Scintillation Properties of CsI:Tl Crystals Codoped With ${\rm Sm}^{2+}$

Despite the acknowledged advantages of CsI:Tl for most scintillator applications, its use for CT and other high-speed imaging has been hindered by a high degree of afterglow in its scintillation decay. We have found that a particularly effective way to suppress this afterglow is to codope the material with certain dipositive rare earth ions capable of trapping the vagrant carriers that give rise to it. We have extensively studied the manner in which one such ion, Eu²⁺, alters the spectroscopic and kinetic properties of the scintillation, and have developed a coherent mathematical model consistent with the experimental results. But the beneficial effect of Eu²⁺ appears to be restricted only to relatively short times (say les 200 ms ) after the end of the excitation pulse. To be effective at longer times, the codopant should also provide some nonradiative means to annihilate the trapped carriers before their escape can enhance the low-level long-term emission. And, as predicted by the model, this is exactly what Sm²⁺ does. In this paper we describe the experimental effort to characterize the behavior of the CsI:Tl, Sm material system. Spectroscopically, we find that the familiar broad Tl emission becomes distorted and progressively shifted to shorter wavelengths. Kinetically, we find that the afterglow of the emission is substantially reduced relative to conventional CsI:Tl (no codopant), and that this effect is always found, regardless of the conditions of excitation. And finally, we find that the material shows virtually no memory of its previous excitation history (the so-called hysteresis phenomenon), in stark contrast with both conventional CsI:Tl and the corresponding material codoped with Eu. Various aspects of these effects and their dependence on the concentrations of the dopants are also discussed.