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The radiations of Se⁷⁵ and Sb¹²⁴ McMahon, Garfield Walter 1955

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THE RADIATIONS OF Se 7 5 AND Sb 1 2 / f by GARFIELD WALTER McMAHON A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of PHYSICS We accept this thesis as conforming to the standard required from candidates f or the degree of MASTER OF SCIENCE Members of the Department of Physics THE UNIVERSITY OF BRITISH COLUMBIA 1955 i ABSTRACT The radiations of Selenium 75 and Antimony 124 were examined with a thin lens magnetic spectrometer at a resolution of 2.% in momentum. Selenium 75 The gamma ray and internal conversion spectra were obtained for Se^ which decays by K-capture to As^. Tran-sitions were observed at the energies 66, 98, 121, 137, 198, 265, 280, 303, and 400 kev. Accurate K/L ratios were ob-tained for the 98 kev. and 137 k e v » transitions. A decay scheme is proposed with tentative spin and parity assignments to several energy levels. Antimony 124 The beta ray and internal conversion spectra of Sb* 2 4 were examined. Five beta groups were observed with end-point energies 2 .315, 1.58, 0.94, 0 .60 , and 0.35 mev. Conversion lines were observed for transitions of energies 604, 651 , and 723 kev. Conversion coefficients were cal-1 culated on the basis of the decay scheme of Langer et al . A l l three transitions have conversion coefficients which would correspond to electric quadrupole radiation. ACKNOWLEDGMENT The research described in this thesis was made possible by the award of a National Research Council Bursary and also through Grants-ih-Aid-of-Research to Dr. K.C. Mann, by the National Research Council. The author wishes to express deep appreciation to Dr. K.C. Mann who supervised this work. TABLE OF CONTENTS Page Abstract i Acknowledgment 11 I. General Introduction 1 II. Equipment 11 III. Experimental Procedures 15 IV. Investigation of Radiations of Se 7^ 19 124 V. Radiations of Sb 30 References 35 TABLE OF ILLUSTRATIONS Figure Facing Page 1. Schematic Diagram of Spectrometer 2. Detector Head 3. ^ Beta-Conversion Source 4. Photoelectron Source . . . . . 75 5. Decay Scheme of Se 6. Decay Scheme of Sb"*"24  Plate I Gamma-Ray Spectrum of S e ^ . 75 II Conversion Electron Spectrum of Se ' . . . 124 III Beta-Conversion Spectra of Sb IV Kurie Plots for S b 1 2 4  1 I. GENERAL INTRODUCTION The aim of nuclear spectroscopy is to obtain as much information as possible on the radiations emitted by radioactive nuclei. It is hoped that ultimately this infor-mation may form the basis of a sound and adequate nuclear theory which at present does not exist. A brief outline of some of the Information which may be obtained by the nuclear spectrocopist is presented on the following pages. Nuclear Energy States It is known that nuclei can exist in a number of energy states the state of lowest energy being called the ground state. A nucleus which is in an excited state w i l l tend to decay to a lower energy state by the emission of a gamma ray or an orbital electron. There are other modes of decay which become important at high excitation energies but they are not important in most cases encountered in beta-ray spectrocopy. The nucleus may decay directly to the ground state or i t may f i r s t decay to an intermediate level before going to the ground state in which case two transitions w i l l be in cascade. The alternative process to gamma ray emission is internal conversion in which the transition energy is imparted to an orbital electron. The emitted electron then has an energy equal to the transition energy minus its orbital 2 binding energy in the atom. For a given transition the ratio of the probability of decay by Internal conversion to the probability of decay by gamma-ray emission is defined as the "conversion coefficient" of that transition. Multipolarity of Transitions In each energy state the nucleus possesses a definite "Spin" (angular momentum) and "parity". Parity is a property of the wave function describing the nucleus. The nucleus may have either "even" or "odd" parity depending on whether the wave function is symmetric or antisymmetric with respect to reflection in the coordinate axes. In a l l cases of transitions from one energy state to another spin and parity must be conserved. It is clear then that a transition from state a higher to a lower energy^is characterized by the energy change, spin change,and parity Awhich accompany the transition. Transitions are conveniently classified as to their spin and parity change, by defining what is called the multlpole order of the transition. For example some of the designations used are electric dipole (El), magnetic dipole (Ml), electric quadrupole (E2), magnetic octupole (M3), etc. Each of these names signifies a definite spin change and parity change ( i f any). (See Table I). 3 TABLE I Spin and Parity changes associated with Multipolarity of Transitions Vector Spin Change 1 2 3 4 5 Multipole Order E l Ml E2 M2 E3 M3 E4 M4 E5 Parity Change Yes No Yes No Yes Although changes in spin and parity are not directly observable, there are certain measurable quantities which are dependent on the multipole order. Some of these are as follows: (i) Lifetimes of the excited states. ( i i ) Angular correlation between two gamma rays emitted in succession, which hence involves the spins and parities of three nuclear energy states. ( i i i ) Conversion coefficients. These have been calculated 2 theoretically and tabulated by Rose et a l . , Spinrad, and others. (iv) K/L ratios, that i s , the ratio of intensities of K-shell and L-shell conversion electrons. Goldnaber 4 / and Sunyar give empirical curves of K/L ratios. The latter two of the quantities mentioned above have been used in the present work to determine the multipole order of some transitions. 