Resonant versus nonresonant nuclear excitation of In-115 by positron annihilation - art. no. 054603

We have measured the resonant cross section sigma (n), for nuclear excitati on of In-115 via the radiationless annihilation of a positron with a K-shel l electron using a monoenergetic positron beam and a thin In target. We fin d 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 <sigma>(beta)= 1.7 x 10(-25) cm(2) determined by two previous measurements of nuclear exc itation 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 unident ified nonresonant channel for nuclear excitation via energetic positrons.