Attempts to detect the interaction of neutrons with electrons

The present paper contains an account of investigations made with a Wilson chamber on the penetrating radiation emitted by beryllium when the latter is bombarded by theα -particles of polonium. Dr. Chadwick has suggested that this radiation consists of a stream of neutrons of unit mass and maximum velocity 3·3 X 109 cm. per second. The neutrons in their passage through matter collide occasionally with the atomic nuclei and produce recoil atoms of short range and great ionising power. The recoil atoms of nitrogen have been studied in detail by Dr. Feather, using an automatic expansion chamber, and the lengths of the recoil tracks are in agreement with the neutron hypothesis. It is of special interest to examine the interaction of the neutrons with electrons. The impact of a similar neutron of the same velocity with an electron might be expected to communicate to the latter velocities up to a maximum of 6·6 X 109 cm. per second, that is, twice the velocity of the neutron for a direct collision, corresponding to a maximum energy of the recoil electron of approximately 13,000 electron volts. According to the data of Nuttall and Williams such an electron has a range of recoil of 3·4 mm. As, however, the total ionisation in this length of track would be only 350 ion pairs the valve counter and automatic expansion chamber are unsuitable for its detection. Further, as will be shown later, it is impossible to detect with certainty the occurrence of such recoil electrons produced by neutrons unless the individual water drops produced in the expansion chamber are clearly photographed. For such experiments, and to a still greater degree for the experiments described in 4, it is also necessary that there shall be no appreciable background of drops in the chamber. Even in the absence of ionisation such a general distribution of drops is usually produced unless special precautions are taken. The detection of individual ions by the condensation on them of water vapour and the photography of the individual drops thus produced, while avoiding the presence of water drops which are not associated with ions, has been a special study of the writer working under the supervision of Professor C. T. R. Wilson.