  The scintillation mechanism in NaI:Tl crystals produces different pulse
shapes that are dependent on the incoming particle type. The time distribution
of scintillation light from nuclear recoil events decays faster than for
electron recoil events and this difference can be categorised using various
Pulse Shape Discrimination (PSD) techniques. In this study, we measured nuclear
and electron recoils in a NaI:Tl crystal, with electron equivalent energies
between 2 and 40 keV. We report on a new PSD approach, based on an event-type
likelihood; this outperforms the charge-weighted mean-time, which is the
conventional metric for PSD in NaI:Tl. Furthermore, we show that a linear
combination of the two methods improves the discrimination power at these
energies.
