Performance of the SABRE detector module in a purely passive shielding

Understand

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 kgxdays 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 $^{40}$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.