Pulse shape discrimination and dark matter search with NaI(T1) scintillator at the Laboratoire Souterrain de Modane

Underground data for WIMP search have been obtained from a 83 days run of a 10 kg NaI(T1) detector in a low activity environment at the Laboratoire Souterrain de Modane (LSM). Emphasis is put in this paper on the understanding of the 2-5 keV energy region, crucial both at establishing limits and at analyzing annual modulation of Spin Independent WIMPs. Part of the PMT noise, dominant in that energy region could induce errors in the results of the modulation analysis. Pulse shape analysis of the data at higher energies have shown an unexpected population of "U" events, also observed by another experiment at the same rate. Additional data obtained at LSM on "U" events are reported. A possible interpretation of these events is related to alpha decays of Radon descendants near the surface of the crystal.