Some measurements of γ - ray energies

Abstract

The measurement of the quantum energies of γ -radiation has presented serious difficulties. Crystal diffraction is capable of high resolution and accuracy, but is at present applicable only when strong sources are available and for quantum energies of less than about 1.0 x 10 6 eV. When dealing with weaker sources the absorption method affords a convenient means of determining the energy of γ -radiation if this should consist of a single line, but is unsatisfactory for the analysis of a more complex spectrum. A somewhat more reliable method is found in the determination of the energies of Compton recoil electrons produced by the radiation. Results obtained by simple absorption methods are again unsatisfactory in the case of complex spectra, but by obtaining curved trajectories in a magnetic field, the energies of the Compton recoil electrons may be more accurately estimated, and hence a knowledge of the quantum energies and relative intensities of the component radiations may be acquired. The recoil electrons may be detected in an expansion chamber or by Geiger-Müller counters. Of these, the counters have the great advantage of being practically con­tinually “sensitive”, whereas the cloud chamber is sensitive only over a relatively short interval, and the difficulty of reducing the statistical fluctuations in the number of particles of different energies observed is correspondingly greater. The following paper describes results which have been obtained by the use of a “ γ -ray spectrograph” in which the magnetically focused recoil electrons are detected by a system of Geiger-Müller counters.