British Association Discussions: Nuclear Physics

Abstract

THE discussion on nuclear physics arranged to take place in Section A (Mathematical and Physical Sciences) on August 18 was introduced by Prof. Niels Bohr, of Copenhagen, who gave an account of the new ideas in nuclear theory which have developed under his guidance during the last few years. The old nuclear theory attempted to explain the interactions of fast particles with nuclei by considering the behaviour of single particles inside the nucleus rather on the same lines as in the theory of the outer electronic system. This picture gave a satisfactory account of the penetration of charged particles into light nuclei but failed to account for many phenomena, in particular the large probability of capture of slow neutrons by nuclei relative to the probability of elastic scattering. These difficulties have been removed by the realization that owing to the tight packing of particles within the nucleus, there is a great facility of energy exchange between the particles. In consequence, when a particle penetrates a nucleus its energy is rapidly distributed amongst all the particles, resulting in a general increase in 'nuclear temperature'. The nucleus then remains in the excited state until sufficient energy is again concentrated on one particle for 'evaporation' or escape to occur. Alternatively, the state of excitation may decay by emission of radiation, but owing to the high symmetry of charge distribution, dipole radiation is in general unlikely and the decay period consequently long. The 'intermediate nucleus' thus exists for a period long compared with the time which would have been taken for the incident particle to traverse the system unhindered.