Validation study of ambient dose equivalent conversion coefficients for radiocaesium distributed in the ground: lessons from the Fukushima Daiichi Nuclear Power Station accident.

Ambient dose equivalent conversion coefficients (ADC) 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, 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 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) and (3) relaxation mass depth recommended by the International Commission on Radiation Units and Measurements before the FDNPS accident (β). When ADC based on β and β were compared to those based on β and β, a positive correlation was found. To confirm the applicability of the ADC based on the three types of β values, radiocaesium inventories were estimated using the air dose rates and ADC, 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 based on β and β and measured by investigating soil samples. By contrast, the radiocaesium inventory estimated using the ADC based on β was overestimated compared with that measured by investigating soil samples. These findings support the applicability of ADC based on β and β in the Fukushima region. Furthermore, the β result suggests that differences in soil characteristics between Japan and other countries should be considered for evaluating ADC.