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 the13C(,)16O reactiontowards its astrophysical-process Gamow peak in the low-background environment of the LaboratoriNazionali del Gran Sasso (LNGS). Eighteen3He counters are arranged in two different configurations (in avertical and a horizontal orientation) to optimize neutron detection efficiency, target handling and targetcooling over the investigated energy range E,lab= 300 − 400keV (En= 2.2 − 2.6 MeVin emitted neutronenergy). As a result of the deep underground location, the passive shielding of the setup and active backgroundsuppression using pulse shape discrimination, we reached a total background rate of1.23 ± 0.12counts/hour.This resulted in an improvement of two orders of magnitude over the state of the art allowing a directmeasurement of the13C(,)16O cross-section down to E,lab= 300keV. The absolute neutron detectionefficiency of the setup was determined using the51V(p,n)51Cr reaction and an AmBe radioactive source, andcompleted with a Geant4 simulation. We determined a (34±3)% and (38±3)% detection efficiency for thevertical and horizontal configurations, respectively, for En= 2.4 MeVneutrons