently i f i s not as easily determined as i n the case of beta particle emission. As before, we can write equation 7 as follows: °C  fv 2  1  :|-H l fi  2  (8)  Nomographs for log_Q^k ( ,Emax) have also been prepared by Moszkowski. z  Nuclear Decay Schemes. A nucleus resulting from a decay process i s frequently l e f t i n an excited state. Such a nucleus w i l l tend to drop to lower levels of excitation and ultimately to the ground state, usually through one of two competing processes. These two deexcitation processes are, emission of a photon (gamma ray), and direct transfer of the excitation energy to an orbital electron which i s then emitted with an energy equal to the trans-  - 10  ition energy minus the binding energy of the electron.  The latter process  i s called internal conversion. The decay and subsequent deexcitation of a nucleus i s conveniently represented by a diagram called a decay scheme, see for example figure 3 . I , _b, I i n figure 3 refer to the spins of the corresponding states and a  c  E i , E2 are the excitation energies. Multipole Radiation. Gamma radiation i s classified by multipole orders L, according to the angular momentum (in units offa) carried off by the photon. For each multipole order there are two classes of radiation;  electric multipole and  magnetic multipole, which differ with respect to parity changes. Electric dipole, quadrapole, octupole, etc. radiations are usually denoted by the symbols, E l , E2, E3, EU, etc. Where the multipolarity i s determined from 2*~ . Similarly Ml, M2, etc. denote magnetic multipole radiations.  Selection  rules governing the possible multipolarities for a specific transition between two states with spins  A"  and J f are:  s (-I)* for electric multipole radiation 1  — TT a (-1)^"-*- for magnetic multipole radiation I f 3 i » 3 f * 0 then a gamma transition i s strictly forbidden. Since the transition probability decreases with increasing multipolarity, one expects only the lowest multipole order permitted by the selection rules ( L ^ X i - J f \ ) ,  Z-H'  _  i stoble c  A Typical Decay Scheme I  Figure 3  I.  i — i  t  |Coincidenee l,2|  T  Basic Angular Correlation System  Figure 4  - 11 -  or perhaps i n some cases a mixing of the two lowest m u l t i p o l a r i t i e s (but of opposite class because of p a r i t y  considerations).  There' are several d i f f e r e n t measurements from which the multipole orders of gamma transitions can be deduced. Some of these are described below. (1)  Conversion C o e f f i c i e n t : If  Xyand  A  c  are the p r o b a b i l i t i e s of gamma and conversion e l e c -  tron emission respectively,  for a p a r t i c u l a r nuclear t r a n s i t i o n , then we  define the t o t a l conversion c o e f f i c i e n t for t h i s t r a n s i t i o n as:  In the majority of cases conversion electrons from innermost (k) s h e l l are the most intense, hence often i t i s more useful t o consider the k - conversion coefficient.  ___  *** ' **  r  (10)  Where )\ ^ i s the r e l a t i v e p r o b a b i l i t y of emission of a k conversion electrons Values of conversion c o e f f i c i e n t s depend upon ( l ) the energy of the t r a n s i t ion, (2)  the atomic number of the nucleus, (3) the s h e l l from which the  electron i s ejected, (U) the multipolarity of the competing gamma t r a n s i t i o n and  (5) the character of the nuclear t r a n s i t i o n i . e . either e l e c t r i c or mag-  netic.  Theoretical K and L s h e l l conversion c o e f f i c i e n t s have been c a l c u l -  ated and tabulated by Rose^ et a l f o r both e l e c t r i c and magnetic t r a n s i t i o n s with m u l t i p o l a r i t i e s up t o 2->.  By comparing the experimental conversion co-  -In-  efficient with these calculations i t i s usually possible to establish the multipolarity and the character of the transition, (2) K/L Ratio. When the intensities of both the K and L conversion transitions can be measured the corresponding raultipolarities may be determined from the ratio of these intensities since K  /L = <___  » ___  (11)  Where K/L i s the intensity ratio of the K and L conversion lines. This method i s sometimes preferred over methd(l) since i t does not involve direct measurement of gamma-ray intensities. Empirical curves of K/L ratios lo  to determine multipolarities are given by Goldhaber and Sunyar. (3) Angular Correlation: The probability of emission of a photon from a nucleus depends i n general upon the angle between the direction of emission and the nuclear spin axis. The actual  -  form of the probability function being determined  by the nuclear spin change and the multipolarity of the gamma transition. Despite this angular dependence of the emission probability, gamma radiation from ordinary radio active sources i s observed to be isotropic, because nuclei making up the source are randomly oriented.  If, however, i t i s poss-  ible to detect only the radiation from those nuclei oriented i n a particular fashion while excluding the radiations from a l l other nuclei, then we would  - 13 -  again have an anisotropic distribution. Using coincidence counting techniques such a selection i s possible for cases where there are two gamma rays i n cascade.  Consider for example, a detection system as shown  in figure 1+a which i s used to study the cascade shown i n figure Ub. Detection of L~_ establishes a reference direction, i . e i only coincident radiation from nuclei oriented i n a particular fashion i s detected, and therefore the second gamma ray _£ w i l l have an angular correlation with respect to the f i r s t . The assumption here i s , of course, that the lifetime of the intermediate state i s short enough to allow no reorientation of the nucleus between emission of Lj_ and L^. The theoretical correlation function i s of the form^, w(e) _  _ A,, p C * 69  p  * 0  ^max  e  ) (12)  :o  Min (2J^,2Li, 2L2)  Where, W(6)ci-Q. i s the relative probability that L2 w i l l be emitted into a solid angle  at an angle 9 with respect to L_. The values of the  coefficients A„ depend upon the spins of the three states involved as well as the multipolarities of the gamma transitions. Theoretical values for 6 A„ have been calculated and tabulated by Rose et a l for various combinations of spins and multipolarities. •Nuclear Models. The lack of a complete nuclear theory has resulted i n the development of various nuclear models, which are used as an aid i n interpreting the  - lli -  date obtained from nuclear experiments,  (1) S h e l l Model, A s u r p r i s i n g l y successful account of a great v a r i e t y of nuclear phenomena i s obtained through the use of the nuclear s h e l l model' ; developed by Mayer e t a l . 