Temporal reduction of the external gamma dose rate due to 137Cs mobility in sandy beaches

In the present paper the contribution to the external gamma dose rate due to ¹³⁷Cs in soil as a function of time is presented. Sampling sites were selected along the Calabria and Basilicata Regions coastal beaches (southern part of Italy) to assess the external gamma dose rate in air, 1 m above the ground level. A convection-dispersion model, with constant parameters was used to approximate the radiocesium soil vertical migration. The model was calibrated using the initial ¹³⁷Cs activity deposition in this region (Chernobyl fallout) and ¹³⁷Cs activity concentration down the soil profile, measured 10 years later. The dispersion coefficient and the advection velocity values, were respectively: 2.17 cm² y⁻¹ and 0.32 cm y⁻¹. The Radionuclide Software Package (RSP), which uses a Monte Carlo simulation code, was used to determine the primary ¹³⁷Cs gamma dose contribution in air 1 m above the ground surface. The resulting ¹³⁷Cs external dose rate ranged from 0.42 nGy h⁻¹ in 1986, to 0.05 nGy h⁻¹ in 2007.