A NOTE ON NUCLEAR-EXCITATION BY POSITRON-ANNIHILATION WITH K-SHELL ELECTRONS

Experimental data for the nuclear excitation of In-115 by positron ann ihilation with K-shell electrons have been examined, taking account of the effective thickness of an In target used, and then the cross sect ion sigma(res) for resonance excitation to the 1078 keV energy level b y positron annihilation has been reevaluated. The cross section for th e process is induced by the measured effective cross section sigma(eff ) of nuclear excitation of the isomeric level In-115m. In this work th e effective thicknesses of the In target for positrons with kinetic en ergies 83.9 and 470 keV have been estimated. The effective thickness d elta r has been determined from a relation delta r = delta E/\dE/dr\, where delta E is the positron energy width at 83.9 or 470 keV, delta r is the mean distance traversed by them, and \dE/dr\ the stopping powe r of indium for them. In the present case, Gamma(K), the atomic-level width of the K-level of indium, is used as delta E. Neglecting a contr ibution from the 1464 keV level being much smaller than that from the 1078 keV level, a reevaluated value of sigma(res) has been obtained as 1.7 x 10(-25) cm(2).