1  For t h i s model i t i s assumed that nucleons move  i n o r b i t s within the nucleus, analagous to electronic motion i n o r b i t s i n the atom.  I t i s further assumed that the motion of any p a r t i c u l a r nucleon  can be determined by considering i t to be moving i n an average  static  nuclear p o t e n t i a l , due t o a l l the other nucleons i n the nucleus. Then the i n t e r n a l motion o f the nucleus i s obtained by a super p o s i t i o n of a l l the quasi ineUpjei^iitmotions of the i n d i v i d u a l nucleons. Each nucleon o r b i t i s characterized by quantum numbers which determine a nuclear energy l e v e l .  According to the P a u l i p r i n c i p l e these l e v e l s may  be f i l l e d with neutrons and protons independently.  Whenever two successive  groups o f l e v e l s are widely separated and the lower group i s completely •populated, there i s said t o be a s h e l l closure at the separation. N u c l e i with a l l nucleons i n closed s h e l l s are exceptionally stable. Such exceptional s t a b i l i t y has been observed f o r n u c l e i with 2, 8, 20, !?0, 82 and 126 i d e n t i c a l nucleons.  In order to p r e d i c t s h e l l closures at these nucleon numbers,  Mayer e t a l assumed strongspin o r b i t coupling.  Thus a l e v e l with angular  momentum ? s p l i t s into two l e v e l s , one with t o t a l angular momentum ]*• \+\ and the other with  j=I ~ £  -15  ^he level with  j = ? •» j  -  i s the lower one, and the separation  from the other i s proportional to 2 ? * l  (i.e. the splitting increases  with ? ). Nuclear spins and magnetic moments are predicted by the shell model, i f i t i s assumed that nucleons making up the nucleus, couple their spins and magnetic moments to zero i n pairs. Thus even A nuclei have zero spins and magnetic moments while odd A nuclei have spins and magnetic moments due only to the last odd nucleon. The predictions of the shell model are i n good agreement with experment for a very large number of nuclei. However, as one proceeds to high A nuclei a more detailed model i s often necessary. Thus we have a slightly more complex model - the collective model. (2) Collective Model. A system of particles held together by their mutual attractions can undergo collective oscillations. This suggests a modification to the shell model. Instead of regarding the nucleons i n closed shells as composing an inert core, Bohr and Mottelson^ assumed that this core might exhibit effects of collective motion of the nucleons with i t .  This motion can take the form  of either a rotation or a vibration, which i s coupledto the motion of the nucleons outside of the closed shell. In general this coupling i s very complex, making the equations i n volved extremely difficult to solve. The coupling, however, takes a simplar form for nuclear configurations with closed or very nearly closed natron and  - 16 -  proton s h e l l s or f o r configurations f a r removed from closed s h e l l s . For these cases the motion of the nucleons outside of the l a s t closed s h e l l and the c o l l e c t i v e motion of the core can be considered  separately.  Thus, the excited states of these n u c l e i can be i d e n t i f i e d with either p a r t i c l e e x c i t a t i o n or a c o l l e c t i v e e x c i t a t i o n . Several methods may be used to d i s t i n g u i s h experimentally,  the  p a r t i c l e states from the c o l l e c t i v e states. One method employs the c o u l omb  e x c i t a t i o n ^ nuclear reaction.  That i s the nucleus i s excited through  an i n t e r a c t i o n between i t s own coulomb f i e l d and that of a c l o s e l y passing bombarding p a r t i c l e . Only c o l l e c t i v e states are excited i n t h i s manner. Therefore, by measuring the energies of the gamma rays remitted by these excited n u c l e i , the energy l e v e l s associated with c o l l e c t i v e motions of the nucleons can be i d e n t i f i e d . Nuclear Spectroscopy. The foregoing discussion has shown that a number of properties of radioactive n u c l e i can be determined by a study of the radiations emitted during decay. In the f i e l d of nuclear spectroscopy we obtain from such measurements on nuclear radiations, information about the excited states of n u c l e i .  This information together with that from other nuclear studies  w i l l , i t i s hoped, eventually l e a d t o the f'or.mul'ettioriof a complete and sati s f a c t o r y nuclear  theory.  - 17 -  I I . EXPERIMENTAL TECHNIQUES  A. Scintillation Spectrometer, (l) General Description. A block diagram of a single channel scintillation spectrometer i s shown i n figure  For gamma ray detection the detector consists of  a sodium iodide crystal (activated with thallium) which i s optically coupled to a photo multiplier tube. When a gamma ray i s absorbed i n the crystal a light scintillation i s produced. The intensity of this scint i l l a t i o n i s proportional to the energy of the absorbed gamma ray. Since the photomultiplier i s a linear amplifying device the voltage pulse produced at the collector/.^ has an amplitude which i s also proportional to the energy of the incident gamma ray. This pulse i s further amplified with a linear amplifier and then fed into a pulse height analyzer. The pulse height analyzer registers an output pulse, which i s recorded by the scaler, only i f the pulse height f a l l s within a predetermined voltage channel. The voltage about which the channel i s centred may be varied from zero to one hundred volts. By scanning over the range of pulse heights from the amplifier one obtains peaks i n the counting rate at voltages which are proportional to the energies of the absorbed photons. Figure 6 i s an example of the type of spectrum obtained i n this way. (2) Detector. Details of the construction of the scintillation detector are shown  SCINTILLATION COUNTER  PULSE  LINEAR  HEIGHT  AMPLIFIER  ANALYZER  SCALER  Scintillation  Spectrometer  Figure  5  60  y S p e c t r u m of Co in i< or (9  z  3 O o  PULSE  HEIGHT  Figure  6  - 18 -  in figure 7. In order to keep the Nal crystal centered on the photo multiplier