Recent developments in the general cavity theory

A model is proposed for the calculation of the stopping power ratio of cavities with any dimension. It is formulated in terms of a general transport equation. This yields two complementary methods of solving the problem, respectively from a ''volume'' and a ''surface'' point of view. These involve the calculation respectively of the electron spectrum within the cavity together with the average energy deposited per unit pathlength and of the net inward electron energy fluence. The application of the volume point of view to small cavities requires the evaluation of the energy deposition of delta-rays generated in the cavity. In the two-step model the delta-rays are assumed to be absorbed on the spot below the cut-off energy Δ. In the Spencer and Attix theory this same model is applied to the wall electrons and thus energy is conserved. The application of the surface point of view to large cavities, requires data on the multiple scattering at the interfaces of cavity and medium. It can be evaluated if sufficient data on energy back-scattering of low energy electrons for isotropic incidence are available. The volume method yields data consistent with an exponential attenuation. This is no longer valid-when the ''surface'' method is applied to the low energy part of the electron spectrum. The values of the cut-off energy and of the effective scattering coefficients should be determined experimentally. A few data are analysed