Pulse shape discrimination and dark matter search with NaI(Tl) scintillator

Abstract

An extensive study of NaI(Tl) as a Dark Matter particle detector is presented. Emphasis is put on the response of the detector, both in energy and pulse shape, to all particles interacting in the detector, namely high energy (MeV) photons, low energy photons (X-rays), betas from external radioactivity, and neutrons, which induce nuclear recoils. The initial hope that the shorter decay times of nuclear recoils induced by WIMPs could be statistically separated from Compton interactions is weakened by the fact that low energy X-rays and betas exhibit pulse shapes similar to recoils. As a consequence, any indication of shorter decay time pulses leads to an ambiguous interpretation. Underground data for the WIMP search were obtained in a low activity environment at the Laboratoire Souterrain de Modane (LSM), with a 10 kg crystal having high photoelectron yield and 2 keV energy threshold. The data contain pulse shapes with decay times shorter than for Compton interactions and are not compatible with calibration reference shapes or a mixture of these. The effect is shown to come from a population with decay times even shorter than nuclear recoils but its origin was not identified. The Compton rejection efficiency is limited to factors ranging from 3 at 5 keV to 8 at 20 keV (electron equivalent energy). It is shown that the sensitivity of NaI(Tl) detectors to the cross section of Spin Independent coupling WIMPs is only slightly improved by the pulse shape analysis, while it is mostly determined by the differential energy rate at threshold. The sensitivity to the cross section of Spin Dependent coupling WIMPs is improved by about an order of magnitude by the pulse shape analysis.