Validation study of ambient dose equivalent conversion coefficients for radiocaesium distributed in the ground:...

Ambient dose equivalent conversion coefficients (ADC_RCs) for converting a radiocaesium inventory to ambient dose equivalent rates (air dose rates) depend on the vertical distribution of radiocaesium in soil. To access the validity of ADC_RCs, the air dose rate at 1 m above ground and the vertical distribution of radiocaesium in the soil around the Fukushima Daiichi Nuclear Power Station (FDNPS) present between 2011 and 2019 were measured in the current study. ADC_RCs were calculated using air dose rates and three different parameters representing the vertical distribution of radiocaesium in soil: (1) relaxation mass depth (β), (2) effective relaxation mass depth (β_eff) and (3) relaxation mass depth recommended by the International Commission on Radiation Units and Measurements before the FDNPS accident (β_ICRU). When ADC_RCs based on β and β_eff were compared to those based on β and β_ICRU, a positive correlation was found. To confirm the applicability of the ADC_RCs based on the three types of β values, radiocaesium inventories were estimated using the air dose rates and ADC_RCs, and the obtained results were compared to the radiocaesium inventory calculated using soil sample measurements. Good agreement was observed between the radiocaesium inventories estimated using the ADC_RCs based on β and β_eff and measured by investigating soil samples. By contrast, the radiocaesium inventory estimated using the ADC_RCs based on β_ICRU was overestimated compared with that measured by investigating soil samples. These findings support the applicability of ADC_RCs based on β and β_eff in the Fukushima region. Furthermore, the β_ICRU result suggests that differences in soil characteristics between Japan and other countries should be considered for evaluating ADC_RCs.