An LNGS Mobile Neutron Detector (ALMOND): Mapping Ambient Neutron Background of Gran Sasso National Laboratory

TL;DR

Ambient neutrons in deep underground labs vary with location and time due to cavern-wall radioactivity, challenging background modeling for rare-event searches. ALMOND provides a mobile capture-gated neutron spectrometer built from 36 plastic scintillator modules with gadolinium capture, enabling direct measurement of ambient neutron flux and energy distribution across LNGS. Through calibration campaigns at KIT, Frascati, and FNG, and a first underground run in Hall A, ALMOND demonstrates the ability to quantify neutron backgrounds and derive energy information from proton recoils and delayed captures. The results lay the groundwork for comprehensive facility-wide neutron background mapping and improved background models for LNGS experiments.

Abstract

In deep underground laboratories, environmental neutrons, which are produced at the cavern walls, introduce a source of background to rare event searches. The flux and spectrum of the ambient neutrons vary considerably with time and location. Precise knowledge of this background is necessary to devise shielding and veto mechanisms, thereby improving the sensitivity of the neutron-susceptible underground experiments. ALMOND, currently in operation, is a low-flux mobile neutron spectrometer developed for the LNGS underground laboratory to measure the ambient neutron background of the entire facility. In this paper, an overview of the design, construction and calibration of ALMOND is given. Furthermore, the result of the first underground neutron measurement is shown along with an outlook for future measurements and analyses.

Figures (5)

• Figure 1: (Left) CAD drawing of ALMOND. (Right) Constructed assembly of ALMOND. The bottom lead plate above the wheels is seen in the picture.
• Figure 2: (Left) $^{208}$Tl Compton edge spectrum obtained with a single ALMOND module. (Right) An example pulse area (S) vs. energy (E) calibration curve.
• Figure 3: (Left) ToF spectrum measured with the tagged neutron source and an individual ALMOND module. The first peak represents the $\gamma$-$\gamma$ coincidence between the BGO detector and the PS unit, respectively, whereas the later distribution shows the neutrons, denoted by $\gamma$-n coincidence. (Right) Time profile of the tagged AmBe neutrons captured by ALMOND. The capture time distribution was empirically fitted with a double exponential function Littlejohn:2012qpa. $\lambda_U$ depicts the rising edge owing to the thermalization of fast neutrons and $\lambda_T$ refers to the falling edge due to the capture of thermalized neutrons.
• Figure 4: ALMOND positioned in Hall A of the LNGS underground laboratory during a first long-term data taking.
• Figure 5: Result in units of count rate per day of the ambient neutron measurement in Hall A