cathode, a fibre centering ring was placed on the photo multiplier. Optical coupling between the photo cathode and the crystal was achieved through a very thin layer of DC200 silicone o i l . After the crystal was put i n place and before i t was wrapped with "scotch" electrical tape, Apeizon Q sealing compound was packed around the centering ring to prevent extraneous light from getting into the photo cathode. The crystal was held firmly against the photo multiplier with two pieces of electrical tape running parallel to the length of the tube and across the crystal. Finally the entire assembly was wrapped with electrical tape as an additional precaution against light leaks. Voltage was applied to the photomultiplier dynodes from a voltage divider network consisting of nine 1 megohm resistors, one. 2 megohm resistor, and one 10 K load resistor (see figure 8). The 2 megohm resistor was used between the cathode and f i r s t dynode, so that the voltage between these two elements would be twice that between other two successive dynodes. The photo multiplier used i n this detector was a DuMont 6292, which has a focussing electrode placed between the cathode and f i r s t dynode. The manufacturer suggests that the potential on this electrode be adjusted to seme value between that of the cathode and f i r s t dynode to optimize the photomultiplier response. It was found, however, that for the particular 6292 used, best performance was obtained when the focussing electrode was connected to the cathode.  £#T\M  A%%tr\«t\  SCOTCH  Et.ttf.ic  W<U>P«0  Tute  Fi«une 7  WITH  Apif2P*/  0  tHT 6292  10 K  i  A  +3 0 0 9 n II  2 MA  Detector E l e c t r o n i c s Figure 8  Figure 9  i  volts  - 19 -  To energize the photomultiplier, 900 volts from a stabilized positive HT power supply was connected across the voltage divider network. The output pulses were then taken off at the collector through a 50 up. fd condenser and fed into a cathode follower preamplifier (figure 8). The cathode follower circuit matched the output impedance of the photo multiplier to the 100 ohm characteristic impedance of the coaxial cables. (3) Electronics. The pulsesfrom the cathodefollower were amplified with an Atomic Instrument Co. Model 20ijB linear amplifier set at a maximum gain and at a 0.8 usee risetime. The pulse height distribution at the output of the amplifier was then analyzed with an Atomic Instrument Co. Model 510 single channel pulse height analyzer. A scale of 6I4. scaler plus register was used to record the output from the pulse height analyzer. (U) Source. (i) Irradiation. The Se75 source used i n the present investigation, was prepared by irradiation of natural selenium with thermal neutrons, i n the Chalk River pile. Other selenium activities produced are short lived, with the exception of Se79, which has a very long half l i f e ( £ 6.5 x 10^ years); a negligible amount of this activity i s produced because of the relatively small 78 capture crossection of Se .  - 20  ( i i ) Preparation for Spectroscopic Study, For a study of i t s radiations the radioactive selenium was deposited as a small spot of red selenium on a thin LC 600 backing, which was supported by a one inch diameter aluminum ring. Preparation of the source i n this manner i s described i n detail by McMahon^, ( i i i ) Collifflation, The gamma ray detection efficiency of scintillation detectors i s best for large source to crystal distances^ i.e. for a parallel gamma ray beam incident on the crystal. However, placing the source at a large distance from the detector increases  the probability that gamma  rays w i l l be scattered into the crystal through compton interactions i n surrounding materials. To preserve the advantage of a large source to detector distance and at the same time eliminate scattering problems, collimation of gamma rays was employed. The arrangement used to accomplish this i s shown i n figure °. (U) Measurement of The Gamma Ray Spectrum. With the pulse height analyzer channel width set at one volt, the counting rates were measured for baseline settings spaced at one volt intervals,from 0 to 70 volts. Since the detection efficiency of the scintillation counters i s very high one minute counts on each point gave a statistical accuracy of better than 1% on the peaks and h% elsewhere.  - 21 -  B. Gamma-Gamma Coincidence Spectrometer, (1) Description. The high detection efficiency of scintillation counters makes them ideally suited as gamma ray detectors i n coincidence counting experiments. In the present investigation, coiicident gamma radiations were measured using the system outlined in the block diagram i n figure 10. Basically the apparatus consists of two scintillation spectrometers, like the one described i n , plus an Atomic Instrument Co. Model 502A A  coincidence analyzer. This instrument has four coincidence channels, only two of which were used i n the present spectrometer. Each channel has an adjustable resolving time from 0.25 micro seconds to 2.0 micro seconds. For reasons which w i l l be explained later the total resolving time (i.e.. the sum of the resolving times of the two channels used) was set at approximately one micro second. In order to get a high coincidence counting rate, the two detectors were placed very close to the so urce. For most measurements, source to crystal distances of one centimeter were used, with detector s on opposite sides of the source. The Se?5 source used was prepared as a small spot on a rubber hydrobromide backing. This backing was supported on a two inch diameter ring made of aluminum wire. The source was supported i n this manner to miiidjnize the possibility of Compton scattering i n the source holder.  