Characteristics of alpha, gamma and nuclear recoil pulses from NaI(Tl) at 10–100 keV relevant to dark matter searches

Measurements of the shapes of scintillation pulses produced by nuclear recoils, alpha particles and photons in NaI(Tl) crystals at visible energies of 10–100 keV have been performed in order to investigate possible sources of background in NaI(Tl) dark matter experiments and, in particular, the possible origin of the anomalous fast time constant events observed in the UK Dark Matter Collaboration experiments at Boulby mine [P.F. Smith et al., Phys. Rep. 307 (1998) 275]. Pulses initiated by X-rays (via photoelectric effect close to the surface of the crystal) were found not to differ from those produced by high-energy photons (via Compton electrons inside the crystal) within experimental errors. However, pulses induced by alpha particles (degraded from an external MeV source) were found to be ∼10% faster than those of nuclear recoils, but insufficiently fast to account for the anomalous events.