Photoactivation investigations withIn115

A ${}_{}{}^{60}{}_{}{}^{}\mathrm{Co}$ source has been used to study the photoactivation of the 336 keV ($T_{\frac{1}{2}}\sim 4.3$h) isomeric level of ${}_{}{}^{115}{}_{}{}^{}\mathrm{In}$. In contrast to earlier investigations, we have considered the possibility of nonresonant processes and have developed a technique which allows us to distinguish between activation via the resonance fluorescence of the 1078 keV ${}_{}{}^{115}{}_{}{}^{}\mathrm{In}$ level, and by nonresonant processes involving the 1172 and 1332 keV ${}_{}{}^{60}{}_{}{}^{}\mathrm{Co}$ photons. We have found that nonresonant processes are the dominant modes of activation. Although various possible mechanisms for the nonresonant excitation have been considered, none seem sufficiently strong to explain our results.