The disintegration of Ga73

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[ Nuclear Physics 10 (1959) 2 8 - - 3 2 ; (~) North-Holland Publishing Co., Amsterdam

4.E

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Not to be reproduced by photoprint or microfilm without written permission from the publisher

THE

DISINTEGRATION

O F G a ~3

L. M A R Q U E Z , E. W . C Y B U L S K A , N. L. C O S T A a n d I. G. A L M E I D A

Centro Brasiteiro de Pesquisas Ftsicas, Rio de Janeiro * and J. G O L D E M B E R G

Departamento de Fisica, Univeraidade de S~o Paulo, ~ o

Paulo

R e c e i v e d 30 O c t o b e r 1958 T h e d i s i n t e g r a t i o n of Ga 78 h a s b e e n s t u d i e d u s i n g g a m m a r a y s p e c t r o m e t e r s , b o t h alone a n d in coincidence; a n d b y t h e a b s o r p t i o n of t h e b e t a r a y s in coincidence w i t h t h e m o s t p r o m i n e n t g a m m a r a y in t h e scintillation s p e c t r o m e t e r . I t w a s f o u n d t h a t t h e r e were t w o g a m m a r a y s ; one w i t h a n e n e r g y of 310 k e V a n d i n t e n s i t y of a b o u t 90 % a n d one w i t h a n e n e r g y of 740 k e V a n d a n i n t e n s i t y of a b o u t 7 %. A s e l f - c o n s i s t e n t d e c a y s c h e m e is p r o p o s e d s h o w i n g one b e t a r a y of 550 k e V a n d i n t e n s i t y of a b o u t 7 %, one b e t a r a y of 1300 k e V a n d i n t e n s i t y of a b o u t 83 % a n d p e r h a p s a t h i r d b e t a r a y of 1600 k e V a n d i n t e n s i t y of a b o u t 10 %. All t h e b e t a r a y s lead to t h e m e t a s t a b l e s t a t e of Ge ~8.

Abstract:

1. I n t r o d u c t i o n The decay scheme of Ga 78 has not been the subject of a thorough investigation. The isotope was found by Siegel and Glendenin 1). They found t h a t it decays with a half life of 5.0 h and they found by absorption methods that it has one beta ray of 1.35 MeV and no gamma rays. It has also been reported 2) that it leads to the 0.53 sec Ge v3, the same as in decay of 76 day As 73. The decay of 0.53 sec Ge v8 has been studied by several workers 2,3,4) and it is known that it decays by the emission of two gamma rays of 53.9 keV and 13.5 keV the first with a conversion coefficient of about 8 and the second with a conversion coefficient greater than 1300. We have found that Ga v3 is followed by a gamma ray of 740 keV with weak intensity and by a gamma ray of 310 keV with strong intensity. The beta ray spectrum of Ga ~8 is complex and our measurements indicate that it has probably three beta ray groups. It is certain t h a t it has two beta ray groups. We propose a decay scheme of Ga ~3consistent with our measurements and from this decay scheme we arrive at a value of the conversion coefficient of the 53.9 keV gamma ray in Ge ~3. * T h i s w o r k w a s done in p a r t u n d e r t h e a u s p i c e s of t h e Conselho N a c i o n a l de P e s q u i s a s of Brazil. 28

