The scattering of α-particles by light elements

Abstract

It is well known that the scattering of fast a-particles by light elements shows very remarkable deviations from the classical law of scattering deduced from the assumption that the nuclei behave as point charges, surrounded by a Coulomb field of force. The first anomalous effects of this kind were obtained in 1919 by Rutherford in an investigation of the collisions of α-particles with hydrogen nuclei. Chadwick and Bieler found in later experiments that the number of H-particles observed at small angles, which, seen from the centre of gravity of the colliding system, correspond to α-particles scattered through large angles, was for the fastest α-particles about 100 times larger than the number calculated on the assumption of Coulomb forces between the colliding particles. They suggested that this effect was due to an oblate spheroidal shape of the α-particle. This assumption also agreed with subsequent experiments by Rutherford and Chadwick on the scattering of α-particles in helium. They found that for high velocities of the incident α-particles the number of particles scattered through 45° was about twenty times larger than expected. For smaller initial velocities and smaller angles this ratio became much less, for certain velocities only one-third. The scattering of a-particles by the nuclei of other light elements has been observed only in two cases, those of magnesium and aluminium. The first investigations of this kind were those of Bieler. He found that the ratio of the observed scattering to that given by inverse square forces is close to unity for small angles, but diminishes as the angle increases, gradually for slow α-particles, more rapidly for the faster ones. Rutherford and Chadwick extended these results and found that for large angles and fast α-particles the ratio of observed to calculated scattering after diminishing to a value of about one-third, began to increase again. In later (unpublished) experiments Chadwick found that the ratio increased in the case of fast α-particles scattered through very large angles to a value considerably higher than unity, An explanation for the anomalous scattering shown by magnesium and aluminium has been advanced by Debye and Hardmeier on general lines. If, as we suppose, the aluminium nucleus is a complex structure of positive and negative charges, this structure may be distorted or polarised in the field of the approaching α-particle. The polarisation gives rise to an attracting force on the α-particle, which varies inversely as the fifth power of the distance. Debye and Hardmeier showed that this hypothesis would account very fairly for the experimental results.