$\frac{M}{L}$ Orbital-Electron-Capture Ratio in ${\mathrm{Ar}}^{37}$ Decay and the Fraction of $K_{\alpha }$ X Rays in the $K$ Series of Chlorine

The ratio of $M$- to $L$-orbital electron capture in the decay of 35-day ${\mathrm{Ar}}^{37}$ has been measured in a single-wire proportional counter containing the gaseous source. This counter was operated with variable paralysis times, 2.0 and 3.8 msec being used in the present experiments to minimize the contribution from afterpulses. Recent investigations of the pulse-height distribution from proportional counters due to single-and multiple-electron events make it possible to measure single-electron spectra down to essentially zero energy with much greater accuracy than heretofore. The experiment was undertaken because of the considerable disagreement between experimental and theoretical $\frac{M}{L}$ capture ratios, particularly in the low-$Z$ region. A survey of all experimental work on $M$ capture is included for comparison with available theoretical results. In the decay of ${\mathrm{Ar}}^{37}$ the experimental value for the $\frac{M}{L}$ capture ratio is $\frac{P_M}{P_L}={{0.104}_{-0.003\mathrm{}}}^{+0.006\mathrm{}}$, where the error limits have been subjected to careful detailed analysis. From this result, a value of $k_{\alpha }=0.980\mathrm{}\pm 0.015$ for chlorine was determined, where $k_{\alpha }$ is the fraction of $K_{\alpha }$ x rays in the $K$ series of chlorine. The theoretical predictions of Bahcall for $\frac{P_M}{P_L}$ lie about 23% above the experimental point, for a theory with exchange correction. The older theoretical value due to Wapstra et al. is in no better agreement with experiment, even if a correction for exhange is applied. An intensive effort is reported to measure the $\frac{L}{K}$ capture ratio in the decay of 53.5-day ${\mathrm{Be}}^7$, as gaseous ${\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_2$ ${\mathrm{Be}}^7$ in the single-wire proportional counter, but owing to the low energy of the $K$-capture events (55 eV) and very low counting statistics, this measurement appears to be beyond the present state of the proportional-counter art.