4 Beta Decay One of the commonest methods of obtaining nuclei in excited energy states is from the beta decay of natural or a r t i f i c i a l radioactive isotopes. Beta decay includes the processes of negatron emission, positron emission and orbital electron capture (K-capture). From the "parent" or radioactive nucleus, one or more beta-decay groups of different energies lead to as many different energy levels of the "daughter" or product nucleus. In positron and negatron emission processes the beta particles have a con-tinuous energy spectrum from zero to a discrete maximum energy, which is called the end-point energy. In the K-capture process there are no primary beta particles and the only observable particle spectrum is that of conversion electrons which follow in the daughter nucleus, should they exist. The Neutrino Hypothesis It has been established experimentally that the transition energy associated with a beta decay group is discrete and equal to the end point energy in the particle emission processes. However, since the beta spectrum is con-tinuous, most of the beta particles have less than the end-point energy. Also no radiation is observed to carry away the transition energy in the K-capture process. To explain this apparent violation of the law of conservation of energy Pauli, in 1927, suggested that a new particle, the neutrino, 5 is emitted during the beta decay process. The neutrino carries away a l l of the transition energy in the K-capture process and shares i t with the beta particle in the particle emission processes. The properties of the proposed neutrino are such as to allow i t very l i t t l e interaction with matter, hence i t is essentially unobservable. It has (1) very small or zero rest mass, (2) no change, and (3) a spin of 1/2-n. The neutrino hypothesis also explains other apparent v i o l -ations of the conservation laws for linear and angular momentum, observed during beta-decay. Fermi Theory of Beta Decay theory of beta decay which predicted the shape of the beta spectrum very satisfactorily in most cases. From his theory the momentum spectrum of the beta particles is given by is the Fermi function, a complex function of energy, E and atomic number, 2 , which has been calculated and tabulated.^ The Fermi function corrects the shape of the spectrum for the electrostatic force between the nucleus and the emitted beta particle. Equation (1) predicts a tangential approach to Eo, which is as observed. Hence a plot of the number of electrons, Using the neutrino hypothesis Fermi developed a ?(p) dp = C F ( Z t E ) p 2 ( £ o - E ) 2 d p 6 versus p makes the determination of Eo very d i f f i c u l t . If however we plot ( M f ) / F ) 1 ^ 2 against energy we see from equation (1) that a straight line should be obtained with an intercept Eo on the energy axis. This straight line plot, known as a Kurie plot, is used to determine the end point energy of beta groups by a simple extrapolation. If there are two or more beta groups, and we assume that the beta groups are independent, the Kurie plot is a straight line only between the end points of the two highest energy groups. The straight line is then extra-polated toward zero energy and the spectrum of the highest energy group is determined using values from the extra-polated line. This group is then subtracted from the total beta spectrum and another Kurie plot is made of the remainder. This process is repeated u n t i l a separate Kurie plot has been obtained for each beta group. It is evident that after two or three aubtractions the errors introduced by this method may become quite large. Beta decay processes are classified as allowed, first-forbidden, second-forbidden, etc. depending on the spin and parity changes undergone by the nucleus during the transition. (See Table II). The theory outlined above holds s t r i c t l y only for allowed transitions. If a beta group f a l l s in one of the forbidden classes, the quantity, C, in equation (1) is no longer a constant, but depends somewhat on momentum, and the Kurie plot deviates slightly from a straight line. 7 Satisfactory expressions for G have been calculated theor-etically? for the f i r s t forbidden cases. Such a correction factor is applied in the present work to the high energy 124 group of Sb to obtain a straight line plot. TABLE II Order of Transition Orbital angular momentum change, A1 Total spin change, A l Parity change ATT Log f t value (approx Allowed 0 0,1 NO < 6 Fir s t forbidden 1 0,1,2 YES - 8 1-forbidden 2 1 NO ~ 6 Second forbidden 2 2,3 NO — 12 Third forbidden 3 2,3,4 YES > 12 A measurable quantity which gives an indication of the order of a beta transition is the "ft-value" or com-parative lifetime. From equation (1) we obtain for the total probability of decay: < ? - c / P * i X F ( z , e V ( E 0 - E ) 2 d P = v t where t is the mean l i f e of the transition. If we define O then f t - 1 (2) C The ft-value thus depends on the nuclear factor, C, which 8 is a constant in the allowed case. Moszkowski gives graphs 8 and nomographs for calculating l o g ^ Q f t