Multiwire-Proportional-Counter Measurement of theMLOrbital-Electron-Capture Ratio inGe71Decay

Abstract

The $\frac{M}{L}$ orbital-electron-capture ratio in ${}_{}{}^{71}{}_{}{}^{}\mathrm{Ge}$ decay has been measured in a wall-less multiwire proportional counter containing the gaseous radioactive compound ${}_{}{}^{71}{}_{}{}^{}\mathrm{Ge}$ ${\mathrm{H}}_4$. The counter was filled to 6 atm with argon containing 15 cm Hg propane. Recent advances in proportional-counter spectrometry below 500 eV have extended the sensitivity for precision measurement to the detection of single- and few-electron events down to essentially zero energy, even in the presence of high intensities of more energetic radiation. These improvements are the result not only of the sophistication of the low-noise electronic system, but also of the present investigation of the degradation spectrum (low-energy tails and afterpulses) from spectral peaks, and of the resolution from each other of single- and few-electron spectra by using computer analysis. A fully transistorized electronic system was developed which includes a special dc-coupled logic unit with zero-time recovery (updating) to perform the following operations: (i) to ensure proper anticoincidence functioning of the ring counter; (ii) to provide paralysis pulses necessary to block possible afterpulses following an ionizing event in either the ring or central counter; and (iii) to eliminate the phenomenon of front- and backedge clipping of large pulses into small ones by the action of the anticoincidence gate. Measurements were made of the $K$, $L$, and $M$ peaks, having mean energies of 10.37 keV, 1298, and 158 eV, respectively, and the resulting orbital-electron-capture ratios were determined to be $\frac{P_L}{P_K}=0.117\mathrm{}\pm 0.001$ and $\frac{P_M}{P_L}=0.162\mathrm{}\pm 0.003$. These ratios are compared with theoretical results. Whereas the present $\frac{L}{K}$ capture ratio is in closer agreement with theory corrected for exchange according to Bahcall, the theoretical prediction of the $\frac{M}{L}$ ratio by Bahcall with correction for exchange lies approximately 7% above the present value, which agrees well with a recent theoretical calculation by Vatai. The half-life of ${}_{}{}^{71}{}_{}{}^{}\mathrm{Ge}$ was found by least-squares fitting to be 11.15 ± 0.15 days, by following the decay of $K$ x rays from a solid source with a Ge(Li) x-ray detector.