Radiative Capture in Few‐Nucleon Systems and Exchange Currents

Abstract

Low‐energy radiative neutron capture by the target nuclei 1H, 2H, and 3He depends on exchange currents. For radiative capture of thermal neutrons in the hydrogen isotopes, contributions of meson‐exchange currents (MEC) range from 10–50%, but these do not explain the photon circular polarization in polarized neutron capture by protons. In order to confirm previous data a feasibility study on the 1H(n↘th,γ↘) reaction has been performed. Experiments on polarized deuterium in a ZrD2 target have been performed in order to unravel the channel‐spin admixtures for the 2H(n,γ) reaction, and to enable a comparison with model calculations. For 3He a radiative thermal neutron capture cross section of 55(3) μb has been measured for single‐photon emission, and 30(80) μb for 3He(n,γγ) double‐photon emission. Accurate cross‐section data for radiative capture of thermal neutrons in 3He are relevant to a solution of the solar‐neutrino problem. Shell‐model calculations, which include meson‐exchange currents, were performed for the 3He(n,γ) reaction and for the so called hep process: 3He+p→4He+e++νe . A theoretical result of 48(17) μb for the thermal neutron capture cross section, which agrees with the experiment, and a much more accurate estimate for the cross‐section ratio of thermal neutron capture‐ and hep‐process were obtained. Together with the present experiments this ratio would imply a hep neutrino flux on earth equal to 5.7(8)⋅104 cm−2s−1. This flux corresponds to about 10 (respectively 3) percent of the measured (respectively calculated) count rate of the 37Cl detector.