values. The order of a beta transition cannot be definitely assigned from i t s ft-value as there is overlapping between orders. Approximate ranges of log f t values are given i n Table II. These values are taken from the observations of Nordheim^. Decay Schemes A decay scheme is a diagram which represents schema-t i c a l l y the decay of a nuclear species. A complete decay scheme should show the end point energies and relative i n -tensities of a l l beta groups, the proper sequence of energy levels in the daughter nucleus, the energies of a l l radiative transitions between the energy levels, and the spin and parity of the nucleus at each energy level. The spin of stable or long lived nuclei only, can be measured directly by spectroscopic or other methods. Recently a method has been described"^ for obtaining certain radioactive nuclei in concentrated form so that the spin may be measured, however this is done only with d i f f i c u l t y . Hence, the spin of the ground state of the parent nucleus is generally not known. The spin of the ground state of most stable nuclei have been measured, and i f not there are certain empirical rules for spin assignments which w i l l be discussed in the following paragraph. Since the daughter nucleus is usually stable the spin of i t s ground state is probably known. One imperical rule for which no exceptions have been found 9 is that a l l even-A, even-Z nuclei have spin zero. Nuclear Shell Model 11, 12, The nuclear shell model of Mayer et a l , has proved useful in setting a spin value on the ground state of nuclei in many cases. Tables of nuclear shell structure 1^ 12 are given by Klinkenberg and Mayer et a l , Examination of Gamma Ray Spectra Since magnetic spectrometers are designed to examine electronic spectra, the gamma-ray spectra may be observed only i f the energy of the gamma-rays is imparted to electrons. The photoelectric effect, in which a photon is absorbed by an atom and an orbital electron is ejected, provides a means for obtaining such an electronic spectrum. The ejected electron has an energy equal to the gamma ray energy minus its orbital binding energy in the atom. Ejection of a K-shell electron is most probable ( i f energetically possible) then L-shell, and so forth. The photoelectric absorption coefficient is highest for high-Z atoms, hence Uranium, Thorium, Lead, etc., are generally used as the "radiator" or source of photo-electrons. The radiator must be thin to minimize self-absorption of the electrons. Also a low- 2 absorber must be placed between the source and radiator to absorb a l l particles emitted by the source i t s e l f , such as conversion electrons or primary beta-particles. Few gamma-rays are absorbed be-cause of their relatively low absorption coefficient. 1G Another gamma-ray interaction process, giving rise to electrons with a continuous energy spectrum, is the Compton process. This process is the elastic c o l l i s i o n of a photon with atomic electrons, mainly in the absorber. The spectrum of the Compton electrons is observed by removing the radiator. The Compton electron spectrum is then sub-tracted from the spectrum observed with the radiator to obtain the true photoelectron spectrum. Relative gamma-ray intertSiities are obtained by dividing the photoelectron intensity by the photoelectric absorption coefficient for the corresponding gamma-rays. Photoelectric absorption co-14 efficients are given by Davisson and Evans, and elsewhere. 11 II. EQUIPMENT The Spectrometer A thin-lens magnetic spectrometer of the type described by Deutsch et a l . , 1 - * was used. A cross-sectional diagram is given in Figure 1. The specifications of the instrument are listed in Table III. TABLE III SPECTROMETER SPECIFICATIONS Tube length 101.5 cm Tube inside diameter 19.6 Source to c o i l centre 52 .0 Window to c o i l centre $1.0 Source diameter 0 . 5 Window diameter 0 .6 Window baffle to window end of tube 2 0 . 9 Source baffle to window end of tube 77 .0 Source baffle slot inside diameter 7.75 Source baffle slot outside diameter 8 .75 Resolution 2.5$ Transmission (approx.) 0.2% The source baffle was the beam defining baffle. The window baffle was set to accept a l l electrons in the beam. SOURCE BAFFLE COILS WINDOW BAFFLE 4 o DETECTOR HEAD WINDOW, • 3 TO VACUUM PUMP Fig- I SCHEMATIC DIAGRAM OF SPECTROMETER 12 Two important characteristics of a l l spectrometers are the resolution or resolving power and the transmission. The resolution in momentum of the thin lens spectrometer is defined as defined as Af/p where Zip ia the momentum range of the particles accepted at any one momentum setting, p , For magnetic spectrometers the resolution is constant, i.e. Z\p/p is constant and hence Af °z p . The spectrometer does not measure the true momentum spectrum, given by N(p) , but measures N7p)Z\p . Hence to obtain the true momentum spectrum of the particles we must divide the counting rate obtained from the spectrometer by &p or (since Apocp) by p . The resolution is observed by measuring the width of a sharp line at half intensity Adividing the width by the momentum at the line. The electron energy of the line must be sufficiently high so that back scattering and self absorption effects are small. These tend to increase the line width and hence increase the apparent resolution figure. The transmission of the thin-lens spectrometer is relatively small; however the design permits the use of larger sources than some other