The High Efficiency Neutron Spectrometry Array (HENSA) is an advanced detection system designed for neutron spectrometry in environments with low radioactivity, such as underground laboratories, as well as for measuring cosmic-ray neutrons.

HENSA is used in underground research to study neutron backgrounds that impact experiments searching for dark matter and neutrinos. These studies take place at the Canfranc Underground Laboratory (LSC, Spain) and the INFN Gran Sasso National Laboratory (LNGS, Italy). In the past, HENSA has also been deployed at the Felsenkeller shallow-underground laboratory in Dresden, Germany, to measure neutron backgrounds.

Beyond underground research, HENSA is also used to study cosmic-ray neutrons, including space weather monitoring and environmental dosimetry during Solar Cycle 25. For these studies, an extended-energy-range version of the detector, HENSA++, operates at the Javalambre Astrophysical Observatory (OAJ) in Teruel, Spain, at 1,957 meters above sea level, with a cutoff magnetic rigidity of 7 GV. Additionally, HENSA++ is being used to investigate neutron backgrounds in solar axion searches, such as the future babyIAXO experiment at DESY.

HENSA has been developed through a collaborative effort involving researchers from multiple institutions: Instituto de Física Corpuscular (IFIC/CSIC-UV, Spain), Technical University of Catalonia (INTE-UPC, Spain), University of Granada (UGR, Spain), Universidad Complutense de Madrid (UCM, Spain), Helmholtz-Zentrum Dresden-Rossendorf (HDZR, Germany).

High energy neutrons are produced as secondary particles from interactions of cosmic-rays with atoms in the atmosphere. Cosmic-ray neutrons are the main component of the ambient neutron background at ground level and high altitudes. The total flux and the spectral distribution of cosmic-ray neutrons depend mainly on: the solar activity, the geomagnetic position and the altitude.  Cosmic-ray neutrons are interesting for a wide range of areas, such as space weather, single event upset (SEU) in microelectronics, cosmic-rays physics and environmental radiation dosimetry.

Neutrons are a challenging source of radiation background affecting experiments dealing with rare event searches. In underground facilities, despite cosmic-ray neutron background is largely suppressed, radiogenic neutrons are still produced in the surrounding materials of the facility  (rocks and walls) by means of (alpha,n) reactions and spontaneous fission. These neutrons have a large penetrability in matter and they are able to induce background signals in the detectors used for nuclear astrophysics, neutrino and dark matter experiments.