Wirkungsquerschnitte für Einquantenvernichtung von Positronen niedriger Energie am Jod

Abstract

Positrons of 200 keV were focused on a slice of NaI. This acted as a scintillation spectrometer and measured the energy loss of the positrons before annihilation. Single-quanta, produced with a probability of only 2.6 · 10^-5, were registered by a second NaI-detector. Two-parameter analysis of the coincidences between these two detectors allowed the determination of six cross sections for single-quantum annihilation between positron energies of 80 and 180 keV. The cross section at 180 keV was 0.09 barn per iodine-atom and decreased with decreasing positron energy, due to the coulombic repulsion. Quantitative agreement was obtained with theoretical calculations within the limits of error.