spectrometers having higher transmission. Detection System The detector used was an R.C.A. 5819 photomultiplier tube with a s c i n t i l l a t i o n crystal of anthracene or potassium iodide. The detector arrangement is shown in Figure 2. A film of high viscosity silicone o i l was used between the 13 crystal and lucite cup and between the lucite and photo-multiplier face to provide good optical coupling. A pre-amplifier with a gain of about five, having a cathode follower output was built into the detector head. The complete detector head was enclosed in a light-tight cylinder of mild steel to provide magnetic shielding for the photo-muitiplier tube. The output pulses from the preamplifier were fed into a remote amplifier having a gain of about 200. From the amplifier the pulses were fed into an "atomic 1G1A" Scale of 64 scaler and then into a register. Thin crystals of about 0.5 mm thickness were used to achieve good light collection of photons generated in the crystal by low energy electrons. Since the low energy electrons are stopped near the surface of the crystal the light has less chance of being absorbed and scattered when travelling through a thin crystal. Because of evaporation of anthracene i t was necessary to replace the crystal after one to two weeks in the vacuum. A potassium iodide crystal was tried but i t proved to be inferior to the anthracene due to a much higher background noise count. Current Regulator The current regulator for the spectrometer c o i l was quite conventional, the current passing through a standard manganin resistance and a bank of 6A$7 regulator tubes. The voltage across the standard resistance was kept equal to the output of a calibrated potentiometer by a control circuit which set the bias on the regulator tubes. Any desired current setting could be obtained by setting the potentio-meter to the proper voltage output. The control system is described in detail by Ozeroff"^ and Daykln"1"7. Sources A l l sources used were five millimeters in diameter. The beta-particle and conversion sources were placed on a film of LC-600 about 0.04 mg./cm thick which was fastened on an aluminum ring. The source arrangement is shown in Figure 3. The aluminum cup was extended over an inch be-hind the source to minimize back-scattering. The sources 2 were approximately 0.2 mg./cm • thick. For the photoelectron source a small brass plate, f i t t i n g into the endplate of the spectrometer, was used as an absorber. The radioactive pellet was held to the brass plate by cellulose tape. A 5 diameter 20 mg./cm.2 thick Uranium radiator was held directly opposite the source with stopcock grease. The thickness of the brass plate between pellet and radiator was 1 mm. which was sufficient to stop a l l particles emitted by the source i t s e l f . The photoelectron source arrangement is shown in Figure 4. SPECTROMETER END - PLATE WINDOW-CRYSTAL-VACUUM li WAX RCA 5819 PM- TUBE LUCITE CUP MILD STEEL Fig- 2 DETECTOR HEAD WAX ^ 1 VACUUM — ? ' ALUMINUM CUP H J - L C - 6 0 0 ^—SOURCE ALUMINUM RING SOURCE PELLET A TAPE WAX VACUUM RADIATOR BRASS PLATE Fig- 3 Fig- 4 15 III. EXPERIMENTAL PROCEDURES Calibration of the Spectrometer The spectrometer was calibrated against the 1.170 60 and 1.330 mev. gamma rays from Co using both lead and uranium radiators as photoelectron sources. The calculations of the spectrometer constant from the four K-peaks agreed to within 0 . 3 $ . The mean value obtained for the spectrometer constant was 0.926 gauss-cm./ volt potentiometer setting, calibrating on the top of the peaks. St a t i s t i c a l Accuracy and Background The counting rates may be obtained from the ordinate of the graphs (Plates I, II, III,). 75 The s t a t i s t i c a l accuracy on a l l points of the Se gamma ray spectrum was better than % with better than 2% on most points including the low energy region. This entailed counting times of at least 20 min. per point with a more careful study of the low energy region and the region about the "303K" peak. The potassium iodide crystal was used and i t was necessary to allow a high background count (150 c.p.m.) to observe the low energy counts. On the Se 7^ conversion spectrum (Plate II) the st a t i s t i c a l accuracy was better than 2% on a l l peaks except those of extremely low counting rate. The counting time was 16 15 minutes per point with 30-40 min. per point in the regions of low counting rate. The thin anthracene crystal gave good low energy counting efficiency with a background of 8 c.p.m. 124 The counting time on the Sb spectrum (Plate III) was varied from 20 min. per point to 40 min. per point to obtain at least 2% accuracy to near the end point. On the conversion peaks the counting time was 40 min. per point with 80 min. per point on the 723 kev. peak. Background was 16-15 c.p.m. Preparation of Sources Selenium 75 - When f i r s t obtained the source was in 75 the form of a pellet of metallic selenium. The Se'^ isotope was prepared by irradiation of natural Selenium with thermal neutrons in the Chalk River p i l e , the reaction being S e 7 + ( h , Y ) Se* Although the natural abundance of Se 7 4" is less than 1%, i t has a high neutron cross-section. Also, a l l other radioactive Selenium Isotopes produced have very short or very long l i f e -times compared to that of S e ^ which i s 125 days. For the photoelectron source the Selenium pellet was used directly as described previously. After completion of the photoelectron spectrum