Nucleon-deuteron capture below the three-nucleon break-up energy

Abstract

Radiative nucleon-deuteron capture is considered at energies below three-nucleon break-up. By using wave function and dispersion theory methods we determine which features of the reaction are independent of the detailed form of the nuclear and electromagnetic interactions. We show that because of the presence of interaction currents, the experimental cross section for thermal neutron capture is at least twice the theoretical value predicted from the neutron and proton magnetic moments. The energy dependence of the M1 capture amplitudes is shown to be determined by the gross properties of the three-nucleon bound state and the S-wave nucleon-deuteron phase shifts. The E1 capture amplitudes are determined by the known properties of nuclear wave functions. In particular the E1 contribution to the isotropic cross section for p-d capture is shown to depend on the asymptotic properties of the D-states of 2H and 3He and not on the D-state probability densities. If the M1 cross section is normalized at 25 keV, we obtain agreement with all the experimental data on the low-energy p-d capture reaction.