RADIANCE AND PARTICLE-FLUENCE

The concepts of radiance and fluence are fundamental to the descriptio n of a radiation field. The International Commission on Radiological U nits and Measurements (ICRU) has defined fluence in terms of the numbe r of the radiation particles crossing a small sampling sphere. A secon d definition has been proposed in which the length of track segments c ontained within any sampling volume are used to calculate the incident fluence. This approach is often used in Monte Carlo simulations of in dividual particle tracks, allowing the fluence to be scored in small v olumes of any shape. In this paper we stress that the second definitio n generalizes the classical (ICRU) concept of fluence. We also identif y the assumptions inherent in the two definitions of fluence and prove their equivalence for the case of straight-line particle trajectories .