DETECTOR  DETECTOR  2048  204B  AMPLIFIER  AMPLIFIER  PULSE HEIGHT  PULSE HEIGHT ANALYZER  ANALYZER DISCRIMINATOR COINCIDENCE  SCALER  SCALER  SCALER  YY  COINCIDENCE  SPECTROMETER  Figure 10 Chonnel W i d t h  Pulse Figure II  Height A n a l y z e r Output  - 22  -  (2) Resolving Time. The use of the Model 5l0 pulse height analyzers i n the coincidence spectrometer outlined i n figure 10 places certain restrictions on the coincidence resolving time that can be used. This results from the fact that the output pulse from the analyzer occurs when the triggering pulse recrosses the baseline discrimination level. That i s , the pulse at the output i s delayed with respect to the input by an amount equal to the width of the pulse above the baseline discrimation level. A fixed delay due to the electronics i s not serious, since compensation for this can be made, however, the above mentioned delay i s i n effect a variable one, depending upon the pulse height. This effect i s illustrated i n figure 11. Pulses "a" and "b" represent the two extreme pulses that w i l l be recorded by the pulse height analyzer for general baseline setting B.L. and a channel width CVJ. Pulse "a" produces an output pulse a time than pulse "bH. The magnitude of  later  i s governed by the channel width and the  slope of the trailing edge of the pulse. The shape of the pulse i s determined by the amplifier circuitry and i s not easily changed. Therefore, to keep small a narrow channel width i s indicated. This was always set at less than 2 volts, which then gave a  of 0.5 micro seconds. This value  represents the minimum resolving time that the coincidence analyzer can have. A resolving time of 1 micro second was chosen so that allowance could also be made for possible additional variations i n delay due to slight d r i f t ing i n the discrimination levels at the inputs to the coincidence analyzer. The dependence of the delay on the discrimation level results from the long  - 23 -  rise time of the pulse height analyzer output pulse. To measure the coincidence resolving time, pulses from a scintillation detector were fed into one amplifier while the output from a pulser was used as the input to the other amplifier. With this arrangement the pulses registered by the coincidence analyzer must be due only to chance. The chance coincidence rate i s related to the resolving time by, N  ch  _% N r 2  (13)  Where, N]_, N2 are the counting rates i n each channel of the coincidence analyzer. Hence T can be calculated. (3) Measurement of Gamma-Gamma Coincidences. With one pulseheight analyzer chanelled on a gamma ray peak the coincidence counting rate was recorded as the other pulse height analyzer was scanned over the entire spectrum. Peaks occur in the coincidence spectrum corresponding to the gamma rays coincident with the one i n the fixed channel. The two largest peaks i n the scintillation gamma ray spectrum of Se?5 are actually composite peaks; one corresponding to a 121 and a 137 kev gamma ray and the other to a 265 and a 280 kev gamma ray. Coincidences with these gamma rays were measured by setting the fixed channel f i r s t on the high side of the peak, so as to accept more of the high energy component and later on the low side to accept more of the low energy component.  - 2k  -  The single channel counting rates were also measured so that the chance coincidence rate could be calculated using equation (13) the calculated chance rate was substracted from the observed coincidence rate, to obtain the significant coincidence rate. C. Beta-Gamma Coincidence Spectrometer. The coincidence measurements just described are limited by the poor resolution of the scintillation counters. For this reason i t was decided to attempt to measure conversion electron - gamma cdhcidences using a magnetic and a scintillation spectrometer.  In this way we  u t i l i z e the high resolution of the magnetic spectrometer and the high transmission of the scintillation spectrometer.  A Block diagram of the  £ - r coincidence spectrometer i s shown i n figure 12. (l) Magnetic Spectrometer. A modified thin lens spectrometer, described i n detail by Milley-^ was used as the conversion electron detection system. This spectrometer differs from the conventional thin lens spectrometer i n two respects; l ) the source and detector are not equidistant from the center of the detection  magnet. 2) Ring focus detection i s used instead of the axial^in conventional spectrometers. These modifications improve the transmission by a factor °\ while not altering the resolution. Ring focus detection i s achiwed with a ring of anthracene crystals  - 2*  -  coupled to a "conical" light pipe with silicone o i l . The light pipe transmits the scintillations from the ring, which i s 5 inches i n d i ameter onto the l|- inch diameter photo cathode of the photo multiplier tube. Milley used a DuMont 6292 photo multiplier, i n the present work, however, this was changed to a lU stage EMI 6262 tube. This change was made i n an effort to get signal pulses from the amplifier with amplitudes sufficiently high to allow the use of a discrimination level that was above the level of most electrical interference, (caused primarily flourescent lights iii the building). Optical coupling between the photo cathode and the light pipe i s secured by coating the end of the photo multiplier with silicone o i l and then pressing' i t firmly against the light pipe. Magnetic shielding i s provided by having the photo multiplier i n side a mild steel tube which i s fastened to the end of the spectrometer. Pulses from the photo multiplier are fed through a cathode follower preamplifier to a Model 20i|B amplifier (set at a 0.2 u sec. risetime). The output from the amplifier i s recorded with a scaler plus register. To provide a necessary time delay adjustment when the spectrometer i s used for beta-gamma coincidences, a variable delay line i s placed at the output of the amplifier. The measurement of beta spectra with the magnetic spectrometer i s accomplished by varying the current through the coil i n steps, and recording the corresponding counting rates, for each current setting the magnetic  Pulse  Coincidence  A Hairier  Variablc  Scaler  Scaler  Scaler  Figure 12 p-y  +*«  COINCIDENCE  i ,Oet«tor :  j  SPECTROMETER  i  4*  Mild Steel  Figure 13  ENDPLATE  MODIFICATION  - 26 -  f i e l d produced focusses beta particles with a particular momentum, on the anthracene crystals. From the above procedure therefore, one can get the momentum distribution of the emitted beta particles. In order to allow the gamma ray detector to be placed near the source for beta-^amma coincidence measurements, the end plate at the source end of the magnetic spectrometer had to be modified. This modification i s illustrated i n figure 13. The mild steel tubing shown i s this figure provides magnetic shielding for the photo multiplier. (2) Preliminary Tests and Adjustments. (i) Single Channel Spectra. A t r i a l conversion electron spectrum of S"d?