A note on nuclear excitation by positron annihilation with K-shell electrons

Experimental data for the nuclear excitation of115In by positron annihilation with K-shell electrons have been examined, taking account of the effective thickness of an In target used, and then the cross sectionσres for resonance excitation to the 1078 keV energy level by positron annihilation has been reevaluated. The cross section for the process is induced by the measured effective cross sectionσeff of nuclear excitation of the isomeric level115mIn. In this work the effective thicknesses of the In target for positrons with kinetic energies 83.9 and 470 keV have been estimated. The effective thickness δr has been determined from a relation δr=δE/ ¦dE/dr¦, where δE is the positron energy width at 83.9 or 470 keV, δr is the mean distance traversed by them, and ¦dE/dr¦ the stopping power of indium for them. In the present case,δK, the atomic-level width of the K-level of indium, is used as δE. Neglecting a contribution from the 1464 keV level being much smaller than that from the 1078 keV level, a reevaluated value ofσres has been obtained as 1.7×10−25 cm2.