Artificial disintegration by α-particles

1. It is well known that the bombardment of certain elements by α-particles gives rise to the emission of protons, which are ascribed to the disintegration of the unclei of these elements. It is generally assumed, on the evidence of Blackett’s expansion photographs of the artificial disintegration of nitrogen, that the emission of the proton is the result of the capture of the α-particle by the atomic nucleus, which is thereby transformed into a nucleus of the element of the next higher atomic number. We should, therefore, expect to observe certain simple relations between the energy of the incident α-particle and that of the emitted proton, and we should find that the change of energy in transforming a given nucleus is a fixed amount. In certain cases at least the experimental results have led to no such simple relation, and in the case of aluminium Rutherford and Chadwick have given clear evidence that the energy change in the disintegration is not always the same. The experiments described in this paper were carried out to investigate this question in more detail. In experiments on artificial disintegration, the emitted protons have usually been detected by the scintillations they produce in a zinc sulphide screen. The scintillation method, though simple and powerful, has certain disadvantages which cannot be avoided. The strain of counting the scintillations is such that the observers must be carefully controlled, and they can be allowed to count only for very limited periods, amounting on the average to about 6 hours per week. The accumulation of results by the scintillation method is thus a long and tedious process. Further, no permanent record of the scintillations can be made.