Household salt as an emergency radiation dosemeter for retrospective dose assessments using optically stimulated luminescence

Sammanfattning: In the event of a radiation accident, it is crucial to make rapid estimates of the absorbed dose received by both emergency workers, so-called first responders, and members of the public. Today, only personnel in rescue teams responding to nuclear emergencies have access to individual dosemeters. To make accurate assessments of the radiological consequences of an emergency situation, methods of retrospectively determining the absorbed dose to individuals and groups of people are needed. The use of optically stimulated luminescence (OSL) in crystalline materials has been considered for retrospective dose assessment after radiation accidents during the past decade. The purpose of this work was to investigate the potential of household salt as a retrospective dosemeter for ionizing radiation using OSL. Measurements of the OSL output as a function of stimulation power, powering mode, preheating temperature, etc., have been performed, and the OSL readout sequence was optimized for salt. Furthermore, the possibility of developing a low-dose (<100 mGy) OSL readout protocol, to be employed with household salt, has been investigated, as have the signal stability and signal preservation in salt in the original salt containers. The OSL response of household salt to different photon energies has also been studied, and compared with the response in tissue-equivalent material. The investigated kinds of household salt, both sea-salt, recrystallized mine salt and non dissolved mine salt, exhibited promising dosimetric properties for low-dose applications, showing a linear dose response in the interval 1-100 mGy, and detection limits down to about 0.2 mGy. A standard SAR protocol for measuring household salt using a Risø TL/OSL-DA-15 reader for low doses was developed. This protocol was found to recover the administered dose within 10% for doses between 1 and 250 mGy for the kind of household salt most extensively studied here, and within 20% for the other three kinds. The OSL signal in household salt when stimulated with blue LEDs did not fade during storage for up to 142 days, but increased slowly during storage. Studies of different packages showed that salt in white plastic packages (although appearing opaque) showed almost complete signal loss due to optical bleaching, both in the laboratory and during field trials in a village contaminated by the Chernobyl accident. The signal appeared to be preserved for several months in salt in other types of packages, e.g. cardboard. The results of measurements using salt as a personal dosemeter in Fukushima, Japan, following the nuclear power plant accident, and in small salt containers in a village contaminated by the Chernobyl accident showed that doses as low as 100 μGy can be assessed. However, measurements of the OSL in NaCl gave a higher absorbed dose than the more tissue-equivalent TL-LiF dosemeter at lower photon energies (<300 keV). The absorbed doses obtain from OSL in NaCl require an energy-dependent conversion factor for photon energies below 600 keV. Hence, knowledge on the average photon energy at the accident site will improve the accuracy of dose estimates when using NaCl as a retrospective dosemeter.