SUBEXCITATION POSITRONS

Lifetime spectra of positrons in positronium (Ps)-forming substances can be affected significantly by the addition of small concentrations of impurities with ionization potentials smaller than those of the pure substances. Such impurities permit Ps formation by subexcitation positrons, i.e., by positrons with kinetic energies less than the threshold energy for the formation of Ps in the pure substance. The impurity-induced Ps yield can increase further under the influence of an electric field at field strengths so moderate that the Ps yield in the pure substance remains unaffected. The quantitative aspects of the impurity-induced electric field effect are indicative of the cross sections for elastic scattering and for energy loss of subexcitation positrons in the substance. Therefore, subexcitation positrons permit the experimental study of electronic subexcitation cross sections in a large class of substances. © 1969 The American Physical Society.