Physics with HENSA

Neutrons in underground facilities

Hall A, Canfranc Underground Laboratory (LSC, Spain)

Deep underground laboratories offer an exceptionally low-radioactivity environment, making them ideal for a wide range of cutting-edge experiments in astroparticle physics, nuclear astrophysics, biology, and geology.

One of the biggest challenges for rare-event experiments is the presence of neutron background radiation. Due to their ability to penetrate materials, neutrons can interfere with experiments searching for neutrino-less double-beta decay and dark matter. In nuclear astrophysics, certain key reactions of the s-process—which governs the formation of heavy elements in stars—require ultra-low neutron background conditions for precise measurements.

Most underground neutron measurements rely on thermal neutron counters or scintillators sensitive to fast neutrons, but fully spectrometric neutron studies remain rare. The HENSA project is helping to fill this gap.

Currently, two HENSA detectors are operational in Europe’s leading underground laboratories: the Canfranc Underground Laboratory (LSC, Spain) and the INFN Gran Sasso National Laboratory (LNGS, Italy). Research with these detectors aims to:

Characterize neutron sources in underground environments
Study potential annual variations in neutron flux
Monitor neutron backgrounds affecting sensitive underground experiments

HENSA has also been used to perform fully spectrometric neutron measurements at the Dresden-Felsenkeller shallow underground facility, providing valuable data for nuclear astrophysics research.

Solar cycle and space weather

NOAA/NASA forecast for Solar Cycle 25. Maximum solar activity expected for July, 2025 (+/- 8 months). The solar minimum between Solar Cycle 24 and 25 - the period when the sun is least active - happened in December 2019.

Neutron background anti-correlation with solar cycle. Cosmic Ray flux from the Climax Neutron Monitor (USA) and re-scaled Sunspot Number.

The Sun magnetic activity is ruled by a cycle of 11 years. Currently, we are starting cycle 25 (Fig. a). The solar activity is monitored by means of the number of sunspots which are areas on the sun where the magnetic field is much bigger than in the rest of the star. The lower number of sunspot, the lower solar magnetic activity effects on Earth. Each solar cycle presents a peak of low and high activity. The total neutron flux induced by solar cosmic-rays is anti-correlated with the solar cycle (Fig. b). Thus, measurements with the HENSA detector during 2020 will provide maximum cosmic-ray induced neutron flux during the solar cycle 25.

Single-Event Upsets in microelectronics

A network of electricity pylons in central Xinjiang, China. (© HUGOCISS/moment/Getty Images)

High energy neutrons, originated from cosmic-rays, are one of the major source of “Single Event Upset” (SEU) events. These malfunctions in microelectronic systems are caused by the collision of neutrons with the chips of the affected electronic device. The collision can cause ionization at a sensitive node of the micro-electronic device. Unlike other related disruptions, SEU events do not generally cause permanent damage, but still can have relevant consequences. Experimental data on cosmic-ray neutrons helps to improve the knowledge on performance and lifetime of strategic infrastructure, such as power grids, communications systems, avionics, defense, etc.

Environmental radiation dosimetry

Neutron conversion factors from fluence to ambient dose equivalent. ICRP74 is the current recommendation (blue). ICRU95 is the new recommendation (red).

There is a need for improved experimental data in order to clarify differences between previous calculation models and previous measurements of natural neutron fluence and dose values at ground and high-altitude levels. In addition, the International Commission on Radiation Units & Measurements (ICRU) have recently introduced new recommendations concerning the operational quantities for radioprotection (ICRU report 95). The new recommended operational quantities will provide a solution to the protection problems in high-energy neutron fields. A revision dosimetric reference values, based on the latest available information, will be required for the assessment of individual exposure at ground level and aircrafts during quiet and active space weather conditions. With the HENSA project we will be able to provide the most recent data at different latitudes in Europe.

Neutron production by lightning discharges

A lightning discharge happening close to a residential area.

Fast neutron burst are generated by natural means in atmospheric discharges (lightning). This radiation is correlated with gamma-flashes observed from satellites (TFG) and ground. Despite of the fact that this phenomenon was proposed in the seventies and first observed in the middle eighties, a satisfactory explanation of the underlying processes consistent with observations has remained elusive for decades. In 2013, the first laboratory scale high-voltage atmospheric discharge producing neutrons was reported (Agafonov et al). In 2017, experimental evidence supporting a mechanism driven by photonuclear reactions in the atmosphere, in particular the 14N(g,n)13N, has been presented for the first time (Enoto et al.). In 2018, simulations focused on the photonuclear mechanism have predicted the existence of a prompt (<1ms, ~0.1-10.0MeV) and a delayed (>1ms, epithermal up to 100keV) component in Lightning Correlated Neutron Burst (LCNB) at ground level (Diniz et al.). Direct spectrometric measurements of LCNB, at ground level and laboratory scale, will provide valuable information in order to elucidate the underlying mechanism of neutron production in lightning discharges.

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