Nuclear Excitation by Positron Annihilation

Abstract

A new mode of positron annihilation has been investigated experimentally. In this annihilation process without emission of radiation, the excess energy liberated is given to the nucleus involved. The experimental evidence for this process was established by observing the conversion electrons from $^{115m}\mathrm{In}$, first excited level (335 keV, ${\frac{1}{2}}^{\ensuremath{-}}$), after irradiation of natural indium foils by positrons from the ${\ensuremath{\beta}}^{+}$ decay of $^{22}\mathrm{Na}$. Using the experimental data obtained, we have attempted to evaluate the cross section for positrons having just the energy corresponding to that necessary to excite $^{115}\mathrm{In}$ from the ground state to an excited level which may decay to the isomeric level concerned. The experimental value of the cross section is found to be \ensuremath{\sim}${10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$, while the theoretical value estimated by Present and Chen is \ensuremath{\sim}${10}^{\ensuremath{-}26}$ ${\mathrm{cm}}^{2}$. Possible reasons for this difference are discussed.