NaI: T1 response to relativistic Ne, Ar, and Fe ions

Data are presented on the response of thallium-activated sodium iodide to relativistic ²⁰Ne, ⁴⁰Ar, and ⁵⁶Fe ions in the energy interval 0-550 MeV/amu. The resulting scintillation efficiency curves are found to rise with decreasing stopping power, contrary to conclusions drawn from lower-energy heavy-ion data. A scintillation model is developed which provides an excellent qualitative fit to data spanning two orders of magnitude in stopping power and which demonstrates a posteriori the failure of the activator depletion mechanism for saturation. This model includes a calculation of the prompt dose profile, the diffusion of excitons resulting from the deposited energy, and a second-order annihilation process which accounts for the observed saturation.