The Natural Radioactivity ofIn115

An investigation of the possible natural radioactivities of the ${\mathrm{Cd}}^{113}$-${\mathrm{In}}^{113}$ and ${\mathrm{In}}^{115}$-${\mathrm{Sn}}^{115}$ pairs of adjacent isobars gave evidence for a natural beta-radioactivity of indium. Four samples of indium differing in geographical origin were subjected to radiochemical purification and then examined in a screen wall counter arranged with anticoincidence counters and steel shielding. Absorption measurements on these samples gave reproducible results indicative of an energetic natural beta-radiation. The energy was measured by comparing the absorption characteristics of the indium radiation with those of ${\mathrm{Tl}}^{204}$ and ${\mathrm{Co}}^{60}$, whose beta-energies have been measured spectroscopically. Measurement of isotopically enriched samples of indium gave conclusive evidence that the origin of the beta-radiation is the ${\mathrm{In}}^{115}$ isotope. Limits were placed on possible accompanying gamma-radiation. The results indicate that ${\mathrm{In}}^{115}$ emits a natural beta-radiation of 0.63±0.03-Mev energy with a half-life of 6±2×${10}^{14}$ years.