Characterization of the LUNA Neutron Detector Array for the Measurement of the 13c(α, N)16o Reaction

We introduce the LUNA neutron detector array developed for the investigation of the C-13(alpha, n)O-16 reaction towards its astrophysical s-process Gamow peak in the low-background environment of the Laboratori Nazionali del Gran Sasso (LNGS). Eighteen He-3 counters are arranged in two different configurations (in a vertical and a horizontal orientation) to optimize neutron detection efficiency, target handling and target cooling over the investigated energy range E-alpha,E-lab = 300 - 400 keV (E-n = 2.2 - 2.6 MeV in emitted neutron energy). As a result of the deep underground location, the passive shielding of the setup and active background suppression using pulse shape discrimination, we reached a total background rate of 1.23 +/- 0.12 counts/hour. This resulted in an improvement of two orders of magnitude over the state of the art allowing a direct measurement of the C-13(alpha, n)O-16 cross-section down to E-alpha,E-lab = 300 keV. The absolute neutron detection efficiency of the setup was determined using the V-51(p,n)Cr-51 reaction and an AmBe radioactive source, and completed with a Geant4 simulation. We determined a (34 +/- 3)% and (38 +/- 3)% detection efficiency for the vertical and horizontal configurations, respectively, for E-n = 2.4 MeV neutrons.