Study of Neutron Induced Reactions on 7Be Using Large Angle Coincidence Spectroscopy

Neutron induced reactions on the radioactive nuclei 7Be represent a unique tool for study of the 8Be nuclear structure in the high-energy region. The interactions of thermal neutrons with 7Be is dominated by the strong 7Be(nth,p)7Li reactions. While 8Be in the ground state disintegrates rapidly into two α particles, the alpha decay from the 2− neutron capture state is prohibited because of the parity violation. The study of the 7Be(nth,α)4He reaction (with two 9496 keV α particles) is then a unique tool to study parity violation in strong interactions. The test was carried out on the collimated thermal neutron beam at NPI Rez using the large angle coincidence spectroscopy. The 7Be nuclei were prepared at the ISOLDE facility by implantation of the 60 keV 7Be ions into 5 μm Al foil. The results showed that the ratio of the cross sections is σ(p1)/σ(p0) = (1.58 ± 0.06) x 10-2, and in the coincidence measurement simultaneous emission of two 8775 keV α particles is observed. This reaction channel corresponds to a two-stage process comprising transition from the neutron capture state (18991 keV) to an intermediate state by emission of γ photon that is followed by the decay into two alphas. The intermediate state in the highly excited 8Be nuclei is the level at 17640 keV (1+), which is in contrast to the previous reported data. In the experiment no α particles with the energy 9495 keV were observed.