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.
Highlights
Femtosecond X-ray lasers have opened up new frontiers in physics, driving a wide variety of applications such as ultrafast crystallography (Patterson, 2014), single-particle imaging using Coulomb explosion (Ablikim et al, 2016) or X-rayinduced photo-electrons (Kastirke et al, 2020), material processing and nanostructuring (Dinh et al, 2019), catalysis (Bergmann et al, 2021), biophysics (Thor & Madsen, 2015), and biomedicine (Melissinaki et al, 2011)
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
Using the Monte Carlo simulation tool XCascade3D, we demonstrated that the size of photo- or electron-induced electron cascades may be larger than the corresponding electron range
Summary
Femtosecond X-ray lasers have opened up new frontiers in physics, driving a wide variety of applications such as ultrafast crystallography (Patterson, 2014), single-particle imaging using Coulomb explosion (Ablikim et al, 2016) or X-rayinduced photo-electrons (Kastirke et al, 2020), material processing and nanostructuring (Dinh et al, 2019), catalysis (Bergmann et al, 2021), biophysics (Thor & Madsen, 2015), and biomedicine (Melissinaki et al, 2011). We applied the XCascade3D code (Medvedev, 2015; Lipp et al, 2017), which is based on the classical asymptotic trajectory Monte Carlo (MC) simulation method It models the following processes: photoabsorption, which excites electrons from core or valence atomic shells of the target; photo-electron propagation accompanied by elastic (change of direction) and inelastic (impact ionization) scattering events; Auger decays of core holes, resulting in emission of secondary electrons; and transport of all the secondary-generation electrons. Since electrons with
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