A high-efficiency gas target setup for underground experiments, and redetermination of the branching ratio of the 189.5 keV22Ne(p,gamma)23Na resonance

The experimental study of nuclear reactions of astrophysical interest is greatly facilitated by a low-background, high-luminosity setup. The Laboratory for Underground Nuclear Astrophysics (LUNA) 400 kV accelerator offers ultra-low cosmic-ray induced background due to its location deep underground in the Gran Sasso National Laboratory (INFN-LNGS), Italy, and high intensity, 250–500 μA, proton and alfa ion beams. In order to fully exploit these features, a high-purity, recirculating gas target system for isotopically enriched gases is coupled to a high-efficiency, six-fold optically segmented bismuth germanate (BGO) gamma-ray detector. The beam intensity is measured with a beam calorimeter with constant temperature gradient. Pressure and temperature measurements have been carried out at several positions along the beam path, and the resultant gas density profile has been determined. Calibrated gamma-intensity standards and the wellknown Ep = 278 keV 14N(p,gamma)15O resonance were used to determine the gamma-ray detection efficiency and to validate the simulation of the target and detector setup. As an example, the recently measured resonance at Ep = 189.5keV in the 22Ne(p,gamma)23Na reaction has been investigated with high statistics, and the gamma-decay branching ratios of the resonance have been determined.