Effect of Tl $^{+}$ and Sm $^{2+}$ Concentrations on Afterglow Suppression in CsI:Tl, Sm Crystals

While the CsI:Tl scintillator is the material of choice for many medical and industrial applications, its use in rapid framing applications (such as CT) has been hampered by its long term persistence or afterglow. We have previously reported that the addition of Eu²⁺ and Sm²⁺ ions reduces this afterglow component, but with unpredictable consequences to the light output. Here we report on CsI:Tl crystals grown by the Bridgman technique in which both Tl and Sm²⁺ dopant levels were varied in order to optimize both the light output and afterglow suppression. Thallium levels were varied from 0.01-0.5 mol% and Sm²⁺ from 0.01-0.1 mol%, and crystals were evaluated in terms of light output by gamma excitation and photopeak position. Afterglow was measured after excitation with an X-ray source 100 ms in duration. The light output was found to increase with Tl concentration up to an optimal level of approximately 0.1%, with greater levels offering no improvement in light signal. The presence of Sm²⁺ has little effect on light output, as long as the concentrations are low; at higher concentrations, however, light output was degraded due in part to the poor solubility of the Sm²⁺ ion into the CsI crystal. Afterglow levels were optimal at Tl levels in the range from 0.04-0.1% regardless of Sm²⁺ concentration, with an approximate five-fold reduction in afterglow with 0.02% Sm²⁺.