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On the decay scheme of ZN"65" Rankin, David 1949

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65 ON THE DECAY SCHEME OP ZN by David Rankin A thesis submitted in partial fulfilment of the requirements for the degree of MASTER OF ARTS in the Department of PHYSICS The University of Br i t i s h Columbia September 1949. ABSTRACT 65 The radiation from Zn has "been investigated in a thin lens beta ray spectrometer. A spiral baffle was used to discriminate between positrons and negatrons. Gamma ray energies of 1.12 and 1.4 mev have been measured as well as . annihilation radiation of .51 mev. A positron end point at .32 mev has also been measured. Fairly intense Internal conversion was found. A decay scheme has been proposed in 65 which Zn decays by K-capture to a 1.4 mev excited state from which i t proceeds to an Intermediary state by emission of a gamma ray. The alternative positron emission i s to the intermediary stage from which both paths descend to the 65 ground state of Cu with the emission of a 1.1 mev gamma ray. ACKNOWLEDGEMENTS This study has been made possible through the Grants-in-AId-of-Research to Professor K. C. Mann from the 65 National Research Council, as well as the loan of the Zn source, without which so detailed a study would have been impossible. The award of a bursary and.a summer scholarship to the author have also greatly f a c i l i t a t e d this work. The author wishes to express his deep gratitude to Professor K. C. Mann for his guidance and encouragement during the progress of this research. TABLE OF CONTENTS Page I. INTRODUCTION 1 II. APPARATUS AND PROCEDURE A Spectrometer 4 Current Regulator 8 Geiger Counter 8 Amplifier 10 Compensating Coils , 11 Source Arrangement 11 Calibration.. 12 St a t i s t i c a l Accuracy... - 12 III. RESULTS 13 Negatron Spectrum 13 Positron Spectrum * 13 Gamma-ray Spectrum 13 IV. CONCLUSIONS 18 V. BIBLIOGRAPHY 20 TABLE OF ILLUSTRATIONS Figure 1 Spectrometer 5 2 Complete Assembly 5 3 Spiral Baffle 7 A Geiger Counter 9 5 Source Arrangement 9 6 Negatron Spectrum 15 7 Positron Spectrum 16 8 Fermi Plot 16 9 Gamma-Ray Spectrum 17 (1) INTRODUCTION 65 Zn i s an a r t i f i c i a l l y radio-active isotope which has been investigated by various methods. This isotope has been produced by the following reactions: 64 65 Zn (d,p) Zn 64 ' 65 Cu (d,2n) Zn 65 65 Cu (p,n) Zn 64 65 Zn (n,y) Zn 65 65 Ga (K-capture) Zn A good deal of confusion attended the earlier inves- tigation of the Cu-Zn-Ga compounds which were produced in the cyclotron by deuteron bombardment. Livingood ^ reporting on one such element, suggests the possibility of a Zn isotope decaying through positron emission with a h a l f - l i f e of 12 hrs. This was un- doubtedly one of the short l i f e a ctivities which i s found in conjunction with Zn a c t i v i t i e s . (2) Perrier, Santangelo, and Segre v ' report, on their examination of the f i l i n g s from the copper deflection plate of the Berkeley Cyclotron, the existence of a 245 day half- l i f e activity. They do not attribute this to Zn^since they quote Livingood »s 12 hr. h a l f - l i f e activity as being Zn^. Barnes and Valley(3) report an activity produced in Cu by proton bombardment with a h a l f - l i f e of approximately (2) 210 days. They suggest a positron and negatron emission in the ratio of 2:1. There is also a strong gamma radiation with a gainma to beta ratio of 60 to 1. The maximum energy of the Beta group i s .7 mev as indicated by i t s absorption in aluminum. Delsasso et alUO report on a C u ^ (p,n) Zn*1-' decay with both positron and negatron activity in addition to strong gamma radiation. They suggest that the decay Is Zn65 Cu65 The f a i r l y intense X radiation i s attributed to both internal conversion of the gamma rays and K-capture with the negatrons due entirely to internal conversion. Livingood and Seaborg (5), using the reaction Z n 6 4 D 2 Zn 6 5 H 1 30 +" 1 3 0 + 1 and 2 Q Cu 65 + - H 1 3 0 Zn 6* ̂ n 1 and either ^ Cu 65 + ^ ^ Zn 6* ̂ n 1 or 2 Q Cu63+ l D2 3 Q z-65 + - suggest the decay as both 3 0 Z n 6 5 2 9 C u 6 5 ^ ' and 3 0Zn 6 5 + e~ 2 9 C u 6 5 w i t h a t o t a l h a l f - l i f e of 250 days. They point to the small number of particles as compared to gamma-rays as evidence of the existence of K- capture as an alternative to positron emission. From a con- sideration of positron and gamma-rays in equilibrium, the ( 3 ) ionization produced by the positron rays should be approx- imately thirty times as great i s that produced by the gamma radiation. Livingood and Seaborg find however, that the gamma-radiation produces the greater part of the ionization. This demonstrates the predominance of K-capture as the mode of decay. From absorption measurements in Pb, a high intensity gamma-ray with an energy of 1.0 mev together with an inapprec- iable amount of .5 mev annihilation radiation tends to confirm the low rate of positron emission to K-capture. Alvarez(6)measured the absorption of the x-rays produced in this reaction. From a comparison of absorption in nickel and copper he deduced that the x-rays were the characteristic C u ^ lines. Sagane et a l ^ u s i n g cloud chamber measurements have found end points for positron emission of .39 and .19 mev. These upper limits were calculated from K.U. plots. Watase et alWhowever, found a single positron with an end point of .47 mev and f a i r l y intense gamma rad- iation of 1.0, .65, and .A5 mev in the ratios approximately 1:1:1. Deutsch, Roberts and E l l i o t t ^ ) r e p o r t on Zn^5 with a h a l f - l i f e of 250 days, and give a gamma-ray energy of 1.1A to better than 1%. Good and Peacock ( 1 0 ) . using a calibrated gamma-ray counter, measured X-gamma and positron-gamma coincidences. They found that 5A% of K-capture occurs in the ground state and A6# in the I.I4 mev excited state; also that 2.2% of the U ) transitions are by positron emission directly to the ground state of Cu64. W. C. P e a c o c k r e p o r t s a positron end point of .320 mev. Jensen, Laslett, and P r a t t C 1 2 ) using a high resol- ution beta-ray spectrometer, give a corrected value for the gamma-ray energy of 1 .11 mev as measured from the photo-elect-, rons ejected from a thin lead radiator. Daykin^ 1^, using the same thin lens spectrometer as the present investigator, found a gamma-ray energy of 1 .11 mev as measured from the photo-electric peak in both lead and uranium and a positron end point of .32 mev. Since this work was done with a source of very low specific a c t i v i t y i t was deemed attractive to repeat this investigation, using a stronger source with a view to a more detailed study of Zn 6^ ac t i v i t y . APPARATUS AND EXPERIMENTAL TECHNIQUE Spectrometer The spectrometer used in this research i s of the "thin lens" type introduced by Deutsch, E l l i o t t and Evans A line diagram is shown in f i g . 1 and a photograph of the complete assembly in f i g . 2 . The spectrometer consists of ah evacuated brass tube 8 n in diameter and A0 n long, with a short F i g u r e 2 - Complete Assembly (6) magnetic c o i l wound around i t s centre section. A system of b a f f l e s defines the t r a j e c t o r i e s of the p a r t i c l e s from the source. These p a r t i c l e s are bent through a s p i r a l bath to focus at the window of a b e l l - t y p e Geiger counter. The centre b a f f l e , which i s shown i n f i g . 3, i s designed to stop either p o s i t i v e or negative beta p a r t i c l e s from passing through the spectrometer, depending on the d i r e c t i o n of the current which i s passed through the focussing c o i l s . Deutsch et a l (U) have shown that the p i t c h i s almost constant f o r t r a j e c t o r i e s having d i f f e r e n t r a d i i . In the course of the present research t h i s b a f f l e has proven to be most e f f i c i e n t i n the elimination of the electrons of the opposite sign. Daykin (12). however, reports a 25% loss in:-the counting rate of the d e s i r e d sign of electrons with the s p i r a l b a f f l e , probably due to too large a p i t c h . The magnet c o i l i s made up i n four co-axial layers which may be used i n d i v i d u a l l y or i n s e r i e s . I t i s e a s i l y shown that the sign of the gradient of the f i e l