A Systematic Study of Quartz Thermoluminescence for Forensic and Retrospective Dosimetry

Abstract

This project uses luminescence techniques to investigate the radiation dose history of materials suspected to have been exposed to man-made (‘artificial’) radiation, from the perspective of two applications: forensic investigation and retrospective dosimetry measurement. The test material used in this project was taken from the concrete bioshield of the ANSTO Moata research reactor decommissioned some 10 years ago. It was subjected to nuclear radiation from the reactor core during its operational life and as such, is an analogue of concrete that would be expected to be sampled from clandestine nuclear facilities. Forensic investigation examines material to ascertain provenance and prior history, and in the context of this project seeks to confirm prior irradiation of concrete by artificial radiation. Retrospective dosimetry measures the absorbed radiation dose in material that is collected after the irradiation has taken place. The concrete was taken from different positions within the bioshield and so had varying absorbed doses of radiation, which again is an analogue of samples likely to be collected from the environs of the site of a nuclear accident or facility. This project detected and analysed the 620 nm (red) /190 °C TL peak in quartz, confirming that the material had been artificially irradiated. This signal was then compared against a dose calibration curve to derive an equivalent radiation dose for individual samples. These results were compared to the position in the bioshield and resulted in a decreasing exponential trend, which confirms the accepted attenuation character of radiation through matter. This characterisation may also be used to determine if thermal erosion of the luminescence has occurred and derive a correction for the sample reading. The quartz study was extended by investigating the 480 nm (blue) emission, found to be universally present in all quartz samples analysed for doses to 64 kGy. Sensitisation of both the 210 °C and 350 °C thermoluminescence peaks from 300 Gy-8 kGy was confirmed, with the sensitisation of the 350 °C peak found to be permanent, whereas the 210 °C peak sensitisation was found to be destroyed by heating and only reconstituted with further high doses. This differing behaviour forms the basis of a quick screening test that can indicate the prior exposure to high artificial radiation events, with a result obtainable in hours rather than days. The TL of un-hydrated cement was investigated and found to be linear to doses of 1 kGy however low sensitivity restricts its use to those scenarios where a large dose has been absorbed.