Photoelectric effect model for gamma

The photoelectric effect is the ejection of an electron from a material after a photon has been absorbed by that material. Depending on the input energy of the incident gamma, the model provides cross-sections based either on the interpolation of tabulated cross-sections data or on the parameterization of cross-sections data obtained through two fits in two different energy ranges (low and high). Note that for this process the lambda table is not built, since the cross-section is not a smooth function of the energy, therefore in all calculations the cross section is used directly. Final state sampling: The incident photon is absorbed and an electron is emitted with a direction that is calculated using the Sauter-Gavrila distribution (for K-shell). The electron kinetic energy is the difference between the incident photon energy and the binding energy of the electron before the interaction. The sub-shell, from which the electron is emitted, is randomly selected according to the relative cross-sections of all subshells, determined at the given energy E0, by interpolating the evaluated cross-section data from the EPICS (Electron Photon Interaction Cross Sections) v.2014 data bank. The interaction leaves the atom in an excited state; the deexcitation process is not implemented yet.