Measurements of range and angle of projection for the protons produced in the photo-disintegration of deuterium

Abstract

The disintegration of a nucleus by the action of γ -rays—the nuclear photo-effect—was first observed by Chadwick and Goldhaber (1934) in the case of deuterium. The products of the disintegration are of course a proton and a neutron. In the first experiments only the protons liberated in the transformation were detected. This was done by means of an ionization chamber and “proportional” valve amplifier, and in this way a rough esti­mate was obtained of the mean kinetic energy with which the protons were ejected. Later the same authors (1935) supplemented the earlier evidence by detecting the neutrons liberated in the disintegration. For this purpose a boron-lined ionization chamber embedded in paraffin wax was used. They were further able to show that, per unit solid angle, the number of neutrons emitted in the direction of the γ -ray quanta was roughly about half the number emitted at right angles to that direction. The present paper describes an attempt to obtain more detailed information concerning the angular distribution of the disintegration particles and their binding energy in the deuterium nucleus. As was pointed out in each of the earlier discussions, such information can be obtained easily only by use of the expansion chamber. With the expansion chamber it is merely necessary to employ a deuterium-rich gas as filling and to be able to take satisfactory photographs of heavy particle tracks in the presence of the relatively intense background of cloud arising from the general γ -ray ionization in the chamber. These two conditions were successfully realized in the early summer of 1935, and a large number of photographs was taken with various arrangements. This work was carried out in the Cavendish Laboratory. Before all the photographs had been measured a preliminary report was made to the Norwich meeting of the British Association in September 1935. About the same time all three authors left Cambridge, and the task of completing the measurements and writing the final account of the research was considerably delayed in consequence. Two of us (J. C. and N. F.), however, made an entirely new set of measurements on all the photographs in March and April 1936. The results of these observations, carried out in the Physics Laboratory of the University of Liverpool, showed good agreement with the earlier incomplete data, and the final range and angular distributions have been based upon them exclusively.