£ was measured to determine the effect, i f any, of the brass and iron placed near the source as a result of the modification described above. The results of this test showed no significant change i n the shape of the conversion spectrum, nor was there any evidence of hysterisis effects due to the magnetic shield. The gamma ray spectrum measured with the scintillation spectrometer does show some distortion due to Compton scattering from either the surrounding brass or iron. Since the photo electron peaks were s t i l l quite prominent, the distortion was not considered to be serious. (3) Delay Time Matching. To compensate for the time delay introduced by the pulse height analyzer, a variable delay line was inserted i n the magnetic spectrometer  -  27-  circuit. The amount of delay required was found by feeding into both 20ljB amplifiers, the output from the pulser. The delay was then varied until a l l of the pulses were counted by the coincidence circuit. (U) Measurement of Beta-Gamma Coincidences. Measurement of conversion electron-gamma coincidences i n the Se?5 spectrum were made both by fixing the magnetic spectrometer on a peak and  scanning  with the scintillation  spectrometer  and visa versa. Because  of the relatively low transmission of the magnetic spectrometer  (~2.5$)  the coincidence counting rate was very low. As a result counting times as long as 100 minutes on a point were often necessary to obtain s t a t i s t i cally significant results. D. Angular Correlation. Some preliminary gamma-gamma angular correlation experiments were performed i n an attempt to gain further information about energy level spins and transition multipolarities. The 137 - 265 kev cascade was chosen as the one to be studied. This experiment yielded a correlation function with negative anisotropy of S.5 %, Interpretation of the results i s , however, not possible at this time because insufficient information has been obtained to allow a correction to be made for the effect of the unavoidable detection of the 121 - 280 kev cascade. Even when this correction can be made, interpretation of the angular correlation results w i l l be difficult because conversion coefficients indicate that both the 137 and the 265 kev gamma rays are probably mixed E2 + Ml.  - 28 -  III. EXPERIMENTAL RESULTS ON THE DECAY OF Se?5  1. Outline of Previous Work. Se?5 (127 days) decays by electron capture to one of several excited states of A ?5. The results of a number of investigations s  of this decay have been published. Good agreement exists on the gamma ray energies of 66, 97, 121, 137, 199, 265, 280, 305 and k02 kev.-'--*-'-^*-^'-'-^-'-* 7  tron spectrum Using a  Cesk et al?"-' have measured the conversion elecspectrometer and photographic detection. They  report conversion lines corresponding to transition energies of 2lu7, 66.2, 80.8, 96.8, 121.2, 136.2, 198.8, 265.2, 280.1, 30U.0, U01.9 kev.  On the basis of these energy measurements a decay scheme i s proposed. Jensen et a l - ^ have also measured the conversion electron spectrum. They did not observe any evidence for the 2U.7 or the 80.8 kev transitions but did find a f a i r l y intense 77 kev transition. From the con^ version electron studies and some preliminary coincidence measurements they propose a decay scheme which differs from Cork's because of the omission of the 2lu7 and the 80.8 kev transitions and the inclusion -i  instead of the 77 kev transition. Temmer and Heyderiburg  o  have induced  by coulomb excitation the 66, 199 and 263 kev transitions. These results are consistent only with Cork's decay scheme. Crystal summing techniques employed by Lu, Kelly and Weideriback show that the highest level i n As"^ resulting from decay of Se^> i s at 7  - 29 -  U02 kev.  The most recent and also the most extensive investigation of the decay scheme has been carried out by Schardt and Welker^-7. They have measured gamma ray energies, internal conversion coefficients, and relative transition intensities.  In addition they have  performed gamma-gamma coincidence and angular correlation measurements. They also studied the decay of Ge?£, which decays by beta emission to Ag 75 . Prom these measurements a level scheme for A 7^ ' i s proposed and p  s  plausible spin and parity assignments are discussed. 2. Results of the Present Investigation. (i) Relative Transition Intensities. The relative gamma ray intensities are obtained from the spectrum measured with the scintillation spectrometer (figure llj.),' The areas under the photo-peak, corrected for the photo-peak efficiency (Appendix I) was taken as a measure of the gamma ray intensity. Compton distributions associated with the (121 - 137) and (265 - 280) kev peaks were calculated using Compton scattering cross sections given by Davisson and Evans ^. These . 2  were subtracted from the spectrum before the areas of the 66 kev and 97 kev peaks x^rere measured. For these two transitions a correction was also made for the escape of the iodine X ray (Appendix I I ) . The relative gamma ray intensities are summarized i n table k column 2. Column 3 summarizes the gamma ray intensities obtained by Schardt and "vlelkerl? from a photo electron spectrum using a lead radiator and a thin lens spectrometer. The 305 and 280  Transition Energy k«v  402  Scintillation r Intensity  Schardt & . Final Welker t Intensity Photoelectron Intensity  0.16  305 280  McMahon Conversion Intensity  K/L  Conversion Coefficient  0.23  0.026K  2.15(-3)  0.02  0.02  0.13K  0.457  0.46  1.00  T r a n s i t i o n Multipole Intensity Assignment  5.66%  Ml  1.28(-l)  0.57%  12?  0.5K  2.15(-2)  11.55%  E2  1.00  LOOK  1.95(-2)  25.0 %  E2*ML  0.087  0.057K  1.23(~2)  2.62%  1.00  1.29  3.78K  5.85(-2)  33.5 %  0.301  0.39  1.57K  8.10(-2)  11.1 %  Ml  0.052  0.20  7.80K  7.70(-l)  8.6%  E2  0.07  0.33K  9.25(-2)  1.87%  K  1.00 265 199  0.06  136  11.4  Ml E2-M1  1.15 121 97  0.14  81  Not observed  66  0.048  25 <0.02 X (-3) = 10,-3  6.8  ~ 7  E2-M1  < 0.029  75 Conversion C o e f f i c i e n t s and T r a n s i t i o n I n t e n s i t i e s f o r As  TABLE 4  g  - 31 -  kev gamma ray intensities are expressed relative to that of the 265 kev gamma ray; while the intensities of the 121 and 97 kev gamma rays are given relative to that of the 137 kev gamma ray. The final relative intensities are arrived at by dividing the intensities of the (265, 280) and (121, 137) composite peaks according to the relative intensities of the two gamma rays i n each peak. Column h of Table U l i s t s the final gamma ray intensities, normalized to that of the 265 kev gamma ray. The conversion electron intensities listed i n column 5 and the K/L ratios i n column 6 were measured by McMahon^- using a thin lens spectrometer set at 2.5$ resolution.  