A simplified method for calculating dose rates from radionuclides deposited on a finite surface soil

A simplified calculation method of dose rates in the environment when radionuclides are deposited on a finite surface soil is proposed. Based on the concept of the point-kernel method, the dose rate from a sufficiently small volume source is calculated by D = KBφ. K is the dose conversion factor. The dose buildup factor B has been examined for major substances and summarized as a function of mean free paths. The formulas expressing the primary photon fluence rate φ have already been derived. In this study, we propose the calculation method of the mean free path until photons emitted from a radiation source reach the detector. When a plane source exists in a single medium, calculation results by the simplified calculation method agree well with those calculated using the detailed Monte Carlo method. In stratified layers of soil and air, the results were greatly influenced by the accuracy of the double-layered dose buildup factor employed. When the radius of the plane source is 100 m or more, the differences from the detailed calculation results were less than 7%. (author)