In the field of rare events searches, a precise knowledge of the environmental backgrounds is crucial for the design of appropriate shieldings for large scale experiments. Among the possible background sources, neutrons are often poorly known due to the lack of an affordable detector technology for the measurement of low-flux neutron spectra from thermal energies to 20 MeV in a short time.

The last decade has witnessed the development of several garnet scintillation crystals containing gadolinium, which are now available with dimensions suitable for the use as particle detectors. Gd₃Al₂Ga₃O₁₂ (GAGG) is of particular interest for its high scintillation light yield, good timing performance, and capability of particle identification via pulse-shape discrimination. Thanks to its high gadolinium content, the application of GAGG as a neutron detector is being considered by the scientific community. In a low-background environment, e.g. an underground laboratory, the distinctive signature produced by neutron capture on gadolinium, namely a γ-ray cascade releasing ~8 MeV of total energy, and the efficient particle identification provided by GAGG would yield a background-free neutron signal.

Exploiting the outstanding scintillation properties of GAGG, the recent availability of crystals of ~100 cm3 volume, and the most up-to-date developments on Monte Carlo simulations of gamma ray cascades produced by neutron capture reactions, we aimed at demonstrating the possibility of neutron spectroscopy in low-background environments using GAGG-based detectors.

We aimed at realizing an array of 10 GAGG crystals read-out with Photo-Multiplier Tubes and operated in Bonner spheres of suitable thicknesses, and at measuring the neutron spectrum underground in the INFN Gran Sasso National Laboratory (INFN-LNGS), reaching a 10% statistical uncertainty with 6 months of data taking.

Such a detector would allow for the first time to perform coherent and consistent measurements of the neutron spectra at different locations of the INFN-LNGS underground laboratory, as well as in other underground laboratories worldwide. Moreover, it would represent a valid alternative to the well-established 3He proportional counters, whose price and availability has varied significantly over the last decades.

The project was structured in three parts: the detector development and characterization with LED pulses and radioactive γ and α sources, the characterization with neutron sources, and the measurement of the neutron spectrum underground.

We envisaged two publications: on the detector characterization and performance, and on the measurement of the neutron spectrum and of the detector intrinsic background.

All hardware operations took place at INFN-LNGS, which is an exceptional environment thanks to the local expertise in the development and fabrication of low-background detectors, and the availability of experimental space underground.

The sudden and unexpected unavailability of GAGG crystals on the market caused by the Rare Earth Ban introduced by the People’s Republic of China in April 2025 made it practically impossible for us to proceed with the last part of the project as originally devised, forcing us to re-design the final setup with mixed configuration of GAGG and NaI detectors.