EFFECTS OF GAMMA RAY IRRADIATION ON STRUCTURAL AND OPTICAL PROPERTIES OF CsI:Tl CRYSTALS

Abstract

In this research work, effects of gamma ray irradiation on structural and optical properties of CsI doped with Tl (CsI:Tl), which is a scintillator material used for radiation detector applications, were investigated by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), UV-VIS spectroscopy and X-ray luminescence spectroscopy. The CsI:Tl crystals used in this study were grown by a modified homemade Bridgman-Stockbarger technique with CsI powder precursors purities of 99.999% and 99.9% and with the same amount of Tl in form powder of Tl. It is observed that CsI:Tl crystal grown with the 99.999% CsI powder precursor exhibited a colorless crystal ingot, while a use of the 99.9% CsI powder precursor resulted in an orange CsI:Tl crystal ingot. To verify a uniformity of CsI:Tl crystals, the ingots were divided into three parts: the top, middle and bottom of the ingots. Both the CsI:Tl crystals exhibited a poly crystalline, which has a cubic structure with a lattice constant in the range of 0.453-0.456 nm. Noted that only a bottom part of an orange crystal ingot exhibited a lower crystallinity and showed XRD feature, corresponding to calcite (CaCO3). This results in a merging of the Tl-related state and the CsI bandgap for the orange crystal. To avoid an effect from impurity, the middle part of both the crystal was selected to irradiate by gamma ray, showing an increase of grain size, a higher crystallinity and a decrease of conductivity observed by FESEM images. Moreover, a merging of the Tl-related state and the CsI bandgap, which was observed in the orange crystal before a gamma ray irradiation, was observed in both CsI:Tl crystals after gamma ray irradiation of 5 Gy. X-ray luminescence spectra showed a large reduction of luminescence intensity after one day irradiation of the gamma ray and then it recovered back to a similar intensity compared to that of before gamma ray irradiation of 1.0 Gy after 7 days. Furthermore, an efficiency of radiation detection and an energy resolution of both the CsI:Tl scintillator were detected. It is found that with increasing gamma ray irradiation doses up to 5.0 Gy, an efficiency of radiation detection was decreased, while an energy resolution was not significantly different. Noted that the colorless CsI:Tl crystal shows a higher energy resolution than that of the orange crystal. As a result, effects of gamma ray on both the structural and optical properties of CsI:Tl crystals, which depended on an impurity level, were directly affected on a performance of radiation detection.