30

J. GOLDEMBERG et ¢~I.

half thickness of Pb corresponding to 310 keV and t h a t it was due entirely or almost entirely to pile up of 310 keV and 740 keV. We carried out measurement with two scintillation spectrometers in which we registered the counting rate of each of them and the coincidences. Both spectrometers were single channel pulse height analyzers. One of them had a NaI (T1) crystal of 1~" diameter and 2" height; the other had a NaI (T1) crystal of 1½" diameter and l " height. They were at 1.7 cm from each other and the samples were at 1.2 cm from the first crystal. We could put absorbers between the sample and the first crystal. The second spectrometer was set to count the line of 310 keV. The spectrum was swept with the first spectrometer. The coincidence counts showed very well the peak at 740 keV clean from the 14 h component. The pile up peak disappeared. The intensity of the 740 keV line was measured and it turned out to be about 7 °/o of all the beta particles. These measurements prove the existence of at least two beta ray groups. One soft beta ray followed by the 740 keV gamma ray and then b y the 310 keV gamma ray; and a harder beta ray followed by the 310 keV gamma ray only. We made three irradiations in which coincidence measurements were carried out between the beta rays and the 310 keV gamma ray. The arrangement had a NaI(T1) crystal of 1½" diameter and 1" height. The crystal was operated as single channel pulse height analyzer and it was set to count the 310 keV peak. At a distance of 2 cm from the crystal there was an end window Geiger counter and the sample was at 1.5 cm from the Geiger. A1 absorbers were placed between the Geiger and the samples. The decay curve for each absorber had to be followed to correct for the 14 h beta activity. We registered the beta activity Np, the gamma activity N~ and the coincidences NpT. A curve of Np~/Npas a function of absorber thickness shows a ratio decreasing with the amount of absorber. The curve indicated that there must be a third group of beta particles. This conclusion is drawn from the fact t h a t the two beta rays previously established were followed by the 310 keV gamma ray. The curve of Np~,/N,scould be fitted with a theoretical curve having 90 % of the beta particles coincident with the 310 keV gamma ray and 10 °/o of a harder beta ray not in coincidence with the 310 keV gamma ray. We do not regard the existence of this third beta ray as definitely established due to the cumbersome procedure of resolving decay curves of 5 h and 14 h from which its existence was inferred. Measurements of the beta ray spectra of Ga ~3 will settle this point. The average energy of the beta ray was determined by measuring their absorption coefficient in A1 as a function of absorber thickness and gives 1.3 MeV. A curve of measured absorption coefficients as a function of maxi-

THE DISINTEGRATION OF Gs ?s

31

m u m beta ray energy was obtained using standard beta ray sources. It follows that the energy of the main beta ray is 1.3 MeV; the energy of the softer beta ray is 0,55 MeV and the energy of the harder beta ray is 1.6 MeV. 3. D i s c u s s i o n

The spin and parity of the ground state of Ga 7s is known to be {% The sequence of the gamma rays of 53.9 keV and 13.5 keV seems to be well established 4,e) and it gives for the corresponding states the choice of spins and parities given by Welker et al.S). The spin and parity of Ga ~s must be { - from shell model considerations. The three beta rays give l o g / t values corresponding to allowed transitions. All these enable us to construct the decay scheme shown in fig. 1. The spin and parity of the two upper levels of

\\ \0.55

0 . 0 5 4

-

0.013

stable Ge73 Fig.

1

the Ge Ts t h a t we have found as a result of this work must be ~-, { - or {-. With the information available it is not possible to decide among them. The only discrepancy present in this decay scheme comes from the measurement of the internal conversion coefficient of Welker a al. s) for the 53.9 keV line. Using their value of 4.7 we find t h a t the line has an intensity of 77 %. In order to get the intensity of 100 ~/o t h a t it should have according to the decay scheme proposed, its conversion coefficient should be 6,4. The 13.5 keV line does not show in our measurements because its large conversion coeffiicent and the soft electrons would not come out from our relatively thick sources.

~2

J. GOLDEMBERG 8~ ~ .

References 1) J. M. Siegel and L. E. Glendenin, NNES 2) S. Johansson, Arkiv Fysik 4 (1952) 273 3) J. P. Welker, A. W. Schardt, G. Friedlander 401 4) R. Barloutaud, R. Ballini and M. Sartori, 5) L. Marquez, Phys. Rev. 86 (1952) 405 6) K. Way, R. W. King, C. L. McGinnes and R. (1955) p. 116

9 (1950) 549 and J. J. Howland, Jr., Phys. Rev. 92 (1953) Compt. Rend. 237 (1953) 836 van Lieshout, Nuclear Level Schemes USAEC