the Selenium pellet was removed and dissolved in concentrated n i t r i c acid. The solution was evaporated almost to dryness and diluted slightly with water leaving a concentrated solution of Selenious acid. The source backing used was a / 2 film of LC-6G0, approximately 0.04 mg/cm thickness, placed on an aluminum ring. A drop of dilute insulin solution was placed on the film and spread out to a diameter of 5 mm. to define the size of the source. After the insulin had dried a drop of the Selenious acid solution was placed on the insulin spot and allowed to dry almost completely. The source holder was then placed on the bottom of a small inverted beaker inside a larger beaker. A few drops of hydrazine were put into the large beaker taking care that none touched the source or holder. A watch glass was placed over the large beaker and the apparatus was l e f t undisturbed for about four hours. The Selenious acid exposed to hydrazine vapor was reduced to red Selenium. The source was allowed to dry completely and a drop of very dilute collodion in ether solution was placed over i t to prevent any flaking off. The reaction between hydrazine and Selenious acid produces only Selenium and volatile substances, thus making the source thinner than i f Selenious acid alone were used. This aids in reducing self absorption and back scattering effects. The activity of the Se 7^ conversion electron source was estimated to be about 30 microcuries. Antimony 124 - The source was obtained in the form 124 of Antimony metal chips. The Sb isotope was prepared by irradiation of natural Antimony with slow neutrons in the Chalk River p i l e . The natural element consists of the two isotopes Sb and Sb . The Sb isotope which is also fom& in the pile has a h a l f - l i f e of only 2.8 days while 124 ; 122 Sb has a h a l f - l i f e of 60 days. The Sb is allowed to decay several half-lives before any observations are made 124 on Sb . 124 To prepare the Sb beta source the chips were dissolved in aqua regia. The solution was evaporated almost to dryness and then diluted slightly with water, leaving a concentrated solution of SbCl^. The source holder was pre-75 pared in an identical manner to that used for the Se conversion source. A drop of the SbCl^ solution was placed on the insulin spot and evaporated to dryness under a heat lamp. A drop of dilute collodion in ether solution was placed over i t immediately, since SbCl^ is deliquescent. The activity of the source was estimated to be about 30 microcuries. 19 IV. INVESTIGATION OF RADIATIONS OF Se 7^ Outline of Previous Work 18 75 (a) Freidlander et al. discusses the production of Se by pile irradiation of natural Selenium. (b) Ter-Pogossian et a l . " ^ examined the photoelectron o spectra with a 180 magnetic spectrometer using both lead and uranium radiators. They report gamma rays at the energies 76, 99(?), 123, 137, 267, 283, 4-05 kev . They did not extend their examination of the low energy region below the 137 kev. peak with the uranium radiator. (c) Cork et al. studied the conversion electron spectrum in a spectrometer with photographic detection. They report transitions of energies 24.7, 66.2, 80.8, 96.8, 121.2, 136.2, 198.8, 265.2, 280.1, 304.0 and 401.9 kev. 21 (d) Jensen et al. examined internal conversion and 7< photoelectron (lead radiator) spectra of Se" with a thin-lens spectrometer. They report transitions of energies 67, 77, 98, 124, 138, 203, 269, 281, 308, and 405, kev. They also carried out absorption coincidence measurements. pp (e) Lu, Kelly, and Weidenbeck have examined the gamma rays of Se?5 with a scintillation spectrometer incorporating a crystal summing technique. (f) Schardt and Welker2^ studied the gamma rays of Se 7^ and GS7^ with a scintillation spectrometer and greg-wedge coin-cidenee analyser. Ge?5 decays to A s ^ by negatron emission. 75 Gamma rays at 269, 203, and 67 kev. were observed from Ge . (g) Aamodt and Fle t c h e r 1 0 have recently measured the spin and quadrupole moment of S e ^ from a sample prepared by cyclotron bombardment of As?-* with deuterons. The spin was found to be 5/2, contrary to the shell model predictions of 9/2 or 1/2. Nucleon configurations which would give rise to the observed spin and quadrupole moment are discussed. Results of Present Work 75 The gamma ray spectrum of Se is shown in Plate I. The energies and relative intensities of the gamma rays c a l -culated from the graph are presented in Table IV. The internal conversion spectrum is given in Plate II. The transition energies,relative Intensities and K/L ratios of the conversion lines are presented in Table V. A preliminary photoelectron spectrum was obtained for S e ^ using a lead radiator. The low energy efficiency of the run was poor. The peaks observed in order of energy were 137L, 137M, 26%, 280K, 303K, 2 6 ? L , 280L, 400K and 400L. The letters K, L, M, refer to the electron shell in the radiator from which the peak originated. A small uncertain peak at an electron energy of 147 kev. was also observed. This would correspond to a 2 3 5 K or a 163L peak. It is interesting to note that the peak labelled 328K in Plate I may also have been identified as 234L. These peaks must be 2 6 5 K SELENIUM 75 GAMMA-RAY SPECTRUM (URANIUM RADIATOR) 20 MGj/CM 2 COMPTON ELECTRONS COMPTON a PHOTO-ELECTRONS 2 8 0 K 2 6 9 L V „ » •14 0(6 MAGNET CURRENT SETTING 0 2 2 0 - 2 6 1— I40Q ' [ IS 00 (GAUSS-CM ) — I — 2 2 0 0 + 2600 SELENIUM 75 CONVERSION ELECTRON SPECTRUM 