L capture ratios in 71Ge decay

Nuclear Physics 36 (1962) 4 9 7 - - 5 0 4 ; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

M E A S U R E M E N T OF THE L/K AND M/L C A P T U R E RATIOS IN ~lGe DECAY C. M A N D U C H I

A N D G. Z A N N O N I Universitc~ di Padova, Italia

and Istituto Nazionale di Fisica Nucleate, Sezione di Padova, Italia t Received 20 1V~arch i962 A direct m e a s u r e m e n t of t h e L / K a n d M / L c a p t u r e r a t i o s of ~lGe h a s been m a d e u s i n g a m u l t i w i r e p r o p o r t i o n a l c o u n t e r . T h e L / K c a p t u r e ratio was f o u n d to be 0.1187 ! 0 . 0 0 0 8 , a n d t h e M / L c a p t u r e ratio h a s been d e t e r m i n e d to be 0.141:k0.010. I n addition, a v a l u e of 0 . 1 0 6 ! 0 . 0 0 3 w a s o b t a i n e d for t h e L / K c a p t u r e ratio of SSFe.

Abstract:

1. Introduction The precision measurements of the ratio of the probabilities of decay by capture of orbital electrons from the L-shell and K-shell for 3~A, 55Fe, 71Ge and 51Cr, are not in accord with the predictions of the theory of Brysk and Rose 2). There appears to be a small but consistent discrepancy of some percent between these measurements and the theoretical results ¢*. The theoretical calculation of the L/K capture ratio carried out b y Brysk and Rose takes into account the mutual repulsion of the K electrons, using screened wave functions, but does not consider the change of the wave function after capture, and the antisymmetry in the coordinates of the electrons. Odiot and Daudel 2) have treated the problem of electron capture taking into account the correlations between the positions of the electrons. Using non-relativistic wave functions, they found that this consideration operates to increase the capture ratio, L/K, tenfold for Z ---- 2 and about threefold for Z ~ 4. The effect falls off rapidly asZincreases and, although it may account for the small discrepancies found in 37A experiments, it is perhaps still insufficient to account for the slight disagreement ascribed to 5~Fe and slCr. On the other hand, for 71Ge the experimental L/K ratio is 20 ~o larger than the theoretical value, and the effect of correlations appears insufficient to account for the singular discrepancy exhibited b y this nucleide. A further study of electron capture in this nucleus seemed therefore desirable, and this is the subject of the present work. t T h i s r e s e a r c h h a s been p e r f o r m e d u n d e r C o n t r a c t E u r a t o m - C N E N . ** See t h e review article b y R o b i n s o n a n d F i n k ') a n d ref. 11). 497

498

C.

MANDUCHI

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ZANNONI

For comparison purposes, the measurement of the L / K ratio was extended to another nucleus, 55Fe, using the same experimental procedure. The contribution of electron capture from shells of principal quantum number greater than 2 is, moreover, not negligible. As a first approximation, one might examine the non-relativistic wave functions for a hydrogen-like atom, taking account of the Slater screening constants. According to Wapstra and Van der Eijk 3), the ratio of capture in the M shell to t h a t in the L shell for ~XGe is 0.148. Alternatively, following the results of the self-consistent field calculations of Hartree and others, Robinson and Fink 4) calculated the total contributions to s-electron capture and p-electron capture from shells of n _> 2. If the dependence on neutrino energy is neglected, the M/L capture ratio of ~lGe appears to be 0.174. In this work we a t t e m p t to verify experimentally the predictions of the theory. The me1 hod employed in the present experiment is based on the measurement of gaseous sources with an anticoincidence arrangement of two concentric proportional counters without an intervening wall. This system enables us to avoid to a great extent corrections due to radiation escape and fluorescence yield.