Relative conversion coefficients  are obtained by dividing the conversion intensities by the corresponding gamma ray intensities. With the identification of the 97 kev transition as being E2, (reasons for this assignment to be discussed later) the relative conversion coefficients can be converted to absolute values. Multiplying the gamma ray intensity by the corresponding conversion coefficient and adding the product to the gamma ray intensity gives the total transition intensity vrhich i s listed i n column 8 of Table h» ( i i ) Identification of Gamma-Gamma Cascades. The existence of various gamma-gamma cascades was established through the coincidence measurements. A typical spectrum obtained with the scintillati. on coincidence spectrometer i s shown i n figure 15.  PULSE  HEIGHT  Figure 15  - 32 -  Considerable care must be taken i n interpreting the coincidence spectra because i n general, pulses due to other than the gamma ray of interest f a l l into the pulse selection channels. For example, when the fixed channel i s set to accept pulses due to the 66 kev gamma ray, i t w i l l accept an almost equal number of pulses due to the 97 and 137 kev gamma rays and the Compton distribution associated with the (265, 280) kev peak. The presence of the 265 and 98 kev peaks i n figure 15 can be explained by detection of these "extraneous" pulses. The possibility of scattering of radiation from one detector to the other must also be considered i n certain cases. I t i s believed that such a scattering process accounts for part of the observed 137 kev coincidence peak when the fixed channel i s set to accept 137 kev pulses. (A small coincidence peak at 137 kev would be expected, since included i n the fixed channel there w i l l be some of the Compton distribution association with the (265, 280) kev photo-peak and these are in cascade:). Related to crystal scattering i s the detection i n one crystal of the iodine X-ray which escapes from the other. The peak at 30 kev i n figure 15 can be attributed to this effect. The results of the gamma-gamma coincidence measurements are summarized in Table 5.'. As can be seen coincidences between the composites peaks of (97, 121, 137) and (265, 280) kev are fairly easily established. To obtain more specific information about the cascades involving these gamma rays, conversion electron-gamma coincidences were measured. The higher resolution of the magnetic spectrometer allowed coincidences with the 97, 121, and 137  - 33 -  TABLE 5  Se ^ Gamma-Gamma Coincidences 1  Selected Event (kev) U02 (265, 280)  200  Coincidences  Comments  None (121, 137)c  There are no coincidences above 137 kev.  66 137  137  66 (265, 280)c No coincidence fiould be established because of interference from other gamma rays  98 (favored) 137, 121 66 280 303 66  Note: c - composite peak.  121 98 (121, 137) 200  - 34  -  kev c o n v e r s i o n e l e c t r o n t o be measured s e p a r a t e l y . F i g u r e 16 shows a t y p i c a l c o i n c i d e n c e spectrum o b t a i n e d i n t h i s manner. The c o i n c i d e n c e peak a t 121 and 137 kev a r e caused b y d e t e c t i o n o f Comptonf s c a t t e r e d 265 and 280 k e v gamma r a y s i n t h e s c i n t i l l a t i o n c o u n t e r . T a b l e 6 summarizes t h e r e s u l t s o f t h e c o n v e r s i o n electron-gamma coincidence  measurements.  M u l t i p o l e Assignment t o 97 k e v . T r a n s i t i o n . I d e n t i f i c a t i o n o f t h e 97 kev t r a n s i t i o n as b e i n g E2 i s b a s e d o n the measured K / L r a t i o and on t h e f a c t t h a t c o n s i s t e n t m u l t i p o l e a s s i g n ments t o o t h e r t r a n s i t i o n s can be made on t h e b a s i s o f c o n v e r s i o n c o e f f icients,  c a l c u l a t e d as a r e s u l t o f t h i s i d e n t i f i c a t i o n .  The K / L r a t i o as measured b y McMahon-^ i s 6.8.  According to  the  e m p i r i c a l K / L r a t i o r a t i o c u r v e s o f G o l d h a b e r and Sunyar t h i s w o u l d mean t h a t t h e 97 k e v . t r a n s i t i o n i s e i t h e r E2, M2 o r M3. m e t a s t a b l e s t a t e w i t h a l i f e t i m e o f 3.5  I f i t were M3 a  minutes w o u l d be  expected -^-.  J e n s e n e t a l - ^ s e a r c h e d f o r such a s t a t e w i t h n e g a t i v e r e s u l t s . i s , therefore,  2  The c h o i c e  narrowed t o E2 o r M2. E2 i s p r e f e r r e d because t h e M2  trans-  i t i o n p r o b a b i l i t y i s t o o s m a l l t o e x p l a i n c o m p e t i t i o n w i t h o t h e r gamma r a y s w h i c h p r o c e e d f r o m t h e U02 k e v . l e v e l .  S c h a r d t and W e l k e r have measured  t h e c o n v e r s i o n c o e f f i c i e n t o f t h e U02 k e v . t r a n s i t i o n r e l a t i v e t o t h e c o n version coefficient  («c - 0.118) o f t h e 66l k e v . C g ^ t r a n s i t i o n . 1  Their  -3  v a l u e o f (2.1; x 10 ) i s i n e x c e l l e n t agreement w i t h t h e v a l u e (2.2 x  -3  10)  o b t a i n e d f r o m t h e p r e s e n t r e s u l t s on the a s s u m p t i o n o f a 97 k e v . E2 t r a n s i t i o n .  p-y  Coincidences  With  St<»tfiliation  S e t on  «PK  MAGNET  Spectrotn«t«  CURRENT SETTING  Figure 16  131  k«*  -35-  TABLE 6 Conversion Electron - Gamma Coincidences: Selected Event  Coincidence  97 kev.Conversion Electron  (265, 280)c Gamma  137 kev.Conversion Electron  (265, 280)c Gamma  121 kev.Conversion Electron  (265, 280)c Gamma  137 kev. Gamma  265 kev. Conversion Electron  280 kev. Gamma  121 kev.Conversion Electron  Note: c - composite peak.  - 36 -  3. Discussion of the Decay Scheme of A 75. s  The energy level sequence with spin and parity assignments, as well as the multipolarities of the transitions i n A 7£ are made on s  the basis of previously published investigations of this decay, i n addition to conversion coefficients, intensity measurements and coincidence measurements of the present work. The level scheme shown i n figure 17 i s identical to that proposed by Cork et a_l£ and differs from Schardt and welkers-^ only i n the omission of the U77 and 628 kev. levels, which are excited through beta decay of Ge7^. A l l of the observed coincidence cascades are accounted for by this decay scheme. I t i s , however, not possible to determine directly the order of emission of the 121 - 280 kev. gamma rays, and the 97 - 305 kev. gamma rays. I t would be possible therefore to replace the levels at 280 and 305 kev. by ones at 121 and 97 kev. T,he latter scheme i s not very likely though, since neither the 97 nor the 121 kev. transition i s excited by coulomb excitation. To be ground state transitions and not be excited by coulomb excitation, the 121 and 97 kev. transitions would have to be pure magnetic multipoles, which does not agree with conversion coefficient data. A level i s placed at 200 kev. rather than at 66 kev. on the basis of Schardt and Welker's ! study of the beta decay of Ge7^. They observe a beta 1  decay to a 628 kev. level i n A s ^ and a U27 gamma ray (among others) i n coincidence with i t .  