21 verified before a positive identification can be made. The decay scheme proposed in this work is based on the identification of the multipole order of transition 2, and on the agreement of this identification with the K/L ratios of other transitions. The K/L ratio of 6.8 obtained in the present work and the high conversion coefficient 21 reported by Jensen et a l . appear to indicate magnetic octupole (M3) radiation for transition 2. However a meta-stable state would then be expected with a lifetime of approximately 5 minutes (calculated from the curves of Gold-4 21 haber and Sunyar ). Jensen et a l searched for such a state with negative results. Also If transition 2 is identified as M3 there is some disagreement between multipole assignments from K/L ratios and conversion coefficients of other tran-sitions, as w i l l be mentioned later. The K/L ratio of 6.8 in close to that expected for E2 or M2 radiation from the empirical curves of Goldhaber and Sunyar4", hence, transition 2 has been tentatively identified as E2 or M2 radiation. Electric quadrupole (E2) appears favoured since the lifetime of an M2 transition of 98 kev. is expected, from the Weisskopf 24 L o formula to be in the region 10""° to 1G"J secondhand De Benedetti and McGowan J found no delayed coincidences in this region for Se^. The theoretical K-conversion co-e f f i c i e n t s 2 ^ for E2 and M2 transitions are 0.7 and 1.1 respectively. A conversion coefficient of 1.0 was assumed for transition 2. Conversion coefficients for the other 22 transitions were then calculated by dividing the K-conversion intensity from Table V by the corresponding gamma ray intensity from Table IV and multiplying by the factor which made for transition 2 equal to 1.0. These K- conversion coefficients together with the multipole order assignment 2 3 obtained from the tables of Rose et a l . and Spinrad are presented in Table VI. The K/L ratios of three transitions were sufficiently accurate to use in identification of multipole order. These are given in Table VI, for comparison with the assignments from conversion coefficients. The agreement is very good, which would not be the case had transition 2 been assigned the multipolarity, M3, with the corresponding conversion coefficient of about 7. It must be pointed out that the relative conversion coefficients in Table VI w i l l be accurate only to a factor of about two due mainly to inaccuracy in the photoelectron intensity of transition 2. The average energy of each transition and the total transition intensity (gamma rays plus conversion electrons) are presented in Table VI. The average energies were c a l -culated from the data in Tables IV and V and rounded to the nearest kev. These energies should be correct to within 1$. 20 They agree very well with the results of Cork et a l . and are in general slightly lower than the energies given by Jensen . , 21 et a l . TABLE IV Gamma-Bay Data (Intensities referred to transition 6) Transition Number Energy from K-Peak L-Peak Gamma Intensity K-Peak L-Peak Mean Intensity 1 ~ 0 . 0 1 2 A 2 97.0 - 0 . 0 5 8 — 0.06 3 121.2 - 0 . 1 0 — 0.10 4 136.5 0.55 0.55 5 £ 0 .02 £ 0.02 6 265.5 264.5 1.00 1 ,00 1.00 7 279.8 (obs) 0.35 0.37 0.35 8 302.8 0 . 35±.©l o.035±.oi 9 328 - 0.02 ~ 0.02 10 / 400.3 401 0.25 0.26 0.25 ft Intensity taken from Schardt and Welker TABLE V/ Conversion Electron Data (Intensities referred to 265-kev. transition) Transition Number Energy from K-peak L-peak K-peak Intensity K/L Ratio 1 6 6 . 0 6 5 . 5 0.33 ~ 7 2 97 .5 98 7.80 6.8±0.6 3 121.3 1.57 4 136.6 136.8 3.78 11.4±1.0 5 198.2 (obs) 0.057 6 265.6 LOG 7 280.1 0 . 5 8 303.7 (obs) 0.13 9 10 400.1 0.026 TABLE VI Transition Average Conversion Number Energy Coefficient 1 66 — 0 . 1 6 A 2 98 1.0 3 121 0.12 4 137 0.069 5 198 ^ 0.02 6 265 0.0077 7 280 0.11 8 303 ~ 0.03 9 328 10 400 0.0008 Transition Multipole Order Intensity from o(.K from K/L ~ 0.02A El,Ml El,Ml ~ 0.12 E2,M2 M3,M2,E2 ~ 0.11 M1,E2 0.58 El,Ml El,Ml $ 0.02 (Ml)? 1.00 Ml 0 .35 E2,M1,M2 ~ 0.04 M2,E3 — 0.02 0.25 El,Ml 23 A Gamma ray intensity from results of Schardt and Welker Fig- 5 DECAY SCHEME OF 'Se 26 The decay scheme proposed for Se^ -* is shown in Figure 5. It is similar to that given by Jensen et a l . 2 1 except for the 77 kev.transition and the sequence of tran-sitions 1 and 5. The 328 kev. transition is not included since the identification of the peak is not certain. A discussion of the spin and parity assignments is presented on the following pages. 26 T r a i l and Johnson have measured the threshold energy of the reaction and obtained 1.674±.005 mev. The energy difference between 75 n< the ground states of As and Se calculated from this values threshold energy is 900 kev. Log ft Acan now be calculated g from the decay scheme using the graphs of Moszkowski . The values obtained are: Group 1, 6.2; Group 2, 7.0; Group 3» 6.8; Group 4, 7.7, assuming only 5$ of the transitions go to the ground state of As' ?. The spin of the ground state 75 f of As has been measured and is 3/2(presumably with odd \ 75 parity, p 3 / 2 , — j . The spin of the ground state of Se 10 recently measured is 5/2 with unknown parity. The K-capture transition to the 400-kev. level is the most intense and is probably allowed. The log ft value is close to that for allowed transitions. The assignment of 5/2 as the spin of the 400-kev. level is consistent, and transition 10 then becomes E l or Ml radiation in agreement with the assignment given in Table VI. Schardt and Welker 2^ report an 11$ beta decay branch 75 from 80 min. Ge y in coincidence with a 265 kev.gamma ray, and with an end point — 2 5 0 kev. below that of the main beta group. The main beta group has an energy of approximately 27 1.2 mev. Log f t values calculated for both beta groups were approximately 5 . 