Abstract ID: 165 Assessment of RBED electron-impact ionization cross sections for Monte Carlo electron transport

Abstract

Electron-impact ionization cross sections for atoms and molecules are essential for the modelling of radiation effects in a wide range of applications. Current state-of-the-art theoretical calculations are based on the distorted-wave Born approximation (DWBA), which provides an improvement in accuracy and validity range over the plane-wave Born approximation (PWBA) by accounting consistently for exchange effects as well as the distortion of the projectile wave functions by the field of the target atom. However, numerical DWBA calculations are elaborate and slow, and tabulated data only exist for integrated cross sections, whereas differential cross sections are also needed in Monte Carlo simulations. The relativistic binary-encounter-dipole (RBED) model combines classical binary-encounter theory with Bethe’s PWBA-based dipole model without the need for any empirical adjustable parameters [1] . The analytical RBED model provides integrated as well as differential cross sections and is much simpler than the DWBA [2] , requiring as input only the binding energy, average kinetic energy, and optical oscillator strength (OOS) for each atomic subshell. Due to the difficulty of obtaining accurate OOSs, simplified models (known as RBEB and RBEQ) which assume a simple functional form for the OOS have also been proposed, and have been employed almost exclusively in studies since then. Here we calculate electron-impact ionization cross sections for a set of atoms spanning the periodic table using the more accurate RBED model, relying on OOSs obtained numerically with self-consistent potentials as described in [3] . Compared to RBEB, we find that the RBED results show better agreement with DWBA data for the K-shell of all atoms and over the entire energy range. The improvement is more pronounced for lower-Z elements, where the agreement between RBED and DWBA is near perfect. For higher shells, there is also better agreement at high energies for all elements, but near the threshold the improvement is less consistent and depends on the specific atomic shells. The disagreements between the RBED and DWBA cross sections near the threshold are due to the interplay between Coulomb and exchange effects, and highlight the limitations of using semi-empirical scaling factors. In the near future we will compile tables of RBED data for biologically-relevant elements, and ultimately implement them in Monte Carlo track-structure codes.