Response of Sodium Iodide Crystals to Alpha Particles and Electrons as a Function of Temperature

In an attempt to determine the mechanism causing the lower scintillation efficiency of $\alpha$ particles in NaI(Tl) crystals relative to the scintillation efficiency of electrons, the response of pure NaI and thallium-activated NaI crystals to $\alpha$ particles and electrons was studied as a function of temperature. Crystals were grown for this investigation both by the Bridgman and Kyropoulos methods. The magnitude of the scintillations per unit of energy loss in the crystals varied with temperature in a way which agreed with predictions of qualitative theories. The ratio of the $\alpha$-particle response to the electron response per unit energy, though a constant with temperature for the crystals grown by the Bridgman method ($\frac{\alpha \mathrm{response}}{e^{-} \mathrm{response}}=\sim 0.7$) showed an interesting temperature dependence for crystals grown by the Kyropoulos method. For a "pure" NaI crystal grown by the Kyropoulos method this ratio attained a value as high as 1.5 at - 150°C, indicating that the $\alpha$ particle was more efficient than the electron at this temperature. In view of this, proposed explanations for the lower efficiency at room temperature such as "local saturation" of scintillation centers or the loss of energy by "hard collisions" are re-examined. A temperature-dependent effect in the resolution of $\alpha$-particle peaks is also reported and discussed.