Neutron Veto Efficiency of the DarkSide-50 Detector

Abstract

DarkSide is a direct dark matter detection experiment searching for evidence of Weakly Interacting Massive Particles (WIMPs), a well-motivated candidate for dark matter. The DarkSide-50 Time Projection Chamber (TPC) has an active volume containing 50 kg of liquid argon and has been in operation at Laboratori Nazionali del Gran Sasso in Italy since late 2013. The DarkSide-50 TPC is surrounded by a 30 t boron-loaded liquid scintillator that acts as a Neutron Veto (NV). The NV is immersed in a 1 kt ultra-pure water Cherenkov detector that acts as a Muon Veto (MV) and passive shielding against external neutrons and γ-rays.

WIMP interactions are expected to be rare due to the low scattering cross section. A majority of the signals in our detector are from background particles. There are four primary sources of background: γ-rays and Cherenkov radiation from radioactive decays in the detector's construction materials, β’s from (_^85)Kr and (_^39)Ar dissolved in the argon target, and neutrons from (α, n) and fission reactions from radioactive contaminants in the detector's materials.

The DarkSide-50 TPC is designed to efficiently reduce background from β’s and γ-rays as well as from Cherenkov radiation by applying pulse shape discrimination and fiducial volume cuts. In this report, I will present detailed estimates of γ-ray activities of the detector components based on spectra and rates measured with the TPC and the NV.

The most important background in WIMP direct detection experiments are neutrons, which can produce elastic nuclear recoils that look exactly like WIMP interactions. In DarkSide-50, we both actively suppress and measure the rate of neutron-induced background events using the NV. The NV response was calibrated using two radioactive neutron sources, (_^241)Am (_^13)C and (_^241)Am (_^9)Be. I will present the neutron veto efficiency based on this calibration data.