Average depths of electron penetration: use as characteristic depths of exposure

The average depth of electron penetration is introduced as the physical quantity useful in electron beam irradiation. It is defined as the average of the maximum depths on the trajectories of electrons passing through finite, semi-infinite or infinite medium. The relation between the transmission coefficient as a function of slab thickness and the distribution of the maximum depths is analyzed, and a semiempirical equation to calculate the average depth of electron penetration is given for 0.1- to 50-MeV electrons incident on materials of atomic numbers from 4 to 92. It is shown that the quantity introduced is usable as the characteristic depths of energy and charge depositions in a target, and can possibly be generalized to the case of heterogeneous targets.