2 , which indicates that both transitions are allowed, one to the 265 kev. level and the other to the 75 ground state in As . From the shell model the ground state of Ge7^ should be 9/2X+ or 1/2 , - . Since there is an allowed 75 transition to the ground state of As then 1/2,- is the most probable configuration. The 265-kev. level is assigned a spin 3 / 2 , - . The beta transition from Ge?5 is then allowed and transition 4 is E l , or Ml in agreement with the assign-ments in Table VI. The assignment of 7/2 + to the 98-kev. level is consistent with the identification of transition 2 as E2 or M2 radiation and with the log f t value for Group 4 which indicates a f i r s t forbidden transition. The parity of the ground state of Se7-' is not known; however, even parity (•+•) appears to be favoured, since Jensen ?1 et a l . report relatively few K-capture transitions to the ground state of As 7^. The transition is probably at least f i r s t forbidden (5/2 , + — * 3 / 2 ! - ) . This is in agreement with a statement by J.P. Welker, quoted by Aamodt and Fletcher 1 concerning the parity of Se?5. 28 A 77-kev. t r a n s i t i o n was reported by two i n v e s t i -g a t o r s ^ ' 2 ^ and was not observed here. The smallest possible conversion c o e f f i c i e n t ( E l or Ml radiation) f o r t h i s t r a n s i t i o n from the tables of Rose et a l . 2 and Spinrad-^ i s — 0 . 1 . For a t r a n s i t i o n of the i n t e n s i t y re-P l ported by Jensen et a l . , the conversion electron i n t e n s i t y would then be at least as great as that of the 265-kev. t r a n s i t i o n using the conversion c o e f f i c i e n t given i n Table VI. f o r t r a n s i t i o n 6. Other i n v e s t i g a t o r s 2 2 ' 2 ^ using s c i n t i l l -ation spectrometers report a much lower i n t e n s i t y f o r the 77 kev. t r a n s i t i o n . The only explanations that can be offered f o r the above res u l t s are that either a f a u l t y assign-21 ment was made to the two peaks observed by Jensen et a l . , and Ter-Pogossian et al.^-9 or that an impurity was present i n the sources used. There i s evidence that the 66 kev. t r a n s i t i o n pre-cedes the 200 kev. t r a n s i t i o n i n the cascade. They have been observed i n coincidence by Schardt and Welker2-^ from both S e ^ and Ge^. In G e ^ the in t e n s i t y of t r a n s i t i o n 5 i n com-parison to that of t r a n s i t i o n 6 i s reported much higher times) than i n S e ^ . This indicates a very low inten-s i t y beta t r a n s i t i o n may take place to the 200-kev. l e v e l . The most important spin and p a r i t y assignments i n the decay scheme have been discussed above. These assignments are not intended to represent completely correct spins or p a r i t i e s but serve to show that the decay scheme i s consistent. There is evidence from research now in progress in this laboratory that the decay scheme of S e " may be even more complex than that proposed in the present work. Considerably 75 75 more study is necessary on the radiations of Se and Ge with coincidence techniques, before an accurate decay scheme can be presented. 30 V. RADIATIONS OF ANTIMONY 124 Outline of Previous Work 28 29 (a) Cook and Langer and Kern et a l report five beta groups and five gamma rays. (b) Langer, Moffat, and Price^° studied the three high energy beta groups and the three low energy gamma rays obtaining the end-point energies 2 . 2 9 , 1.69, and 0 . 9 5 for the beta groups, and 607, 653> and 730 kev. for the gamma.rays. A search for the presence of orbital electron capture produced negative results. o-, 32 (c) Metzger-5,1- and Hutchinson and Wiedenbeck studied the 607 kev. transition carefully, identifying i t as electric quadrupole (E2) radiation. (d) Langer et a l * obtained end point energies 2.317, 1.602, 0.966, 0.61, and 0.24mev. for the five beta groups. A C ^ (Al = i, YES) was applied to the Kurie plot of the high energy group and a linear plot resulted. Gamma rays are reported at the energies 0.603, 0.641, 0.716, 1.68 and 2.09 mev. (e) Langer and Starner^ using triple coincidence techniques showed that the three gamma rays at 0.603, 0.641, and 0.716 mev. are in cascade. (f) Tomlinson et a l . ^ 4 " examined the beta and conversion spectra of Sb 1 2 4" in a high resolution magnetic spectrometer. 31 They observed conversion lines for transitions at 604, 648, 725, and 1697 kev. They report that the second most energetic beta group as well as the most energetic group has a Kurie plot indicative of a forbidden transition. 