Such a gamma transition can be accounted for, i f there  i s a level at 200 kev.  39 A s  7 8  94  KEV 402 137 MI*EZ 906  3L i  260 266  •i (t)  i  i  66 E2«MI  199  402 Ml  STABLE—*  266 E2>MI  199 Ml  60  A. 306 E2?  280 E2  * 76 DECAY SCHEME OF SELENIUM  Figure 17  76  Se  - 37 -  The spin of Se'- has been measured using microwave spectroscopy 3  techniques and found to be 5/2. The shell model predicts a spin of 22  either \ or 9/z, Trail and Johnson ^ have measured the threshold energy of the 2  reaction, A ^ (p, n) Se?5 7  s  and obtained 1.67U- 0.005 Mev. states of As' and 5  The energy difference between the ground  calculated from this threshold energy i s 900 kev.  From this information the log f t values for decays to the various levels 3  can be calculated using the graphs of Moszkowski . In particular the log f t value for the decay to the U02 kev. level i s 6.2, which indicates that this decay i s probably allowed. Hence the i;02 level could have spins of 7/2, 5/2 or 3/2. The conversion coefficient for the U02 kev transition agrees very well with the theoretical value for an Ml transition. Therefore, since the spin of the ground state of A ^ i s 3/2 i,-) ^, the 7/2 7  s  2  spin choice for the U02 kev. level i s eliminated. Spins of 5/2 and 3/2 remain as possibilities with the 5/2 being more likely because of the Ml ground state transition. This choice i s also indicated by the studies^ of the beta decay of Ge75 to As -*. 7  7  The ground state of Ge?£ i s very l i k e -  ly l/2 and thefore i f the U02 kev. level of As"^ were 3/2, one would expect a beta decay to this level. Since no such decay i s observed the argument in favour of the 5/2(--) choice i s strengthened. The 97 kev. transition was identified as E2, which means that the  - 38 -  305 kev. level can have a spin of 9/2, 7/2, 5/2, 3/2 or l / 2 . From the conversion coefficient data, the 280 kev. transition was identified as E2. Therefore the possible spins of the 280 kev. level are 7/2,  5/2, 3/2 or l / 2 .  The conversion coefficient for the 265 kev. transition indicates that i t i s rather pure E2, but some Ml mixing wouM be consistent with the results. With an E2 + KL identification for this transition the possible spins of the 265 kev. level are 5/2, 3/2, ori/2. The log f t value for beta decay from Ge  p  to the 265 kev. level i s 5«6  (i.e. an allowed  beta decay). Hence the spin of the 265 level i s either 3/2 or l / 2 . The value of l / 2 would be preferred since the 265 dev. transition i s an E2 type and goes to the 3/2 ground state. This choice would require the multipolarity of the 137 kev. transition to be at least E2. However, the measured conversion coefficient for the 137 kev. transition i s near the theoretical value for Ml but a factor 10 smaller than the theoretical value for an E2 transition. Tentatively, therefore, a spin of 3/2 i s assigned to the 265 kev. level. The 200 kev. transition has a measured conversion coefficient which agrees very well with an Ml assignment. Thus the possible spins for the 200 kev. level are 5/2, 3/2 or l / 2 . Unfortunately, the measurei conversion coefficient for the 66 kev. transition isnot very accurate. I t appears to be closest to the theoretical value for Ml which would be consistent with spin assignments of the 265 and 200 kev. levels.  -  39  -  Looking at the 30!? kev. transition, i t i s seen that i t could be M3, E2 or Ml with M3 more likely on the basis of spin change alone. This assignment agrees, within experimental error, with the measured conversion coefficient. Again, however, this conversion coefficient i s not known too accurately because of the 16 Wintensity of the 30£ kev. transition. An assignment of M3 would mean that this transition would 21 have an expected lifetime of 0.08 seconds. The existence of such an isomeric state i s not expected. Feenberg 26 ?7 and Hammack  and Nordhienr' have pointed out that "islands of isomerism"  exist for odd A nuclei with 39<odd Nor Z) * h9 and for 63*(odd N or Z)«8l. A -* with 33 protons and even N does not f i t into either one of these 7  s  islands. Therefore, i n the absence of definite evidence for the existence of an isomeric state, the 30J> kev. transition has tentatively been labelled E2. I t might, however, be noted here that identifying the 30J> level as an isomeric state would explain the pronounced 97 kev. peak i n the summed spectrum ^. 1  The 80 kev. transition was not detected i n the present investigation but i s reported to be weak.^ This vjould be expected from the spin assign7  ment of the 280 and 200 kev. levels.  -Uo  -  Conclusion. The results of this investigation are summarized by the decay scheme i n figure 17. The sequence of energy levels i n As * seems to be 7  fairly well established. The spin assignments to the various levels are made on the basis of foregoing argu ments, and differ i n some respects from those proposed by Schardt and Welker.^ In cases where unambiguous 7  assignments could not be made, the most probable spin i s indicated f i r s t , with the less probable ones enclosed i n parenthesis. I t i s expected that more accurate measurements on the low intensity transitions w i l l provide the necessary information to remove these ambiguities.  - la -  APPENDIX 1 Gamma Ray Detection Efficiency of a N*I (Eh) Crystal Gamma rays that are incident on a scintillation crystal may be . absorbed through one of three processes; they are, photo electric effect, Compton effect and pair production. The latter i s possible only i f the gamma ray energy i s greater than 1.02 mev. Since the present work deals only with gamma rays having energies less than U00 kev., absorption by pair production need not be considered here.  