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PITL and PTTL characteristics of TLD-100 and TLD-700 TL detectors

The phenomenon of thermoluminescence when excited by light (photostimulated thermoluminescence) has two aspects, photoinduced thermoluminescence (PITL) and phototransfered thermoluminescence (PTTL). PTTL refers to thermoluminescence (TL) induced by light as a result of photo transfer of charge carriers from one or more traps to other traps. Usually this transfer takes place from high temperature deep traps which have been filled earlier by irradiation with ionizing radiation, to lower temperature shallow traps which have been emptied thermally, leaving the deep traps filled. PITL is thermoluminescence directly induced by light in a fully annealed material. Photostimulated thermoluminescence has been found useful in the study of electronic processes, both PITL and PTTL have been tried for the measurements of ultraviolet radiation and PTTL has been observed and studied for a wide variety of materials and its measurement has been suggested as a technique in radiation dosimetry and for dating ceramic artefacts. In radiation dosimetry, PTTL found its use for dose reassessment: after a first dose evaluation, by a normal TL measurement, the same dosimeter is subjected to the PTTL dose reassessment procedure with the intention of obtaining a new estimation by a completely independent measurement procedure. Recently, some experimental data indicate that PTTL could be used as a tool for dose discrimination in a mixed field using single TL detector. In this work, the characteristics of PITL and PTTL after irradiation with 137Cs gamma rays were determined using a Camag UV lamp (600  Wcm-2 for 254 nm) for TLD-100 and TLD-700 TL detectors. Subsequently, detectors were irradiated with 14.5 MeV neutrons produced by Texas Nuclear Corporation 300 keV electrostatic accelerator and PTTL was also determined in addition to TL. The results are given for 10 minutes exposure with 254 nm UV irradiation at room temperature. Detectors were at the distance of 17 cm from the lamp. Measured values for PITL were equivalent to 0.1 mGy and 0.05 mGy for TLD-100 and TLD-700, respectively. After gamma or neutron irradiation detectors were read out, then exposed to UV and read out again. From the second reading PITL was subtracted, the result was divided by the first reading and the relative PTTL values were obtained. For 137Cs relative PTTL values were about 5% and 2% for TLD-100 and TLD-700, respectively. For 14.5 neutrons relative PTTL values of about 12% and 5% for TLD-100 and TLD-700 respectively, were obtained. Difference in the sensitivity for neutrons for both signals, PTTL and TL, enables determination separately gamma and neutron component of the dose for TLD-100 as well as for TLD-700. The PTTL signal is more pronounced for TLD-100 detectors.

TL; PTTL; Mixed field dosimetry; TLD-100; TLD-700

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