d i n the r a d i a l d i r e c t i o n Is opposite to that required f o r the best momentum discrimination. This e f f e c t can be minimized by using only the outermost layer of the c o i l , which of course requires much heavier currents. Therefore a compromise must be made between the consideration of focussing and the conven- ience of covering an ample range of momenta. In t h i s work only the two outermost sections were u t i l i z e d . (7) (8) Current Regulator Only those particles, travelling in the correct direction, with momenta such that they w i l l he focussed at the counter window, w i l l satisfy, the focussing conditions determined .by the strength of the magnetic f i e l d of the focussing magnet. Since there Is no ferro-magnetic material in or near the spectrometer the strength of the f i e l d w i l l be directly proportional to the current through a standard manganin resistance of approximately .08 ohms in series with the c o i l . The voltage drop across this resistance i s compared to the voltage set on a Rubicon Potentiometer and the unbal- anced voltage used to drive the current stabilizer described by Lindenfeld, Mathews, Ozeroff and Daykin(H). Geieer Counter The thin window Geiger Counter shown in f i g . 4 was designed in this laboratory to eliminate as much as possible the use of wax seals. The only wax used is the very thin layer which seals the mica window between the brass flanges. The copper anode is f " in diameter. The .005" tungsten anode wire is hard soldered to an advance wire which in turn i s soft sold- ered to the top end of the Kovar seal through which i t projects and which in turn Is soldered to the top of the metal outer case of the counter. A short section of Nonex glass tube i s fastened to the glass sleeve of the Kovar seal which i s then soldered upside down onto the outer case of the counter. This assembly (9) F i g u r e 4 GEIGER COUNTER MICA _ WINDOW FLANGE* BRASS CASING COPPER ANODE FILUNG TUBE ADVANCE F i g u r e 5 F i g u r e 5(b) URANIUM RADIATOR -SOURCE BRASS ABSORBERS -SOURCE FOR ELECTRONS FOR GAMMAS (10) i s used to f i l l the counter and the glass tube Is then sealed off, thus eliminating the use of a stopcock and the possible leakage thereof. The 2.8 mg/cm mica window i s sealed to the base of the counter with Plicene wax which has been dissolved in boiling turpentine and peinted onto the brass flanges. The counter was f i l l e d with 1.5 cm. of alcohol and 8.5 cm. of argon. Amplifier The arrangement of the laboratory required the use of a ten foot cable to carry the counter pulses to the amplif- i e r . To avoid the loading of the counter by the cable, a cathode follower i s used to feed the pulses through the cable Into the amplifier. A matching resistance of 100 ohms Is used at the putput end of the cable. Amplifier i s a two-stage grounded grid triode type preceded by a cathode follower. The pulse i s sufficiently amplified by the f i r s t stage to saturate the second stage and provide 60 volt pulses of equal amplitude for the scalar. This amplitude of pulse allows a scalar dis- crimination bias of 15 volts which i s quite sufficient to eliminate most of the counts due to stray pickup, while at the same time keeping the counting rate completely independent of discriminator bias fluctuations. The plateau obtained with this circuit and the counter described above has been satisfact- ory and has remained stable over the whole period during which this research has been carried out. (11) Compensating Coils The horizontal component of the earth's magnetic f i e l d is,compensated for by a pair of Helmholtz coils wound on a frame about the spectrometer table. The current is regulated against line voltage variations by the use of two ballast tubes in series with the 10 ohm c o i l s . The excess of the normal 1.7 amps above the required .94 amps i s shunted through a rheostat. This regulation i s sufficient to care for normal hour to hour line voltage fluctuation. Source Arrangement The 1 millicurie Zn 6? source was produced by slow neutron irradiation in the Chalk River pile of a sample of pure Zn 6* in the form of a thin square f o i l , 25 mg/cm2. This form of a source was most adaptable for the various types of invest- igation which were required in this research. For counting positrons and negatrons the source was fastened to a 2 mg/cm2 mica backing as illustrated in f i g . 