Evidence forK-Shell Ionization Accompanying the Alpha-Decay ofPo210

The radiations of ${\mathrm{Po}}^{210}$ have been studied using NaI scintillation counters. In agreement with Grace, Allen, West, and Halban, it is concluded that the soft electromagnetic component probably entirely consists of x-rays of lead. The region from 25 kev to 2.5 Mev has been examined and, with the exception of the known $\gamma$-ray of 800 kev, no nuclear $\gamma$-rays were observed. The ratio of the number of $K$ x-rays to the number of 800-kev $\gamma$-rays was measured as 0.134±0.025 to 1. The $K$ shell internal conversion coefficient of the 800-kev transition has been reported as about 0.05, and hence the $K$ x-ray intensity is too great to be explained by internal conversion alone. The residual x-rays are attributed to the process whereby the emission of the alpha-particle causes ionization of the atom. Comparison of the relative intensity of $K$ x-rays and alpha-particles shows qualitative agreement with the probability of this ionization process as calculated by Migdal.