NUMERICAL SIMULATION OF DIRECT MEASUREMENT TO DETERMINE THYROID super(131) I CONTENT OF TWO TEPCO WORKERS CONSIDERING INDIVIDUAL TISSUE THICKNESS

After the Fukushima Daiichi Nuclear Power Station accident, the National Institute of Radiological Sciences examined seven heavily exposed emergency workers and performed internal dose estimations. The largest dose contributor was found to be super(131) I, which was detected by thyroid monitor with an HPGe detector. Different energy peaks from super(131) I were simultaneously identified in the pulse-height spectra of the two subjects with the highest doses regardless of late measurements. A closer look at the spectra indicated that the count ratio of the two peak areas at 80.2 and 365 keV differed somewhat between the individual workers, suggesting a difference in attenuation in the overlaying soft tissue and in the thyroid itself. In this study, the relationship between the count ratio (80.2/365 keV) and the thickness of soft tissue overlying the thyroid was investigated by means of numerical simulations performed using the Japanese Male (JM) phantom varying the thickness of the overlaying tissue. From the measured count ratios, it was possible to estimate that the overlaying tissue was thinner for Worker 1 (difference from the JM phantom: -0.34+ or -1.29 cm) and thicker for Worker 2 (diff.: 2.5+ or -1.2 cm). The thyroid super(131) I contents evaluated taking into account the individual thicknesses were 4.3 kBq for Worker 1 and 8.4 kBq for Worker 2, resulting in a significant increase for Worker 2 compared with the content based on the default counting efficiency at 365 keV of the original JM phantom. However, the results have large uncertainty factors of 1.4 for Worker 1 and 1.3 for Worker 2 and should be carefully considered together with other factors influencing the attenuation.