Performance of a prototype active veto system using liquid scintillator for a dark matter search experiment

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We report the performance of an active veto system using a liquid scintillator with NaI(Tl) crystals for use in a dark matter search experiment.

• When a NaI(Tl) crystal is immersed in the prototype detector, the detector tags 48% of the internal K-40 background in the 0-10 keV energy region.
• We also determined the tagging efficiency for events at 6-20 keV as 26.5 +/- 1.7% of the total events, which corresponds to 0.76 +/- 0.04 events/keV/kg/day.
• According to a simulation, approximately 60% of the background events from U, Th, and K radioisotopes in photomultiplier tubes are tagged at energies of 0-10 keV.