Cross section of 3He(α, γ)7Be around the 7Be proton separation threshold

Abstract

Background: The He-3(alpha, beta)Be-7 reaction is a widely studied nuclear reaction; however, it is still not understood with the required precision. It has a great importance both in Big Bang nucleosynthesis and in solar hydrogen burning. The low mass number of the reaction partners makes it also suitable for testing microscopic calculations. Purpose: Despite the high number of experimental studies, none of them addresses the He-3(alpha, beta)Be-7 reaction cross sections above 3.1-MeV center-of-mass energy. Recently, a previously unobserved resonance in the Li-6(p, gamma)Be-7 reaction suggested a new level in Be-7, which would also have an impact on the He-3(alpha, beta)Be-7 reaction in the energy range above 4.0 MeV. The aim of the present experiment is to measure the He-3(alpha, beta)Be-7 reaction cross section in the energy range of the proposed level. Method: For this investigation the activation technique was used. A thin window gas-cell target confining He-3 gas was irradiated using an alpha beam. The Be-7 produced was implanted into the exit foil. The Be-7 activity was determined by counting the gamma rays following its decay by a well-shielded high-purity germanium detector. Results: Reaction cross sections have been determined between E-cm = 4.0 and 4.4 MeV with 0.04-MeV steps covering the energy range of the proposed nuclear level. One lower-energy cross-section point was also determined to be able to compare the results with previous studies. Conclusions: A constant cross section of around 10.5 mu b was observed around the Be-7 proton separation energy. An upper limit of 45 neV for the strength of a He-3(alpha, beta)Be-7 resonance is derived.