Rapid method of gamma-ray does rates for distinction between natural and artificial radioactive nuclides using portable NaI(Tl) scintillation measurement system

When evaluating an environmental γ-ray dose rates, it is important to determine the individual contributions of both natural and artificial radioactive nuclides to the overall measured rate. In areas where nuclear contamination is suspected, however, it can be difficult to rapidly ascertain these relative contributions. This paper describes the use of a portable NaI(T1) scintillation spectrometer to quickly and accurately measure γ-ray dose rates at 16 locations in Japan, in order to construct a contamination map on an emergency basis. The survey data showed that, in locations with differing energy distributions, there were noticeable correlations between the total γ-ray dose and the count rates within the specific energy ranges of 1400 - 3000, 1650 - 3000 and 1800 - 3000 keV, and these correlations differed markedly between natural and artificial nuclides. These observations suggested that the artificial nuclide contribution to the γ-ray dose rate could be calculated using the correlations, and subsequent analysis of the data confirmed that this was possible. The resulting estimates were accurate to within approximately 3.3, 0 and 10% within the energy ranges of 1400 - 3000, 1650 - 3000 and 1800 - 3000 keV as the 30 nGy/h, respectively. (author)