Positron Emission fromAg105and Related Aspects ofAg105Decay

Positron emission in the decay of 40-day ${}_{}{}^{105}{}_{}{}^{}\mathrm{Ag}$ has been discovered by detection of the 180°-coincident annihilation $\gamma$ rays. The intensity is 8.6±1.4 positrons per million ${}_{}{}^{105}{}_{}{}^{}\mathrm{Ag}$ disintegrations. Of these, only about (2±2)% can be attributed to processes associated with $\gamma$ transitions, leaving 8.5±1.5 positrons per million disintegrations attributable only to positron emission as a mode of $\beta$ decay of ${}_{}{}^{105}{}_{}{}^{}\mathrm{Ag}$. By $\gamma \gamma \gamma$ triple-coincidence measurements, it was found that these positrons (or the bulk of them) are not in coincidence with $\gamma$ rays, and hence that the transition is probably between the ${}_{}{}^{105}{}_{}{}^{}\mathrm{Ag}$ and ${}_{}{}^{105}{}_{}{}^{}\mathrm{Pd}$ ground states. The spectrum of the positrons was obtained by $\beta \gamma \gamma$ triple-coincidence measurements, and it was found to have an endpoint of 325±25 keV. The implied $log{f}_{1}t$ value (8.5) is within the range predicted for the expected but hitherto-undocumented unique first-forbidden transition between the ${}_{}{}^{105}{}_{}{}^{}\mathrm{Ag}$ and ${}_{}{}^{105}{}_{}{}^{}\mathrm{Pd}$ ground states, the spins and parities of both of which are known ($\frac{1}{2}-\to \frac{5}{2}+$). Radiations observed by means of NaI(Tl), Ge(Li), and Si(Li) spectrometry were the Pd $K$ x rays (abundance 78% of the ${}_{}{}^{105}{}_{}{}^{}\mathrm{Ag}$ disintegrations) and the following $\gamma$ rays: 64.07 (11%), $\approx 155 (\approx 0.5\%)$, $182.7 (\approx 0.5\%)$, 280.5 (32%), 306.3 (0.7%), 319.4 (4%), 331.6 (4%), 344.5 (42%), 360.7 (0.6%), 370.2 (0.7%), 392.8 (2%), 415.5 (0.2%), 443.4 (11%), 526.4 (0.09%), 560.4 (0.5%), 617.8 (1%), 644.4 (10%), 649.6 (3%), 673.2 (0.9%), 726.3 (0.2%), 743.1 (0.5%), 807.3 (1%), 961.5 (0.1%), and 1087.8 (4%) keV. By x-$\gamma$ coincidence measurements, it was shown that the lifetime of the 344-keV level of ${}_{}{}^{105}{}_{}{}^{}\mathrm{Pd}$ is short ($t_{\frac{1}{2}}<6\mathrm{}\times {10}^{-8\mathrm{}}$ sec), in contradiction to recent assertions. The