Radioactivity Induced by Nuclear Excitation I. Excitation by Neutrons

It is shown that the 4.1-hr. period of indium can be produced by nuclear excitation of indium and is to be attributed to an excited metastable state, ${\mathrm{In}}^{115,*}$ of the stable ${\mathrm{In}}^{115}$. This result is obtained by studying the radioactivity produced in indium by neutrons of different energy distributions and by studying the chain reactions produced in cadmium by fast neutrons. It is found that ${\mathrm{In}}^{115*}$ can be produced from ${\mathrm{In}}^{115}$ by 2.5-Mev neutrons, but not noticeably by strong sources of photoneutrons which have energies of a few hundred thousand electron volts. A radioactive ${\mathrm{Cd}}^{115}$ of 2.5-day half-life time is found to transform with emission of negative electrons into ${\mathrm{In}}^{115*}$. ${\mathrm{Cd}}^{115}$ was produced by neutron loss from ${\mathrm{Cd}}^{116}$ and by neutron capture from ${\mathrm{Cd}}^{114}$.