We have measured the resonant cross section ${\sigma }_n$ for nuclear excitation of ${}^{115}\mathrm{In}$ via the radiationless annihilation of a positron with a K-shell electron using a monoenergetic positron beam and a thin In target. We find an upper limit on the resonant cross section ${\sigma }_n<4.3\mathrm{}\times {10}^{-26}{\mathrm{cm}}^2$ at a 99% confidence level, compared to the cross section ${\sigma }_{\beta }=1.7\mathrm{}\times {10}^{-25}{\mathrm{cm}}^2$ determined by two previous measurements of nuclear excitation of ${}^{115}\mathrm{In}$ using the broad spectrum of positrons from the beta decay of ${}^{64}\mathrm{Cu}.$ Together these results imply the existence of a hitherto unidentified nonresonant channel for nuclear excitation via energetic positrons.

• Received 4 December 2000

DOI:https://doi.org/10.1103/PhysRevC.64.054603