  We have studied the scintillation of pure and doped sodium iodide crystals
handled in low humidity conditions under external $\alpha$ and $\gamma$
excitation from room temperature down to 4K. The light yield of pure sodium
iodide was seen to increase at low temperatures by a factor 30 compared to room
temperature, up to a maximum of 40 photons/keV under $\gamma$ excitation at
60K, stabilizing to 30 photons/keV at lower temperatures. Thallium doped sodium
iodide fluctuates by 20% around the room temperature value, stabilizing at 40
photons/keV at low temperature. $\alpha / \gamma$ quenching factor stabilizes
at roughly 0.7 for both materials beneath 50K. Time constants of both materials
slow greatly at low temperature, reaching tens of microseconds. Cryogenic
applications of these materials are complexified by their mechanical fragility
and hygroscopicity.
