Commissioning of a mobile neutron spectrometer for LNGS

TL;DR

The paper addresses the challenge of characterizing underground neutron backgrounds relevant to rare-event searches by presenting ALMOND, a mobile neutron spectrometer that employs capture-gated spectroscopy with gadolinium-coated plastic scintillators. It documents calibration campaigns at KIT and ENEA Frascati, including time-of-flight and capture-time analyses, and reports the first commissioning results at LNGS Hall A, demonstrating sensitivity to ambient neutron flux. The work provides a validated approach for mapping the neutron background across LNGS, with implications for shielding and veto design, and outlines plans to extend measurements to Hall C and other LNGS locations. This advances the ability to design robust background mitigation strategies for underground experiments.

Abstract

Environmental neutrons are a source of background for rare event searches in underground laboratories. Since the majority of the neutron background comes from the cavern walls due to the intrinsic radioactivity of concrete and rock, the flux is known to be time and location dependent. Therefore, a precise knowledge of the spectrum and of the total flux is needed to devise shielding and veto mechanisms for rare event searches. Here ALMOND (An LNGS Mobile Neutron Detector) is presented. It is a mobile neutron spectrometer, based on capture-gated spectroscopy and comprised of an array of plastic scintillator bars wrapped with gadolinium foils. The detector has been calibrated with Americium-Beryllium source at Karlsruhe Institute of Technology and with an Americium-Boron source and a D-D generator at ENEA Frascati. The results of the neutron calibration with the time of flight method and the D-D generator are shown here, alongside the first results on capture time profile. Moreover, the first results from the neutron background run in Hall A at LNGS are presented.

Figures (4)

• Figure 1: Left: scheme of a single ALMOND module with cross sectional view of a corner near the PMT. In blue, the EJ-200 scintillator, in orange, the reflector foil, in green, the 100µ m gadolinium foil and in black the low background ET 9302B PMT. Right: the detector in the calibration setup. A Bosch profile structure houses an array of $6 \times 6$ modules, DAQ board and HV supply on top, here without Pb sheets on side walls.
• Figure 2: Time difference between the module and the BGO in the time-of-flight measurement. The peak on the left is due to coincidence between two gamma rays, the structure on the right is given by delayed coincidence between the neutron and the 4.4MeV gamma.
• Figure 3: Top: fit of the proton recoil edge after background subtraction leading to an energy deposit of 654 ± 4keV_ ee produced by 2.2 MeV D-D neutrons. Bottom: comparison of the capture time profiles between the AmBe calibration at KIT and the AmB calibration at FNG.
• Figure 4: Neutron candidates and accidental coincidences per day for the neutron background run in Hall A at LNGS. The region in yellow was excluded due to the presence of a neutron source in proximity of the detector.