Resonant versus nonresonant nuclear excitation of In-115 by positron annihilation

We have measured the resonant cross section sigma (n), for nuclear excitation of In-115 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 x 10(-26) cm(2) at a 99% confidence level, compared to the cross section (beta)= 1.7 x 10(-25) cm(2) determined by two previous measurements of nuclear excitation of In-115 using the broad spectrum of positrons from the beta decay of Cu-64. Together these results imply the existence of a hitherto unidentified nonresonant channel for nuclear excitation via energetic positrons.