Abstract
Methods of evaluating factors relating to the concentration of uranium, radium, thorium and potassium and the exposure rate at ground level are chronologically reviewed from the beginning of this century up to the present. Time dependence of the values of these factors is illustrated in a single figure. The values estimated before 1956 are extremely low as compared with recent ones. Each of these values, however, shows recently a convergence to a constant value.
As a result of the chronological survey of the conversion factors, the following points are presented as future problems: (1) benchmark calculations using various computational methods are required, (2) direct estimates of the factors by experimental measurements are necessary, and (3) these factors should be evaluated taking into account the effect of radon emanation.
