Cloud chamber studies
Supporting evidence regarding the nature of the artificial disintegration was obtained in 1925 by Prof. P. M. S. Blackett, who became President of the Royal Society in 1965.
Blackett shot ex-particles into a cloud chamber containing various gases and was able to study ordinary collision processes as well as disintegration.
Fig. 46.9 illustrates the common straight-line a-particle tracks which are observed. Occasionally, however, a cloud track displays a forked end (Fig. 47.3). This results from one of the rare occasions when an ex-particle approaches so close to the nucleus of a gas atom that it is deflected out of its path. The spur track is caused by the recoiling nucleus.
Thus, when helium was bombarded, the two tracks were approximately at -right
equal mass (an IX-particle is a helium nucleus). But in the case of oxygen the spur track is much shorter owing to the greater mass of the oxygen nucleus compared with that of the e-particle.
Blackett’s most striking achievement was the evidence he obtained to confirm the disintegration of nitrogen by alpha bombardment. After many trials he obtained a photograph which showed the path of the ejected proton on leaving the nitrogen nucleus (Fig. 47.3 (c)). The thin track going off to the left is that of the proton, and the thicker track that of the oxygen nucleus which results from the disintegration of the nitrogen nucleus.