35 124. (g) Lazar examined the gamma rays of Sb with a sc i n t i l l a t i o n spectrometer. Two new gamma rays at 0,99 and 1,38 mev.are reported in addition to the five mentioned above,-Results of Present Work The beta and internal conversion spectra of S b 1 2 4 are shown i n Plate III. Kurie plots were made, using C^*^ 7 corrections on the two most energetic groups, and five groups were observed. The results are presented in Table VII. The Kurie plots are shown on Plate IV. A Kurie plot of the highest energy group without the C^1^ correction is shown for comparison. Although the spread in points for Group 2 is too great to notice any improvement in linearity i t was f e l t that the correction may affect the end point energies of the three lower groups. The end-point of the least energetic group is very uncertain after the four subtractions are made. The internal conversion data is presented in Table VIII. The conversion coefficients were calculated assuming the decay scheme shown in Figure 6 which is essentially the same as that proposed by Langer et a l . * The results agree closely with those of Tomlinson et a l . ^ 4 " which are shown ANTIMONY 124 KURIE PLOTS GROUPS 18 2 C? CORRECTION NO CORRECTION ENERGY (UNITS m.c*) IV Fig- 6 DECAY SCHEME OF Sb 32 for comparison. Except for the 651 kev. transition the energies should be accurate to within 1%. TABLE VII Beta Decay Data Group End-point Intensity Log f t energy (Mev) (%) Value 1 2.315 ±.01 15% 10.45 2 1.58 +.02 8% 10.1 3 0.94 ±.03 Q% 9.25 4 0.605 ±.01 52% 7.9 5 - 0 . 3 5 17% 7.*5 Transition Energy Present Tomlim-Work 604(K) 65100 72300 604 (L+M) son et a l 604(K) 648(K) 72500 TABLE VIII Internal Conversion Data Conversion Intensity Present Tomlinson Work 1.00 ~0.06 0.09±.02 604(L+M)0.18±.02 et a l 1.00 .054 .078 0.16 Conversion CoefficientXIO" Exper. value 3.7 -2.9 2.1+.5 Theoretical 2 E2 4.2 3.4 2.7 Ml 5.8 4.8 3.7 33 The assignment of the configuration 3>- to the 124- 1 ground state of Sb is discussed by Langer et a l . This assignment is made on the basis of the shell model and the 12 empirical rules of Nordheim, and also on the observation 124 , that beta transitions to the ground state of Te (even A, even2 nucleus, hence, spin 0,«0 should be observed i f a smaller spin were assigned. The multipole order of the 604-kev.transition has been established as electric quadrupole by a number of authors^-1'»32,34 and has been verified here. This sets the 604-kev.level as 2,*-. The conversion coeffic-ient of the 723 kev. transition indicates again electric quadrupole radiation. Hence the spin of the 1327-kev.level is probably 4 , * . The two most energetic beta-transitions w i l l then be both Al - 1, YES transitions. Assuming the decay scheme to be correct as far as i t goes, i t would appear from the high log f t values that a l l beta groups are forbidden, the three most energetic being 1st forbidden and the other two being at least 1-forbidden. These log ft values are in close agreement to those of Langer et a l . * Sb 1 2 4" has proved to be an extremely d i f f i c u l t nucleus to which to assign a comprehensive decay scheme, largely because of the complexity of i t s radiations and the low internal conversion coefficients of most of i t s 34 transitions. The only good agreement among investigators has concerned the two most energetic beta groups and the energies of some of the gamma rays. That portion of the decay scheme dealing with the higher excited states of 124 Te is s t i l l open to question. The results of the present work serve largely to confirm the results of Tomlinson u 1 et al.- 5 and of Langer et a l . 35 REFERENCES 1. L.M. Langer, N.H. Lazar, R.J.D. Moffat, Phys. Rev. 5 1 , 338, (1953). 2. M.E.Rose, G.H. Goertzel, B.I. Spinrad, J. Harr, P. Strong, Phys. Rev. 26, 1883, (194-9). 3 . B.I. Spinrad, Phys. Rev. , 2 8 , 1302, (1955). 4. M. Goldhaber, A.W. Sunyar, Phys. Rev. 8^, 906, ( 1 9 5 D . 5. E. Fermi, Z. Physik, 88, 161, (1934) 6. Tables for the Analysis of Beta Spectra, U.S. Dept. of Commerce, Nat. Bur. of Standards (1952) . 7. E.J. Komopinski, Phys. Rev. 88, 1266, (1952) . 8. S.A.Moszkowski, Phys. Rev. 82, 35, ( 1 9 5 D . 9. L.W.Nordheim, Phys. Rev. £8, 294, (1950) 10. L.C. Aamodt, P.C. Fletcher, Phys. Rev. ^ 8, 1224, (1955) . 11. M.G. Mayer, Phys. Rev. 2§» 16, ( 1950) . 12. M.G. Mayer, S.A. Moszkowskl, L.W. Nordheim, Revs. Mod. Phys. .24, 315, ( 1 9 5 D . 13. P.F.A. Klinkenberg, Revs. Mod. Phys. 24, 6 3 , (1952). 14. CM. Davisson, R.D. Evans, Revs. Mod. Phys. 24, 79, (1952) . 15. M. Dentsph, L.S. E l l i o t t , R.D. Evans, R.S.I. 1£, 178, (1944). 16. M.J. Ozeroff, M.A. Thesis, University of Br i t i s h Columbia. 17. P.N. Daykln, M.A. Thesis, University of B r i t i s h Columbia. 18. N.J. Friedlander, L. Seren, S.H. Turkel, Phys. Rev. 22, 2 3 , (1947). 19 M. Ter-Pogossian, J.E. Robinson, C.S. Cook, Phys. Rev. 21, 955, (1949). 36 20. J.M. Cork, W.C. Rutledge, C.E.Branyan, A.E.Stoddard, J.M. Le Blanc, Phys. Rev. 22> 889, ( 1950) . 21. E.N. Jensen, L.J. Laslett, D.S.Martin, P.J. Hughes, W.W. Pratt, Phys. Rev. ,90, 557, (1953). 22. D.C. Lu, W.H. Kelly, M.L. Widenbeck, Phys. Rev. 97, 139, ( 1955) . 23 . A.W. Schardt, J.P. Welker, Phys. Rev. 21, 916, (1954). 24. J.M. Blatt, V.F. Weisskoff, "Theoretical Nuclear Physics" John Wiley and Sons, Inc. New York, 1952, p. 627. 25. S. De Benedetti, F.K. McGowan, Phys. Rev. 7_4, 728, (1948). 26. CC. T r a i l , C.H. Johnson, Phys. Rev. $1, 474, (1953). 27. G.T. Seaborg, J.J. Livingood, G. Friedlander, Phys. Rev. 22» 320, (1941). 28. C.S.Cook, L. M. Langer, Phys. Rev. 22> 1149, (1948). 29. B.D. Kern, D.J. Zaffarana, A.C.G.Mitchell, Phys. Rev. 21, 1142, (1948). 30. L.M. Langer, R.D. Moffat, H.C Price, Phys. Rev. 7_9_, 808, ( 1950) . 31. F.R. Metzger, Phys. Rev. 86j. 435, (1952). 32. D.R.Hutchinson, M.L. Weidenbeck, Phys. Rev. 88, 699, (1952). 33. L.M. Langer, J.W. Starner, Phys. Rev. 253, (1954). 34. E.P. Tomlinson, S.L. Ridgeway. K. Gopalakrishan, Phys. Rev. £1, 484, (1953). 35. N.H. Lazar, Phys. Rev. J9J>, 192, (1954). 


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