  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
$\mu$A, proton and $\alpha$ 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 well-known $E_p$ = 278 keV
$\mathrm{^{14}N(p,\gamma)^{15}O}$ 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 $E_p$ =
189.5 keV in the $^{22}$Ne(p,$\gamma$)$^{23}$Na reaction has been investigated
with high statistics, and the $\gamma$-decay branching ratios of the resonance
have been determined.
