Nucleon capture by 3He and the production of solar hep-neutrinos: Cross-section measurements and shell-model calculations

Abstract

An experimental and theoretical investigation on low-energy nucleon capture by 3He is reported. The extremely small cross section of the 3He(n, γ) reaction has been measured with thermal and with 24.5 keV neutrons, yielding gs n γ(th) = 55 ± 3 μb and σ n γ(24.5 keV) = 9.1 ± 0.8 μb, respectively. Shell-model calculations that include the contributions of two-body meson-exchange currents are presented for the radiative capture of thermal neutrons by 3He as well as for the weak 3He(p, e + ν e) reaction. With the ratio of the corresponding magnetic dipole and Gamow-Teller matrix elements, and the experimental radiative thermal neutron cross section, a value S = (5.7 ± 0.8) × 10 −19 ke V · b is obtained for the astrophysical S- factor of the neutrino-emitting reaction. This indicates that roughly 10% of the measured solar-neutrino flux in the Davis' experiment can be ascribed to the high-energy 3He + p neutrinos.