Characterization of the LUNA neutron detector array for the measurement of the 13C(a,n)16O reaction

TL;DR

The paper describes the development and characterization of the LUNA neutron detector array, optimized for measuring the 13C(a,n)16O reaction at low energies relevant to astrophysics, achieving unprecedented low background and high efficiency.

Contribution

The paper introduces a novel neutron detector array with optimized configurations and background suppression, enabling direct measurement of the 13C(a,n)16O cross-section at astrophysically relevant energies.

Findings

Achieved a background rate of 1.23 counts/hour, two orders of magnitude lower than previous setups.

Determined neutron detection efficiencies of approximately 34% and 38% for two configurations.

Enabled direct measurement of the 13C(a,n)16O reaction cross-section down to 300 keV.

Abstract

We introduce the LUNA neutron detector array developed for the investigation of the 13C(a,n)16O reaction towards its astrophysical s-process Gamow peak in the low-background environment of the Laboratori Nazionali del Gran Sasso (LNGS). Eighteen 3He counters are arranged in two different configurations (in a vertical and a horizontal orientation) to optimize neutron detection effciency, target handling and target cooling over the investigated energy range Ea;lab = 300 - 400 keV (En = 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 13C(a,n)16O cross-section down to Ea;lab = 300 keV. The absolute neutron detection efficiency of the setup was determined using the 51V(p,n)51Cr 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 En = 2.4 MeV neutrons.