A Scintillation Spectrometer for High-Energy Gamma-Rays

Abstract

IT has been shown^1,2 that energy measurements with scintillation counters can be made with considerable accuracy with the aid of the ‘photo-electron lines’ in the pulse–height distribution. But the application of this method to energies exceeding 2 MeV. is attended by difficulties. In this energy-range it is better to measure the energy of the electron-positron pairs produced by the gamma-rays. The following procedure was adopted for this purpose. Collimated gamma-rays are allowed to impinge upon the sodium iodide crystal of a scintillation counter (Fig. 1). The crystal is flanked by two other scintillation counters placed opposite one another. The purpose of these counters is to record the two annihilation quanta emitted from each electron-positron pair produced in the central crystal. Coincidences between these two counters are used to separate pair-production pulses from other pulses of the central counter. Only the former are counted. The pulse – height distribution is studied with the aid of a discriminator. The apparatus was calibrated by measurements made with the central counter only, in the manner described earlier¹.