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In general, superior spatial resolution is expected from the direct detection type, in which Ion-chamber-type detectors and Si diodes based on the radiation–ionization phenomenon are used for high-energy dose detection. However, because the ion chamber has a high work function, the speed for collecting electrons and holes is slow, and thus, the dose-detection characteristic deteriorates. In addition, because of the low electron–ion-pair detection rates in ionization chambers, signal detection may decrease in response to the temporal changes during the continuous detection of high-energy radiation, resulting in a possible decrease in the reproducibility and sensitivity to signals. In this paper, the thin coplanar lead iodide(PbI_2)films as a photosensitive converter requiring only a few tens of volts of bias, associated with a thick columnar coating of phosphor layer, were simulated and designed. PbI_2, which was used in this study, is a very important material with technological applicability as a room-temperature radiation detector. It is a wide-band-gap semiconductor (Eg > 2.0 eV) with a high environmental stability efficiency[1]. In this structure, gamma rays are converted into visible light on a thick Gd_2O_2S:Tb phosphor layer which is then converted to electric charges in a thin PbI_2 layer. The electron-hole pairs can also be generated from gamma-ray interaction in the PbI2 photoconductor, which can improve the generation efficiency of electric charges. To optimize the thickness of the phosphor coupled PbI_2 multilayer structure in range of iridium-192 gamma ray energy, the gamma-ray absorption was estimated using the MCNPX code. In addition, the photoluminescence and electrical measurements of phosphor coupled PbI_2 dosimeter were evaluated. From the experimental results, the 180 μm Gd_2O_2S:Tb coupled 10 μm - PbI2 dosimeter proposed in this work exhibited a low dark current and excellent gamma-ray sensitivity, and in particular, excellent linearity to x-ray exposure dose. The measured dark currents were below 100 pA/cm2 at an electric field of 1 V/μm for PbI_2. The preliminary sensitivity measurements give a signal in the range of about 12.6 and 4.2 nC/cm^2 for 250μm Gd_2O_2S:Tb / PbI_2 and 250 μm PbI_2 at the exposure conditions respectively. The results of this research suggest that the new coplanar gamma -ray dosimeter with a hybrid-type structure can resolve the following problems: high sensitivity from the conventional dosimeter, and low conversion efficiency from the indirect conversion method.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(NRF-2020R1I1A1A01074908)
[1] R. Ahuja et al., Electronic and optical properties of lead iodide, J. Appl. Phys. 92(12) (2012) 7219-7224.
