Quantifying electron cascade size in various irradiated materials for free-electron laser applications

Abstract

Studying electron- and X-ray-induced electron cascades in solids is essential for various research areas at free-electron laser facilities, such as X-ray imaging, crystallography, pulse diagnostics or X-ray-induced damage. To better understand the fundamental factors that define the duration and spatial size of such cascades, this work investigates the electron propagation in ten solids relevant for the applications of X-ray lasers: Au, B4C, diamond, Ni, polystyrene, Ru, Si, SiC, Si3N4 and W. Using classical Monte Carlo simulation in the atomic approximation, we study the dependence of the cascade size on the incident electron or photon energy and on the target parameters. The results show that an electron-induced cascade is systematically larger than a photon-induced cascade. Moreover, in contrast with the common assumption, the maximal cascade size does not necessarily coincide with the electron range. It was found that the cascade size can be controlled by careful selection of the photon energy for a particular material. Photon energy, just above an ionization potential, can essentially split the absorbed energy between two electrons (photo- and Auger), reducing their initial energy and thus shrinking the cascade size. This analysis suggests a way of tailoring the electron cascades for applications requiring either small cascades with a high density of excited electrons or large-spread cascades with lower electron densities.

Non-thermal structural transformation of diamond driven by x-rays

“…The effective x-ray dose D eff is calculated as follows: where E is the pump pulse energy, w x is the pump horizontal x-ray focus (width at the 1/e level), w y is the pump vertical x-ray focus (width at the 1/e level), λ e is the electron cascade size at the pump photon energy of 7 keV (0.41 μ m according to Refs. 20 and 33 ), λ x is the x-ray penetration depth at the pump photon energy, and ρ a is the atomic density. The pump pulse energy is evaluated from the pump pulse intensity observed by the spectrometer, which is then normalized by the signal of a beamline intensity monitor and corrected by the reflectivity of the Kirkpatrick–Baez focusing mirrors.…”

“…(1) represents an approximation because λ e is a maximum distance at which the electrons lose their energy below a cutoff. 20 The electron cascade size increases the volume of energy deposition especially for the smaller x-ray spot sizes, i.e., higher effective doses. In addition, the pump and probe beams have Gaussian intensity profiles, which results in different regions of the sample being excited by a range of x-ray doses.…”

“… 8 It exceeds the graphitization damage threshold observed experimentally by Gaudin et al 6 (∼0.7 eV/atom). The threshold from Gaudin et al has the advantage that at XUV photon energies, the electron cascade size in diamond is in the nm range 20 and does not have a significant influence on the x-ray dose. The long timescale required to observe the graphite diffraction peak implies that graphitization may occur by a thermal process, rather than non-thermal one.…”

“… 19 Meanwhile, initiated by 10 keV photoelectrons, the spatial distribution of the secondary electron cloud expand to 600 nm. 20 …”

Picosecond to microsecond dynamics of X-ray irradiated materials at MHz pulse repetition rate

“…In the present example of the application of the NanoDiff code, we used the initial conditions estimated with our in-house simulation tool, XCASCADE. For more realistic initial conditions, in future, we plan to utilize our in-house code XCASCADE-3D 32 , taking into account ballistic electron transport 33 , beam polarization and a non-uniform spatial pulse shape. Its final energy and particle distributions can serve as the input for NanoDiff.…”