Measurement of Na and I recoil discrimination and detection efficiency in a cooled ‘UVIS’ NaI crystal for dark matter searches

The UVIS dark matter detector, proposed by Spooner and Smith (Phys, Lett. B 314 (1993) 430), consists of < 1000 ppm T1 doped NaI scintillator operated at 100-200 K, with measurement of UV and visible scintillation components used to discriminate between electrons (gammas) and Na or I recoils. Presented here are results of measurements of the gamma/nuclear recoil discrimination power of a Kyropolous NaI (0.5 ppm T1) UVIS test detector operated at 160 K, with monoenergetic neutrons used to induce nuclear recoils via elastic scattering, Defining the statistical gain factor for N events as C-m/N-1/2, the coefficient C-m was measured as a function of photoelectron pulse height. At 40 photoelectrons C-m was found to be 0.5 (corresponding to 90% rejection of gammas with 20% loss of neutron events). The scintillation efficiency for Na and I recoils relative to electrons was also measured at 160 K and found to be 35 +/- 5% for Na recoils and 10 +/- 2% for iodine.