The Nuclear Isomers ofIn116

Abstract

The isomeric periods of ${\mathrm{In}}^{116}$ (13 seconds and 54 minutes), produced by the capture of slow neutrons by ${\mathrm{In}}^{115}$, have been studied with the purpose of determining the energy level scheme for ${\mathrm{In}}^{116}$. The $\gamma$-ray of energy 1.4 Mev previously measured by the authors has been shown to have a period of 54 minutes. No $\gamma$-ray of 13 second period could be found. The percentage transmission by boron of resonance neutrons which effect the two periods has been measured and found to be the same within the experimental error, for both periods. The value obtained for the percentage transmission of 0.0252 g/${\mathrm{cm}}^2$ $B$ (uncorrected for obliquity) is 73.5±3.0. The branching ratio $\frac{R(13\mathrm{} \sec .\mathrm{})}{R(54\mathrm{} \min .\mathrm{})}$ has been shown to be independent of the method of excitation and has the value 1.12±0.06. From the known beta-ray end points and the energy of the gamma-ray an energy level diagram has been drawn up which accounts for all the experimental facts. The two excited levels of ${\mathrm{In}}^{116}$ from which disintegration takes place are separated by 0.3 Mev, the level corresponding to the 13 second period lying higher. Gamma-ray transitions between these two levels can be forbidden by choosing quantum numbers which differ by 5 units.