2. Experimental Apparatus The cross-sectional diagram of the counter assembly is shown in fig. 1. The end plates are made of 2 cm thick lucite held apart by supporting rods. The centre counter is 27.5 mm in radius and 88 cm in active length, with 32 wires as the cathode; the anode is mounted between teflon stoppers and fitted with field-correcting tubes. The number of cathode wires seems sufficient to remove the coupling effect between the central and the ring counters. Twelve wires define the volume of each shielding counter. The cathode wires are arranged so that the field distribution around the eight counting wires is not much distorted. The outer cathode wires are 0.1 mm harmonic steel arranged in a circle of 153 mm diameter; all other cathode and anode wires are 0.05 mm wolfram. The entire assembly fits inside a 190 mm in diameter brass cylinder which will withstand 10 atm pressure. Either anode or cathode wires can be operated at earth potential. The surrounding counters are proportional in operation and they can operate at the same voltage as the main counter. The block diagram of the electronics is given in fig. 2. The signals from the central counter are fed, through a low-noise preamplifier and a non-blocking double-line linear amplifier, to the TMC Model CN-110 256-channel analyser. The signals from the ring counters are fed, through a non-overloading preamplifier, into a pulse-height selector coupled to a 3 #sec resolution coincidence circuit. To the other input of the coincidence circuit are sent, through a pulseheight selector, the signals from the central counter. The coincidence pulses are fed to a fast trigger circuit used to control the late-anticoincidence gate of the

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C. M A N D U C F I I

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G. Z A N N O N I

analyser. Thus, if a K= or Kp X-ray, escaping the centre counter, is absorbed in a periferic counter, the residual L or M pulse in the centre counter is cancelled. The intensity of the L or M peak is then associated with L or M capture only. The K peak is due partly to the events in the ordinary counter and partly to the K photons coming from the outer counter, which compensate for the K photons that escape from the central counter.

3. E x p e r i m e n t a l Procedure Germanium hydride gas has been prepared b y the action of sodium amalgam on a solution of germanium oxide. Hydrogen was pumped off with the germanium hydride frozen at liquid nitrogen temperature. No chemical reaction happened with the counter materials. A volatile iron compound, Fe (C5H5)2, was prepared b y the action of monomer cyclopentadiene on ferrous chloride (from freshly reduced ferric chloride) in tetrahydrofuran, in the presence of diethylamine. The compound has been purified b y sublimation. No effect on the operation of the counter was found to arise from the compound kept at very low partial pressure. The measurements have been carried out at an argon pressure of 6 atm for 71Ge and 5 atm for 55Fe pulse analysis. Methane at a pressure of 20 cm Hg was used as quenching gas. The filling gas was previously passed into a mixing tank through a Ca purifier at 300 ° C and a liquid nitrogen trap. The pulse-height selector connected with the ring counters was biased at 3.8 keV. The typical 71Ge and 5~Fe X-rays spectra observed with the counter design described above are shown in figs. 3 and 4, respectively. The K-peak resolution in the ring counter was about 15 °/o. The true L/K capture ratio is then obtained from the observed value NL/N K b y substitution in the formula L/K = NL/NK[1-- (P2+Pa)OKJ-- (P~+P2+Pa)coKk~, while the true M/L capture ratio is obtained from the observed value NM/N L b y substitution in the formula M/L = NM/NL[1+K/LPoKk=]--K/LPco s ka. Here P1 is the probability that a K~ X-ray will escape the centre counter through the ends; P~ is the probability that the X-ray will escape the centre counter and hit a cathode wire; P3 is the probability that the X-ray will escape the centre counter and pass through the ring counter without being absorbed; P4 is the probability that the X-ray escapes the centre counter and hits the containing cylinder. The sum PI+P2+Pa is indicated b y P, while k~ and kp are the fractions of K~ and Kp X-rays in the K series; coK is the K-fluorescence yield of the daughter element. The small escape of M and L X-rays is neglected.

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Fig. 4.55Fe d e c a y X - r a y s s p e c t r u m f r o m c e n t r a l c o u n t e r in a n t i c o i n c i d e n c e w i t h t h e r i n g c o u n t e r (background not subtracted).

4. R e s u l t s Germanium 71 decay is a simple electron capture transition with a half-life of 12.5 d. From the internal bremsstrahlung spectrum associated with the electron capture, the transition energyis found to be 5) ( 2 3 1 i 3 ) keV and the decay is allowed with log ]t = 4.6. From the curves of Brysk and Rose the theoretical value of L / K ratio for 71Ge is 0.106. Langevin e), using a proportional counter containing the source in gaseous form, observed a higher value of 0.30. More recent work by Drever and Moljk 7), using a special proportional counter system which avoids the effects of X-ra V escape, ascribed to 71Ge a value of 0 . .1~+o.0o5 . . --0.003 for the L/K capture ratio.

