Systems for detecting and measuring backgrounds with the SABRE South experiment

TL;DR

The paper addresses the need to test the DAMA/LIBRA dark matter modulation claim with a model-independent approach using NaI(Tl) crystals in two hemispheres (SABRE North and SABRE South). It describes the SABRE South veto systems, including a 12 kL LAB-based liquid scintillator veto with 18 PMTs and an above-shield muon veto of 8 plastic scintillator panels with 2 PMTs each, designed to tag radiogenic and cosmogenic backgrounds. Calibration strategies, such as in-situ optical calibration with a 445 nm laser and radioactive calibration tubes, are presented, along with PSD studies using a small LS test vessel demonstrating gamma/neutron discrimination and a boosted decision tree classifier. Altogether, the work lays groundwork for background rejection and reconstruction essential for the dual-hemisphere DAMA test and long-term operation in SUPL.

Abstract

The SABRE (Sodium iodide with Active Background REjection) experiment aims to detect an annual rate modulation from dark matter interactions in ultra-high purity NaI(Tl) crystals which will provide a model independent test of the signal observed by DAMA/LIBRA. SABRE will consist of two separate detectors in the Northern and Southern hemispheres. SABRE South will be located in the newly completed Stawell Underground Physics Laboratory (SUPL), the first deep underground laboratory in the southern hemisphere. The combination of SABRE North and South is intended to disentangle seasonal or site-related effects from the dark matter-like modulated signal. Measuring and understanding backgrounds is essential for the reliability and consistent performance of these searches, and as the first large detector in SUPL SABRE South will also be used to measure backgrounds from radiogenic and cosmogenic sources. The SABRE South veto system is designed to detect the signals generated by radiation and cosmic rays using a 12 kL linear alkyl-benzene (LAB) based liquid scintillator (LS) detector contained in a steel vessel and instrumented with 18 Hamamatsu R5912 photomultiplier tubes (PMTs), alongside a plane of 8 plastic scintillator modules (instrumented with 2 R13089 PMTs) located above the vessel to reliably detect muons from cosmic-rays with a position resolution of 5 cm.

Figures (5)

• Figure 1: A render of the complete SABRE South detector. The active LS veto surrounds the NaI(Tl) detectors, with the muon veto placed on top of the shielding.
• Figure 2: (Left) The proposed design for the in-situ optical calibration system. (Right) The number of PEs detected by the LS veto detector for the energy depositions corresponding to each source.
• Figure 3: Probability heatmaps for PE detection probability within the LS veto. The right plot represents a top-down slice in the x-y plane, and the left shows a side-on slice in the x-z plane showing 2 of the 18 veto PMTs.
• Figure 4: (Left) The ROC curve resulting from the BDT classification of neutron/gamma events in the small LS test vessel. (Right) Example average gamma/neutron pulses in the test vessel, with the prompt and delayed charge windows.
• Figure 5: (Left) The charge histograms resulting from a $^{60}$Co calibration with the linear stage --- 150 cm is the centre of the panel. (Right) A plot of the time difference between the signals detected by each PMT on the muon panel, for different longitudinal positions of a $^{60}$Co placed on the panel --- 150 cm is defined as one end of the panel, with 0 cm the centre.