The High Efficiency Neutron-Spectrometry Array (HENSA) is an extended energy detection system for neutron spectrometry based on the Bonner Spheres principle [THO02]. The current version of HENSA is composed of ten different individual neutron detectors using 60 cm active length cylindrical thermal neutron counters (3He-filled tubes). To provide sensitivity in different energy regions, each tube is embedded in a matrix of different materials, including high-density polyethylene (HDPE) moderators, cadmium shielding and high-energy lead neutron converters. The use of long 3He-filled tubes gives HENSA a neutron response 5 to 15 times higher than that of conventional Bonner sphere systems in the energy range from thermal to 10 GeV.

Neutron characterisation in the central area of Hall A is being performed with a configuration consisting of one bare 3He counter and three others in a polyethylene matrix with cross-sectional sizes of 45×45, 70×70 and 120×120 mm2. This configuration provides accurate neutron sensitivity from thermal energies up to 20 MeV by applying Pulse Shape Discrimination (PSD) techniques. The average measured thermal flux is (3.5 ± 0.8) 10-6 n/cm2/s. Seasonal variations in the neutron flux can mimic an annual modulation comparable to that expected in dark matter detectors. The joint project of the HENSA and ANAIS collaborations proposes to characterise the energy spectrum and time evolution of the neutron flux in Hall B. The neutron monitor, which includes a bare counter and a polyethylene matrix with cross sections of 45x45mm2 and 180x180mm2, has been taking data since 2021.

HENSA Detectors