Stopping Power and Luminescent-Response Calculation for Channeling in NaI(Tl) and CsI(Tl)

Stopping powers of the most prominent axial channels in NaI(Tl) and CsI(Tl) are calculated for wellchanneled ions (He⁴-Ne²⁰) over the velocity range E/A=1-10 MeV/amu. An electron-gas model is applied to a treatment of the excitation of valence electrons, and an additional term is constructed which accounts for core-electron excitation. Nonparticipating electrons are excluded by application of an adiabaticity criterion. The channel-stopping powers that result are typically 20-25% of those characteristic of random incidence, indicating a relatively pronounced channeling effect for the alkali iodides. The calculated curves are combined with appropriate scintillation-efficiency data (dL/dE versus dE/dx) corresponding to bombardment of NaI(Tl) and CsI(Tl) scintillation counters, in order to obtain a prediction of the luminescent response (L versus E) to the channeled particles, and thus to account quantitatively for a reported anisotropy in the scintillation spectra. Higher than normal scintillation efficiency is predicted for the channeled particles, indicating that the crystal channels in NaI(Tl) and CsI(Tl) are axes preferential for luminescence. This effect may be explained qualitatively by reduced ionization density for channeled particles, which favors radiative recombination of electrons and holes at Tl⁺ sites over competing non-radiative recombination.