5a. For counting the photo- electrons ejected from a uranium radiator, the source was fastened to one side of a 1/16" thick brass plate with the uranium radiator of 90 mg/cm2 on the other side. The thickness of the brass absorber was calculated from the Feather formula to be sufficiently thick to stop the most energetic beta particles which were observed in the negatron spectrum. In order to obtain the Compton background an identical brass absorber was instituted without the uranium radiator. This (12) arragement is shown i n f i g . 5b. Care was taken to ensure that the position of the source for beta counting was identical to the position of the uranium radiator in order to provide a cross check for the calculations of energies. Calibration of the Instrument This instrument was calibrated on the basis of the .607 mev gamma line of radium. This energy was obtained by Ozeroff(3) 0n the basis of the well known F line of thorium B as measured in a similar instrument. The potentiometer read- ing corresponds then to the energy of gamma line minus the binding energy of the correct level of the radiator, since the photo-electric effect is used. In the above cases the K shell of lead with a binding energy of 87.5 kev is appropriate. The potentiometer setting can then be translated directly into Ef values. St a t i s t i c a l Accuracy The s t a t i s t i c a l accuracy of a l l points on the spectra was better than 2%, while those points in the regions from which important data might be expected, had a s t a t i s t i c a l accuracy approaching 1$. This entailed a minimum of 20 minutes counting per point in the f i r s t case and as much as 60 minutes in the latt e r . (13) RESULTS Negatron Spectrum The negatron spectrum i s shown in the graph in f i g . 6. A Fermi plot of this spectrum was prepared and nof d i s t - ribution could be recognized, indicating the continuum was due largley to Compton recoil electrons. In other words any (5~ emission must be of so low an activity as to be lost in the Compton distribution which is produced by gamma-rays in the Zn source. A very pronounced spectral line i s in evidence which can be identified as an internal conversion line of C|i^. With the binding energy for Cu as calculated, from the data in the Handbook o.f Physics and Chemistry at .088 mev, this leads to a gamma-ray energy of 1.13 mev Hi .005. Positron Spectrum The positron spectrum i s shown in f i g . 7 and the Fermi plot of this spectrum i s shown in f i g . 8. The extrapol- ated end point i s f i t t e d by the method of least squares, using the 11 points indicated by the arrows in f i g . 8. The results indicate an end point energy of .320 _T .003 mev. Gamma-Ray Spectrum The gamma-ray spectrum shown in f i g . 9 displays the (14) high intensity photo-electric peak from the K shell of uranium, as well as the less pronounced L shell peak. A rather weak peak at .3 V. corresponding to a gamma-ray energy of .513 .003 mev i s identified as annihilation radiation. Using the value .114 mev for the binding energy of the K shell and .020 mev for the L shell, the gamma-ray energy Is i n both cases 1.12 .005 mev. A close examination of the high energy end of the Compton distribution in f i g . 9 shows a spectral configuration which may be due to a low intensity gamma-ray. Using a re- arranged version of the well-known Compton scattering formula, we can determine the gamma-ray energy responsible for the maximum recoil electron. This may be expressed as: - where = energy of the gamma-ray Em = maximum energy of the Compton recoil electron in the forward direction (which in this case is 1.14 mev). This leads to the result that: E y = 1.4 - »1 mev. COUNTS PER MINUTE (16) F i g u r e 7 80c| POSITRON SPECTRUM OF Z N 6 5 POTENTIOMETER VOLTAGE F i g u r e . 