Estimation of immediate fallout after the accident at Fukushima Daiichi Nuclear Power Plant by using HPGe detector and EGS5 code

•Emergent measurement was conducted immediately after the accident at Fukushima Dai-ichi nuclear power plant.•Sampling methods were decided flexibly according to situation that the authors were in.•Utilization of EGS5 code was necessary to determine activity.•Variations and the amount of fallout during almost a month after the accident were shown.

After the accident at the Fukushima Daiichi nuclear power plant, we managed to carry out emergency measurements of the radioactive fallout. The included nuclides were identified via gamma-ray spectrometry using an HPGe detector. Quantifications of each radionuclide in the fallout were determined based on the efficiency calibrations and relevant corrections. The collected samples had a variety of shapes, densities, and compositions. EGS5 Monte Carlo code was used for the flexible estimation of these parameters. The measurement results show the temporal changes in the fallout quantity about a month after the accident.