Ultraviolet response of CaSO4 : Dy

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Abstract
A simple method for measuring ultraviolet exposure using the thermo-luminescent properties of CaSO4 : Dy is presented in this paper. In general, three peaks at 145, 215 and 365 °C were observed by ultraviolet irradiation of virgin phosphor. The peak at 365 °C was found to be very small and that at 145 °C fades away within a week; no appreciable fading was observed for the 215 °C peak. Further, the peak at 215 °C was found to coincide with the gamma dosimetric peak and this peak was chosen for ultraviolet exposure measurements. A standard quartz pen-ray lamp was used for calibrating the phosphor both by peak height measurement and by integration. The dosimetric peak has been found to be linear from the detection threshold of 400 erg mm-2 to 4 x 104 erg mm-2. Beyond this exposure the phosphor behaved supralinearly to ultraviolet exposure up to the studied range of 5 x 105 erg mm-2. The phosphor exhibited strong dependence on photon energy. The TL sensitivity increases sharply by a factor of 300 as the wavelength is decreased from 250 to 200 nm. The response is negligibly small above 350nm. Other important characteristics such as particle size dependence, fading characteristics, thickness dependence and re-usability are also presented. The ultraviolet sensitivity increased by a factor of 3 when the grain size range was changed from 105-210 μm to 0-37 μm. Self-shielding by a factor of 2 was observed when the sample thickness was increased from 10 to 72 mg cm-2.

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