Environmental ionizing radiations were surveyed in Okayama city during December of 1992, using a NaI (Tl) scintillation survey meter (TCS-161; Aloka, Japan) and a NaI (Tl) γ spectrometer (JSM-102; Aloka, Japan) ; measerments were carried out in fine days. Equivalent dose rate of environmental ionizing radiations was in the range of 0.048-0.171μSv/h, and the average was 0.082±0.019μSv/h. The dose rate was higher in hills than in fields. Therefore, it appears as if rocks have more radioactive materials than soils do. The distribution of the dose rates was analyzed from geological points of view. Mesozoic and Paleozoic layer was in the range of 0.067-0.095μSv/h, and the average was 0.080±0.008μSv/h. Rhyolitic layer was in the range of 0.076-0.105μSv/h, and the average was 0.088±0.010μSv/h. Granitic layer was in the range of 0.057-0.171μSv/h, and the average was 0.098±0.022μSv/h. Neogene Period layer was in the range of 0.057-0.114μSv/h, and the average was 0.076±0.015μSv/h. Quaternary Period layer was in the range of 0.048-0.114μSv/h, and the average was 0.076±0.013μSv/h. By γ spectrometry, 40K, 208Tl, 212Bi, 214Bi, and228Ac were detected at Mt. Kaigara (0.171μSv/h), Tsushima (0.076μSv/h) and Fujita (0.048μSv/h), especially, 226Ra was detected at Mt. Kaigara. It seems that40K was distributed uniformly in Okayama city area. Thas, the difference in the dose range is attributable to nuclides other than40K. Now, we conclude that there has been no change of the dose rates of environmental radiation in Okayama city these 10 years.
For practical use of dual energy gamma-ray transmission techniques in various fields, necessary conditions have been examined experimentally. The study revealed the necessary conditions shown in the following : (1) the shape and size of the collimator aperture for a gamma-ray beam must be changed according to those of a sample to be measured, (2) a sufficiently intense gamma-ray source is required which provides counting rates of tens of thousands per minute, (3) the ratio of N/N0must be in a range of 0.8-0.1, and (4) the value of R with a minimum error calculated should be adopted.