Scattering of Hard Gamma Rays by Lead, and the Annihilation of Positive Electron

Abstract

THE following experiment was carried out to make a further test of the hypothesis, proposed by Blackett and Occhialini, that the nuclear scattering of hard rays by heavy elements is due to the annihilation of positive electrons produced by the rays1. rays from an emanation tube were used, the source being. placed at S (Fig. 1). Observations were made on the ionisation produced in the chamber I due to the introduction of a thin lead foil (0·1 mm.) into the position BB first, with an aluminium sheet (3 mm. thick) at AA, secondly without the aluminium sheet. According to the above hypothesis, the ionisation should be greater in the first case, because the positive electrons which are produced in the lead foil and escape from it in the forward direction, are stopped in the aluminium sheet. In the second case they are practically lost from the neighbourhood of the ionisation chamber, and therefore also their annihilation radiation. Apart from this, the only difference between the two cases is that, in the first, the lead foil absorbs some of the radiation scattered by the aluminium. This tends to make its effect smaller than in the second case, and is therefore in the opposite direction to the above positive electron effect. Quantitatively, the absorption effect is, however, quite small, being only 2 per cent of the total effect of the lead foil.