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While a wide variety of new scintillators are now available, CsI:Tl remains a highly desired material for medical and
industrial applications due to its excellent properties, low cost, and easy availability. Despite its advantages, however,
its use in high-speed imaging applications has been hindered by an undesirably high afterglow component in its scintillation
decay. To address this specific issue and to make the material suitable for applications such as volumetric CT and
high-speed radiography, we have discovered that codoping the material with certain dipositive rare earth ions is particularly effective for such afterglow suppression. 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.
Unfortunately, the beneficial effect of Eu2+ appears to be restricted only to relatively short times (say ≤200 ms) after
the end of the excitation pulse. At longer times, the carriers whose diversion into deep traps is responsible for suppression
of the short-term afterglow begin to escape those traps, resulting in enhancement of the low-level persistence on a
time scale of seconds or minutes. What is needed is to 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 mathematical model,
this is exactly what Sm2+ does.
In this paper we compare the respective effects of the two additives on the afterglow and hysteresis characteristics of
the host CsI:Tl material system. We find that while Eu begins to exert its afterglow-suppressive effect sooner after termination
of excitation, the influence of Sm lasts much longer. Moreover, the suppressive effect of the latter is always
found, regardless of the conditions of excitation, and becomes more profound the greater the duration of the exciting
pulse. Various aspects of these effects and some their consequences for imaging performance are also discussed.
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