CAPTURE RATIOS IN 71Ge DECAY

503

A mean of the experimental values obtained in the present measurement (table 1) gives, for the measured ratio NL[NK, a value of 0.19.27. Approximate calculations show t h a t the fraction of K X-rays which escape through the ends of the centre counter is P1 -~ 6 • 10~3, and the fraction which hit the cathode wires of the centre counter is P2 ---- 2 • 10-3. The calculated probability for K X-rays passing through the ring counter without being absorbed is P3 = 0 . 7 . 1 0 -3, and the probability for hitting the containing cylinder is P , = 0.1 • 10-3. The values taken for the K-fluorescence yield of gallium and for the fraction of K~ X-rays in the K-series were coK = 0.458 and k = 0.885 (ref. 8)). TABLE 1 L / K c a p t u r e ratio of ~lGe Activity (counter p e r 12842 12842 12842 17520 23459 23459

ram)

NLIN K

L/K

0.1230 0.1228 0.1231 0.1229 0.1221 0.1225

0.1190 0.1188 0.1191 0.1189 0.1181 0.1185

When these corrections are taken into account, the L / K capture ratio of ~XGe was found to be 0.1187±0.0008. The following facts were considered in estimating the limits of error: i) the standard error in the ratio due to the counting error amounted to about 0.4 %; ii) the errors in the tabulated values for o~K and k are estimated to be less than 3 %, and the probabilities Ps were calculated with an estimated accuracy of some 10 %; iii) the possibility of systematic errors, arising mainly in the determination of the number of counts in the Lpeak, has been carefully examined and estimated to be less than 0.5 %. Corrections for dead-time in the electronics are negligible. The L / K capture ratio of ~lGe obtained here is in slightly less disagreement with the theoretical results of Brysk and Rose. It seems, however, unlikely that the electron correlation factor may account for the discrepancy between the latter and the experimental result. The M/L capture ratio was found to be 0.1414-0.010, in gross agreement with the theoretical ratio. In establishing the limits of error the possibility was considered of a systematic error distorting the shape of the low energy part of the M-peak. The value chosen for the fraction of Kp X-rays in the K-series was s) 0.115. Iron 55 decay is a simple electron capture transition with a half-life of 2.60 y. The transition energy obtained from the internal bremsstrahlung spectrum associated with the electron capture is found to be ~) (226-¢-10) keV and the

504

C. M A N D U C H I A N D

G. Z A N N O N I

decay is allowed with log/t -~ 6.0. From the curves of Brysk and Rose, the theoretical value of the L/K ratio for ~SFe is 0.0944, which is slightly lower than the experimental value of 0.108:J:0.006 obtained b y Scobie et al. lo) in a multiwire proportional counter. In the present work the L/K capture ratio of 55Fe has been determined from several separate measurements to be 0.106±0.003, in good agreement with the results of the latter authors. The values taken for the K-fluorescence yield of manganese and for the fraction of K~ X-rays in the K-series were 0.261 and 0.899, respectively 8). We are greatly indebted to Prof. A. Rostagni and Prof. C. Villi for useful discussions. References H. Brysk and M. E. Rose, Revs. Mod. Phys. 30 (1958) 1169 S. Odiot and R. Daudel, J. phys. et rad. 17 (1956) 60 A. H. Wapstra and W. van der Eijk, Nuclear Physics 4 (1957) 325 B. L. Robinson and R. W. Fink, Revs. Mod. Phys. 32 (1960) 117 A. Bisi, E. Germagnoli, L. Zappa and E. Zimmer, Nuovo Cim. 2 (1955) 290 M. Langevin and P. Radvanyi, C. R. Acad. Sci. 241 (1955) 33 R. W. P. Drever and A. Moljk, Phil. Mag. 2 (1957) 427 A. H. Wapstra, G. J. Nijgh and R. van Lieshout, Nuclear spectroscopy tables (North-Holland Publ. Co., Amsterdam, 1959) p. 81 9) W. S. Emmerich, S. E. Singer and J. D. Kurbatov, Phys. Rev. 94 (1954) 113 10) J. Scobie, R. B. Moler and R. W. Fink, Phys. Rev. U 6 (1959) 657 11) C. Manduchi and G. Zannoni, Nuovo Cim., submitted