Ultra-sensitive gamma-ray spectroscopy set-up for investigating primordial lithium problem

To precisely determine BBN Li-6 production, the cross-section of the nuclear reaction H-2(alpha, gamma)Li-6 must be directly measured within the astrophysical energy range of 30-400 keV. This measure requires an ultra low gamma-ray background in the experimental set-up. We have realized the conditions matching these very strict requirements at LUNA, the deep underground accelerator laboratory active in the INFN Gran Sasso National Laboratory (LNGS), Italy: the gamma-ray spectrometer background has been reduced down to reach unmatched low levels, comparable to the good ones experienced in dedicated off-line underground ultra low gamma counting rate. We present and discuss the gamma-ray background reduction reached in the HpGe spectrometer, where most of the remaining gamma-ray background seen in the spectra are coming from the energetic deuterons scattered in the gas target by the alpha beam. Thanks to the low neutron environmental background at LUNA, the effect of this weak flux of 2-3 MeV neutrons on HpGe detectors has been studied in details and the results are presented and discussed.