Measurement of Na and I recoil discrimination and detection efficiency in a cooled “UVIS” NaI detector for dark matter searches

The UVIS dark matter detector, proposed by Spooner and Smith (Phys. Lett. B314 (1993) p 430), consists of <1000 ppm Tl 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 Tl) UVIS test detector operated at 160 K, with monoenergetic neutrons used to induce nuclear recoils via elastic scattering. A parameter C_m, defined to quantify the discrimination capability 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 sodium recoils and 10+/-2% for iodine