8 ° o X 601 4 0 f N p hfl 2 0 FERMI PLOT OF THE POSITRON SPECTRUM • 1 8 0 • « 0 0 2 2 0 M " ENERGY IN MEV- 2 8 0 3 0 0 3 2 0 3 4 0 cm GAMMA" SPECTRUM OF Z N 6 S 4 1 •» ( D POTENTIOMETER VOLTAGE (18) CONCLUSIONS The results obtained in this research agree i n their main features with those of the most recent investigations. The rather high intensity internal conversion electrons found, confirm in part the assumptions of Delasso et a l ^ regarding the presence of negatrons. The positron end point agrees with the results of W. C. Peacock^ 1 1) and i s 7$ lower than that of P.N. Daykin^ 1 3} The gamma-ray energies .513 - .002 mev and 1.12 - .005 agree well with those of Jensen, Laslett and P r a t t b u t are slightly lower than the results of Deutch, E l l i o t t and Roberts ^ ) . However the gamma energy as measured from the int- ernal conversion line 1.13 .005 agrees closer with the result of the latter investigators. The findings of Sagane et a l ^ w i t h respect to the positron end points seem most unlikely, in view of the results of this investigation, while the three gamma-ray energies reported by Watase et a l (8) in the ratio 1:1:1 are even more improbable, since the energies 1.0, .65, and .4.5 mev are well within the possibility of detection in the intensities claimed, with our instrument. The gamma-ray energy of 1.4. t .1 mev f i r s t reported here must be considered with caution in view of the low activity source available for the measurement of this reaction. (19) This gamma l i n e i s i n h e r e n t l y weaker than the 1.1 mev. l i n e i f the hypothesis which i s put forward below has any s i g n i f i c a n c e The c o n f i r m a t i o n of the 1.4 mev l i n e must await e i t h e r a beta, ray spectrometer i n v e s t i g a t i o n w i t h a stronger source, or perhaps b e t t e r , an i n v e s t i g a t i o n u s i n g a p a i r spectrometer w i t h the source i n a strong f l u x o f neutrons. A decay scheme based on these r e s u l t s can be t e n t a t i v e l y advanced: Cu°5 Zn 65 K-capture .51 mev - N ) 1.4 mev V P o s i t r o n .32 +.51 mev 1.1 mev V Before t h i s scheme can be accepted w i t h l e s s than the utmost c a u t i o n , f u r t h e r research must be undertaken. The r a t i o of gamma t o p o s i t r o n emission must be re-examined.' I t should be noted that the arrangement of the source m i l i t a t e s a gainst counting the a n n i h i l a t i o n r a d i a t i o n , a l s o the presence of strong i n t e r n a l conversion must a f f e c t the r a t i o p r e v i o u s l y accepted. Not only X-gamma and positron-gamma-but a l s o gamma- gamma coincidences must be measured, before a comprehensive a n a l y s i s of the decay scheme o f Z n 6 5 can be v e r i f i e d . ( 2 0 ) BIBLIOGRAPHY 1 . J . J . L i v i n g o o d Phys. Rev., 5 0 , 4 - 2 5 , 1 9 3 6 2 . -C. P e r r i e r , M. S a n t a n g e l o , E. Segre Phys. Rev., 5 3 , 104, 1 9 3 8 3 . S. W. B a r n e s , G. V a l l e y Phys. Rev., 5 3 , 9 4 6 , 1 9 3 8 4 - L. A. D e l s a s s o , L. N. R i d e n o u r , R. S h e r r , M. G. White Phys. Rev., 5 5 , 1 1 3 , 1 9 3 9 5 . J . J . L i v i n g o o d , G. T. Seaborg Phys. Rev., 5 5 , 4 5 7 , 1 9 3 8 6 . L. W. A l v a r e z Phys. Rev. 5 4 , 4 8 6 , 1 9 3 8 7 . R. Sagane, S. K o j i m a , G. Miyamoto P r o c . Phys. Math. Soc. Japan, 2 1 , 7 2 8 , 1 9 3 9 8 . Y. Watase, J . I t o h , E. Takeda P r o c . Phys. Math. Soc. Japan, 2 2 , 9 0 , 1 9 4 0 9 . M. Deutsch, A. R o b e r t s , L. G. E l l i o t t Phys. Rev., 6 1 , 3 8 9 , 1 9 4 2 1 0 . W. M. Good and W. C. Peacock Phys. Rev., 6 9 , 6 8 0 , 1 9 4 6 1 1 . W. C. Peacock P l u t o n i u m P r o j e c t R e p o r t , Mon. N - 4 3 2 , 5 6 , 1 9 4 2 . ( p r o b . r e s t r i c t e d c i r c u l a t i o n ) 1 2 . E. N. J e n s e n , L. J . L a s l e t t , W. W. P r a t t Phys. Rev., 7 5 , 5 2 9 , 1 9 4 9 1 3 . P. N. D a y k i n M. A. T h e s i s , U n i v e r s i t y o f B r i t i s h Columbia P. L i n d e n f e l d , M. A. T h e s i s , U n i v e r s i t y o f B r i t i s h Columbia M. J . O z e r o f f , M. A. T h e s i s , U n i v e r s i t y o f B r i t i s h Columbia 1 4 . M. Deutsch, L. G. E l l i o t t , R. D. Evans, R. S. I . , 1 5 , 1 7 8 , 1 9 4 4 .


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