Absolute Method for Measuring Environmental Radioactive Materials Using Imaging Plates

Abstract We had previously developed a measurement method using an imaging plate (IP) to evaluate and address surface contamination caused by the release of radioactive materials during the Fukushima nuclear accident. The measurement units for the surface contamination density were in relative values [described as relative luminescence levels measured in luminescence arbitrary units (LAU)], but the evaluation was required in absolute values, such as Bq cm−2, to enable appropriate control of exposure doses. This study establishes a method for converting the IP measurements of surface contamination density due to environmental radioactivity into absolute values. Soil contaminated with radioactive materials from the Fukushima nuclear accident was collected to create a working reference material (WRM). The conversion coefficient for surface contamination density was calculated using the WRM values measured with an IP and high-purity germanium detectors. The IP measurement values were converted into the surface contamination density using the conversion coefficient. The WRM values measured with the IP and high-purity germanium detectors were 324.1 LAU and 32.22 ± 2.27 Bq cm−2, respectively. The surface contamination density conversion factor was calculated as 0.0994. The surface contamination density on the roof of the Tsukuba City facility was re-evaluated using the conversion factor. The average value of 29,972 Bq m−2 matched the amount of radioactive material fallen in Tsukuba City. By standardizing the measurement conditions for surface contamination when using IPs, we successfully quantified the surface contamination density with an accuracy comparable to that of conventional methods. This method is expected to make a significant contribution to efficient radiation safety management.