In a photo electric i n -  teraction the entire gamma ray energy i s transferred to a bound electron. Thus the intensity of the scintillation produced i s proportional to the gamma ray energy (provided that the X-ray produced i s also absorbed). The Compton effect on the other hand results i n only part of the original gamma ray energy being transferred to an electron, the remainder appearing as a scattered photon. Escape of the scattered photon from the crystal, then results i n a scintillation, the intensity of which i s proportional to the energy transferred to the electron. Since this energy i s not fixed, but depends upon the scattering angle, Compton processes result i n a rather broad pulse height distribution, unlike the sharp peaks produced by photo electric processes. The scattered photon may however undergo further interactions in the crystal and ultimately the entire gamma ray energy may be absorbed in the crystal. This multiple process results i n the same pulse height as  - U2 -  i f the gamma ray had been absorbed through a photo electric process directly. Such a process i s quite desirable since i t increases the intensity of the photo electric peak, and as we shall see below, only this peak i s of interest i n calculating relative intensities of gamma rays. The photo electric peaks have a shape which i s indist. inguishable from a Gaussian curve. It i s therefore a f a i r l y simple matter to obtain the areas under these peaks. This area i s proportional to the number of pulses i n the photo-peak, and when corrected for photo-peak detection efficiency one has a measure of the relative gamma ray intensity. Using total absorption cross sections for Nal , Bell- - has pre1  2  pared a set of curves, for a number of source td crystal distances, giving the total detection efficiency as a function of gamma ray energy for a 1" x i f " crystal. A variation i n efficiency with source position results from crystal edge penetration effects, i.e. for moderate source distances the thickness of crystal seen by the gamma ray varies considerably with different angles of emission. By multiplying the total efficiency factor obtained from these graphs by the fraction of pulses that are i n the photo-peak, one gets the corresponding photo-peak efficiency. The peak to total ratio as a function of energy has been measured by B e l l ^ 2  for the 1" x i f " crystal. A measured rather than a calculated ratio must be used because the amount by which the intensity of the photo-peak i s increased by multiple absorption processes i s difficult to calculate.  -U3 -  To summarize then, relative gamma ray intensities are obtained from a scintillation spectrum as follows: 1) Gaussian curves are fitted to the photo electric peak and the area under the peak determined. 2) From Bell's curves the appropriate total efficiency i s determined and multiplied by the peak to total ratio. 3) The photo-peak area i s divided by the photo-peak efficiency (determined i n 2) to give a measure of the relative gamma ray intensity.  APPENDIX I I  Correction for the Escape of the Iodine X-ray Low energy gamma rays are absorbed through a photo electric process very near the surface of the crystal, consequently the I X-ray so produced has a good chance to escape from the crystal. This then results i n an escape peak at 30 kev. below the main phdbo electric peak. The ratio of the number of X-rays that leave the crystal to the number of gamma rays incident has been measured ? as a function of 2  gamma ray energy. In the case of the Se * spectrum the escape peaks of the 7  (121, 137) kev. gamma rays f a l l very near to the 97 kev. photo-peak,, and the escape peak of the 97 kev. gamma ray f a l l s on the 66 kev. peak. Because of the presence of the escape peaks the intensities of the 66 and 97 kev. gamma rays i n measured using the following procedure: Gaussian curves are fitted to the spectrum at (121, 137), 97 and 66 kev. The half width of the Gaussians at 97 and 66 kev. are determined by using the fact that the resolution of the scintillation spectrometer i s inversely proportioned to J~I} where E i s the gamm ray energy. The area of the (121,137) peak i s multiplied by the fraction (0.02) of the X-rays that escape. The resulting product i s then subtracted from the measured area of the 97 kev. peak. This area i s then corrected for the fraction (0.06) of the X rays that escape from the 97 kev. peak. The result i s then taken as the actual  area of the 97 kev. photo electric peak, and from which the relative intensity i s obtained. Using the corrected photo peak area for the 97 kev. gamma ray the intensity of the 66 kev. gamma ray i s obtained in a similar fashion.  BD3LI0GRAPHT  1. 2. 3. U. 5. 6. 7. 8. 9. 10. 11. 12. 13. 111. 15. 16. 17. 18. 19. 20. 21. 22. 23. 2k. 25. 26. 27. 28. 29.  -46-  Fermi Z.Fhys.k. 88, l 6 l , 193k National Bureau of Standards; Tables for the Analysis of Beta Spectra. Moszkowski Phys.Rev. 82, 3$ (1951). Rose and Goertzel Appendix 17 K.Seighahn and ray Spectroscopy. Frauenfelder page 5U9 K. Seighahn and ray Spectroscopy. Biedenharn and Rose. Rev.Mod.Phys. 25, 7U6 (1953). Mayer Phys.Rev. 78, 16, 1950 Haxel, Jensen Suess Phys.Rev. 75, 1766L (19U9) Z.Physik. 128, 295, (1950) Bohr and Mottelson DaniMat.Fys.Medd. 27 (1953) K.Seigbahn and ray spectroscopy (Chap.17) Alder, Bohr, Huus, Mottelson, Winther and Zupancic Rev.Mod.Phys. as ,+%t. 0»*O Goldhaber and Sunyar Phys.Rev. 83, 906, 195l. McMahon Master's Thesis U.B.C. RSS.Bell p.l5U K.Seigbahn Beta and Gamma Ray Spectroscopy. Milley Master's Thesis U.B.C. Ter-Pogossian, Robinson and Cook. Phys.Rev. 75, 995 (19H9). Cork, Rutledge, Branyan, Stoddard and LeBlanc. Phys.Rev. 79, 889 (1950) Jensen Laslett Martin Hughes and Pratt Phys.Rev.90, 557 (1953). Schardt and Welter Phys.Rev. 99, 810 (1955) Temmer and Heydenburg Phys.Rev. 93, 35l (195U). Lu Kelly and Weidenback Phys.Rev. 97, 139 (1955). Davisson and Evans Rev.Mod.Phys. 2k, 79, 1952. Moszkowski Seigbahn Beta and Gamma Ray Spectroscopy, page 391 A amodt, Fletcher, Silvey and Townes Phys.Rev. 98, 122U (1955) Trail and Johnson Phys.Rev. 91. U7U (1953). KLinkeriberg Rev.Mod.Phys. 2U, 63, (1952). DeBenedett: and McGowan Phys.Rev 7U, 728, (19U8). Feenberg and Hammack Phys.Rev. 75, 1877 (19U9). Nordhiem Phys.Rev. 75, 189U (19U9) Bell, P.R. Seigbahn - Beta and Gamma Ray Spectroscopy, page 139 Bell, P.R. lot. t i t . page 155.  


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