A measurement of the residual radioactivity with NaI survey meter and its MC simulation

The present study shows how Monte Carlo method using EGS4 code can be useful for evaluating the residual radioactivity induced in shielding concrete of accelerator facilities. By using EGS4 code, the response of NaI survey meter concerning the shielding concrete block radius and thickness are calculated. It is found that the surface dose rate is saturated when the shielding concrete radius and thickness reach 40 cm and 30 cm respectively. Under the above-mentioned conditions, the surface dose rates at position north-8 and south-9 of SF cyclotron in University of Tokyo, are investigated, and compared with the experimental results obtained by NaI survey meter. It is shown that the calculation results are in good agreement with the experiments. In addition, the conversion factor of the surface dose rate to surface residual radioactivity is obtained to be 0.90(Bq·g-1)·(μSv·h-1)-1 for 60Co and 1.26(Bq·g-1)·(μSv·h-1)-1 for 152Eu. By using this conversion factor, we can easily evaluate the radioactivity induced in shielding concrete of accelerator facilities from the experimental results of NaI survey meter. (authors)