Neutron induced by an α-beam incident on a deuterium gas target, and the background for a study of the ²H(α,γ)⁶Li reaction at LUNA

The production of the stable isotope ⁶Li in standard Big Bang nucleosynthesis has recently attracted much interest. This is so because some recent observations in metal-poor stars suggest that a cosmological ⁶Li plateau may exist. If true, this plateau would come in addition to the well-known Spite plateau of ⁷Li abundances and would point to a primordial origin of ⁶Li, contrary to the results of standard Big Bang nucleosynthesis calculations. Therefore, the nuclear physics underlying Big Bang ⁶Li production must be revisited. The present work reports on the neutron-induced background encountered in a new study of the ²H(α,γ)⁶Li reaction. In the experiment, an α-beam from the ultra-low background underground accelerator LUNA in Gran Sasso, Italy, and a windowless deuterium gas target are used. A low neutron flux is induced by energetic deuterons from elastic scattering and, subsequently, the ²H(d,n)³He reaction. Due to the ultra-low laboratory neutron background at LUNA, the effect of this weak flux of 2-3 MeV neutrons on well-shielded high purity germanium detectors has been studied in detail. Data have been taken at 280 and 400 keV α-beam energy and for comparison also using an americium-beryllium neutron source. The ramifications for the planned ²H(α,γ)⁶Li measurement are discussed.