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

Background: The 3He(α, γ )7Be 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 3He(α, γ )7Be reaction cross sections above 3.1-MeV center-of-mass energy. Recently, a previously unobserved resonance in the 6Li(p, γ )7Be reaction suggested a new level in 7Be, which would also have an impact on the 3He(α, γ )7Be reaction in the energy range above 4.0 MeV. The aim of the present experiment is to measure the 3He(α, γ )7Be 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 3He gas was irradiated using an α beam. The 7Be produced was implanted into the exit foil. The 7Be activity was determined by counting the γ rays following its decay by a well-shielded high-purity germanium detector. Results: Reaction cross sections have been determined between Ecm = 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 μb was observed around the 7Be proton separation energy. An upper limit of 45 neV for the strength of a 3He(α, γ )7Be resonance is derived