XCASCADE (3D)

XCASCADE (3D) is a Monte Carlo (MC) code realizing an individual-particle event-by-event MC simulation scheme. It treats a solid as a composition of atoms distributed at a certain density, and models X-ray or high-energy electron triggered electron cascades in this solid for non-relativistic energies of the impact particles (up to ~30 keV). X-ray induced electron cascade starts with an absorption of an X-ray photon, which releases an energetic photoelectron from a valence or a core hole of an atom in the target. In the latter case, the hole undergoes single- or multistep decay (depending on the target and photon energy) creating one or several secondary electrons. Photo- and secondary electrons scatter on atoms inelastically, creating more secondary electrons until they can no longer excite more electrons (i.e., until their energy falls below the lowest ionization threshold of the target's atoms). It is also possible to initiate a cascade with an incident electron instead of a photon.

The program implements a “standard” MC algorithm [1-5] based on the atomistic approximation described above. This approximation holds at sufficiently high photon or impact electron energies (higher than a few times the band gap or the width of the valence band of the material). The solid target is characterized by its atomic composition, density, and atomic energy levels. The laser pulse is characterized by its duration, shape, fluence and photon energy.

The XCASCADE (3D) code uses independent-cascade approximation, i.e., it assumes that each photon is absorbed independently and the corresponding electron cascade develops in a neutral undamaged material, without interactions between the cascades. This assumption holds only at low X-ray fluences. For reliability of the results obtained, the density of excited electrons has always to be lower than the atomic density. This should be checked, while analyzing the simulation results.

In the present version, BEB atomic cross-sections are used [6]. The simulation relies on the EPICS2017 databases [7] for the photoabsorption and electron scattering cross sections in the atomistic approximation as well as for the atomic ionization potentials.

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[7] International Atomic Energy Agency, Nuclear Data Services, https://www-nds.iaea.org/epics/