Ambient dose equivalent conversion coefficients for radionuclides exponentially distributed in the ground

Abstract

Conversion coefficients of radionuclide deposition density to the ambient dose equivalent rate at 1 m height above ground were calculated for exponentially distributed sources in the ground. First, Monte Carlo transport simulations assuming exponential distributions in the ground were performed to obtain ambient dose equivalent for mono-energetic gamma-ray sources having different relaxation depths; next, on the basis of the simulated data, conversion coefficients for radionuclides were composed considering recent nuclear decay data. The ambient dose equivalent rates were then compared to the effective dose rates for reference adults and a new-born baby as well as to air kerma rates quoted from previous studies. It was confirmed that the ambient dose equivalent sufficiently overestimates effective doses, independently of age, for sources exponentially distributed in the ground. Furthermore, the air kerma was found to also overestimate the effective doses for all ages in the same conditions. In order to verify the computed conversion coefficients, the ratio of ambient dose equivalent to air kerma obtained by simulation was compared to the ratios measured at hundreds of locations in Japan which have been contaminated with radioactive cesium after the accident at the nuclear power plant in Fukushima Prefecture, Japan, in 2011; a good agreement was observed.

Evaluation of ambient dose equivalent rates influenced by vertical and horizontal distribution of radioactive cesium in soil in Fukushima Prefecture

“…The soil sampling campaigns by Matsuda et al [19] commenced in December 2011, so cannot provide measurements to derive a mean βḢ * (10) eff value applicable to this period. The circles in Fig.…”

“…6). Conversion coefficients published for various values of β can be used to evaluate the 1 m ambient dose equivalent rate given the radionuclide inventory per unit area of soil [10]. In contrast, two parameters characterize the penetration of the radionuclides within soil for the hyperbolic secant distribution -a relaxation mass depth β and a mass depth ζ 0 for the peak in activity concentration below the surface (Eq.…”

“…A source of uncertainty in the decay and migration model is the choice for mean βḢ * (10) eff for the first air dose rate survey campaign (June 4 to July 8, 2011 - Table III). The soil sampling campaigns by Matsuda et al [19] commenced in December 2011, so cannot provide measurements to derive a mean βḢ * (10) eff value applicable to this period.…”

“…The dates of the air dose rate surveys and the mean air dose rates at flat, undisturbed fields are listed in Table III. We modelled the decrement in dose rates due to radioactive decay and cesium migration deeper within soil. First, we matched the mean βḢ * (10) eff values from the soil sampling campaigns to the periods of the air dose rate surveys (Table III). We then modelled exponential distributions with β parameters equal to the mean βḢ * (10) eff values with the tool.…”

“…Previous authors have published conversion factors between the concentration of radionuclides within the ground and various air dose rate quantities [5][6][7][8][9][10][11]. These conversion factors assume spatially constant radionuclide inventories and depth distributions.…”