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Thallium-doped cesium iodide (CsI:Tl) single crystal is a well-known scintillator that has found many applications in nuclear science, radiography, and active interrogation of cargo in transit. It has relatively high density, high light yield, light emission matched with photodetectors and is less hygroscopic than sodium iodide. On the other hand, it has been hampered by a persistent afterglow, attributed to thermal ionization of trapped electrons (Tl0) followed by radiative recombination with trapped holes, which causes pulse pile-up in high count- rate applications. However, codoping by an appropriate modifier ion, especially divalent samarium and europium, has been found to be quite successful in suppressing this afterglow. But this effect had not yet been demonstrated in the crystal sizes and excitation energies relevant to real-time scanning of cargo. It is the purpose of this work to address this issue.
In this work, codoping of CsI single crystal with Sm2+ or Eu2+ was carried out using the vertical Bridgman technique. Large diameter CsI crystals, in the range of 1 to 3 inches, were grown for application in sentry portal detector. The crystals were cut in the form of pillars suitable to use in the detector module and tested for after-glow. At 2 ms after excitation cut-off, the codoped CsI:Tl crystal pillars showed afterglow on the order of 0.5-0.8 % compared to 2% for CsI doped with Tl alone. In scaling up the crystal growth process for larger diameter, it was observed that Eu2+-codoped crystals had much better reproducibility than those codoped with Sm2+.
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