https://doi.org/10.1140/epjc/s10052-022-11108-z
Regular Article - Experimental Physics
Performance of the SABRE detector module in a purely passive shielding
1
Physics Department, Princeton University, 08544, Princeton, NJ, USA
2
Dipartimento di Fisica, Università degli Studi di Genova and INFN Genova, 16146, Genoa, Italy
3
INFN-Sezione di Roma, 00185, Rome, Italy
4
INFN-Sezione di Milano, 20133, Milan, Italy
5
Dipartimento di Fisica, Università degli Studi di Milano, I-20133, Milan, Italy
6
INFN-Laboratori Nazionali del Gran Sasso Assergi (L’Aquila), I-67100, L’Aquila, Italy
7
Dipartimento di Fisica, Sapienza Università di Roma, I-00185, Rome, Italy
8
Department of Physics, University of California Berkeley, 94720, Berkeley, CA, USA
9
INFN-Sezione di Roma, 000185, Rome, Italy
Received:
23
August
2022
Accepted:
5
December
2022
Published online:
21
December
2022
We present here a characterization of the low background NaI(Tl) crystal NaI-33 based on a period of almost one year of data taking (891 kg
days exposure) in a detector configuration with no use of organic scintillator veto. This remarkably radio-pure crystal already showed a low background in the SABRE Proof-of-Principle (PoP) detector, in the low energy region of interest (1–6 keV) for the search of dark matter interaction via the annual modulation signature. As the vetoable background components, such as 
K, are here sub-dominant, we reassembled the PoP setup with a fully passive shielding. We upgraded the selection of events based on a Boosted Decision Tree algorithm that rejects most of the PMT-induced noise while retaining scintillation signals with > 90% efficiency in 1–6 keV. We find an average background of 1.39 ± 0.02 counts/day/kg/keV in the region of interest and a spectrum consistent with data previously acquired in the PoP setup, where the external veto background suppression was in place. Our background model indicates that the dominant background component is due to decays of 
Pb, only partly residing in the crystal itself. The other location of Pb is the reflector foil that wraps the crystal. We now proceed to design the experimental setup for the physics phase of the SABRE North detector, based on an array of similar crystals, using a low radioactivity PTFE reflector and further improving the passive shielding strategy, in compliance with the new safety and environmental requirements of Laboratori Nazionali del Gran Sasso.
© The Author(s) 2022
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Funded by SCOAP3. SCOAP3 supports the goals of the International Year of Basic Sciences for Sustainable Development.
