Background studies at the LUNA underground accelerator facility

Complete text of publication follows. The LUNA (Laboratory Underground for Nuclear Astrophysics) facility has been designed to study nuclear reactions of astrophysical interest. It is located deep underground in the Gran Sasso National Laboratory, Italy. Two electrostatic accelerators, with 50 and 400 kV maximum voltage, in combination with solid and gas target setups allowed to measure the total cross sections of the radiative capture reactions 2H(p,γ)3He and 14N(p, γ)15O within or close to their relevant Gamow peaks. While the laboratory background can be reduced by proper shielding, it is difficult and in some cases impossible to shield the detector against γ-rays arising from parasitic reactions induced by the ion beam incident on the target system. The proton beam induced back- ground for a setup with solid state target has previously been investigated in the energy region from Ep 140 - 400 keV. It was found that the principal background reactions were 11B(p,γ)12C, 18O(p,γ)19F, and 19F(p,αγ)16O. They originated from the target itself, and a reduction in their yield was achieved by making adjustments in target production and preparation. Here, we report on the gamma background at the gas target system in the Gran Sasso Laboratory measured by HPGe and BGO detectors, with an incident proton beam. A spectrum obtained with the HPGe detector at Ep = 200 keV proton energy and with 1mbar of nitrogen gas is shown at the top of Fig. 1. In the spectrum, the most important background lines as well as the lines from the 14N(p,γ)15O reaction are identified. At the bottom of Fig. 1, two spectra obtained with the BGO detector at Ep = 200 keV are shown: one denoted as 'nitrogen' with 1mbar nitrogen as target gas, the other denoted as 'helium' with 1mbar helium as target gas. It is clear that high resolution germanium detector is needed in order to identify the background visible in the spectra taken with the BGO detector. The localisation of the background sources has been done using the Doppler shift of the γ lines. For a given transition and beam energy, the sign and magnitude of the shift depend only on the angle of emission θ as measured from the beam direction. This allowed to localize in several cases the source of the γ-rays. In conclusion, the intensities and the sources of the beam induced background lines have been determined at the gas target system of LUNA. These results help to design further underground experiments and reach even lower background at LUNA. (author)