Anomalous scattering of α-particles and long-range nuclear forces

Extract

The recent experiments of Mohr and Pringle on the anomalous scattering of α-particles in hydrogen, deuterium and helium have revealed the existence of appreciable deviations from Coulomb's law at quite large distances (of the order 10⁻¹² cm.). One explanation of this result which immediately suggests itself is that these deviations arise from ordinary electrostatic polarization of the colliding nuclei due to their electric charges, an effect considered originally by Debye and Hardmeier by classical methods to explain anomalous scattering. It is fortunately easy to examine whether this is the correct explanation. If we assume polarization to be the main cause of long-range anomalies in nuclear interaction, it is clear that these anomalies must fall off inversely as the fifth power of the distance. From the observed scattering of α-particles in helium which have √20 units of angular momentum relative to the struck nuclei and so pass at distances where the long-range anomalies are alone important, it is possible to determine the constant in the inverse fifth power law. From this constant the polarizability of the α-particle necessary to give rise to the observed effects can be determined. Now although it is not possible to calculate the polarizability of an α-particle with any degree of accuracy, our knowledge of its structure not being sufficiently definite, we can perform the calculation for the deuteron with some accuracy. Since the a-particle is certainly a more compact structure than the deuteron there is no doubt that it has a much smaller polarizability. Comparison of polarizabilities derived from the scattering and calculated from the known structure of the deuteron can therefore decide for or against this explanation of the anomalies.