Muon veto system for the CROSS double-beta decay search experiment

TL;DR

The CROSS experiment faces a substantial muon-induced background due to the relatively high residual muon flux at Canfranc. The authors design and optimize a dedicated muon veto surrounding the cryogenic detector facility, combining lateral and bottom polystyrene scintillator modules read by SiPMs with a top PMT-based panel, plus a sophisticated electronics/DAQ to enable trigger logics and timing correlation with the bolometers. Geant4-based simulations, validated by dedicated RUN data, show that muon-tagging in coincidence with the detector multiplicity cuts can suppress muon-induced events to the level of ~2×10^-3 cnts/keV/kg/yr in the ROI around 3 MeV, albeit with an ~18% dead time. This veto performance, validated in RUN 13 with CUPID-Mo modules, ensures CROSS readiness for a high-sensitivity 0νββ search by effectively mitigating the dominant cosmic muon background in an underground environment.

Abstract

In preparation to the CROSS experiment at the Canfranc underground laboratory (Spain) $-$ aiming to search for neutrinoless double-beta ($0νββ$) decay of $^{100}$Mo using low-temperature detectors with heat-scintillation readout $-$ we report on development of a dedicated muon veto system. The need for the muon veto in CROSS is caused by a comparatively high residual cosmic muon flux at the experimental site ($\sim$20 $μ$/m$^2$/h), being a dominant background in the region of interest (ROI) at $\sim$3 MeV. Thus, we installed the muon veto system around the CROSS low-background setup, forming four lateral, one top, and four bottom sectors. In this paper we describe the design, construction and operation of the CROSS muon veto system, as well as its optimization and validation by comparing dedicated Monte Carlo (MC) simulations of muons with low-temperature measurements in the setup. We demonstrate a stable operation of the veto system with the average trigger rates compatible with MC simulations. Also, we investigated two muon trigger logics based on coincidences with either 2 sectors or a single sector of the veto. The MC study shows that, in combination with the multiplicity cut of thermal detectors, these trigger logics allow to reject 99.2\% and 99.7\% of muon-induced events in the ROI, respectively. Despite a comparatively high dead time ($\sim$18\%) introduced by coincidences with any of nine sectors of the veto $-$ the adopted strategy $-$ the muon-induced background in the ROI of the CROSS experiment can be reduced down to $\sim$2 $\times 10^{-3}$ cnts/keV/kg/yr, i.e., an acceptable level compatible with a high-sensitivity $0νββ$ decay search foreseen in CROSS.

Figures (24)

• Figure 1: Design of the main parts of the CROSS muon veto: (Left) View of the Top, Lateral, and Bottom parts of the CROSS muon veto and the hut (without a roof for visual purpose), where the cryogenic facility is placed (not shown); (Middle) View on the the Lateral veto with respect to the position of the three pillars of the cryostat and the hut's entrance; (Right) Top view on the Lateral and Bottom veto sectors.
• Figure 2: Drawing of the Lateral muon veto made of 28 scintillating detector modules, the readout of which is split to 4 sectors (S0, S1, S2, and S3 in figure \ref{['fig:Muon_veto']}) for the muon trigger logic; see details in text.
• Figure 3: Single section of the Lateral veto before (top) and after (bottom) the assembly.
• Figure 4: Single panel of the Bottom veto before (left) and after (right) the assembly. Splitter boards are the two long boards with eight RJ45 connectors, while the PCBs connected to the SiPMs are the 15 narrow boards on the bottom of the bars.
• Figure 5: Image of the CROSS setup, taken from the entry of the hut, showing a frame of the cryostat together with fragments of the Lateral and Bottom parts of the muon veto.