Estimation of Very High Energy Electron (VHEE) beam parameters using an in-air screen

Abstract

Very high energy electron (VHEE) beams offer several promising properties for radiotherapy. Accurate beam delivery requires a beam model that accounts for scattering along the beam propagation path. This work develops and validates a beam model incorporating Multiple Coulomb Scattering (MCS) to estimate beam parameters. MCS is incorporated as an additive term to the covariance matrix transformation for a drift space, allowing conventional quadrupole scans to estimate beam parameters in the presence of a downstream scatterer. The scattering correction factor is fitted to Monte Carlo (MC) simulations for a range of materials and thicknesses. The model is then validated by recovering the initial beam parameters from simulated quadrupole scans of a realistic beamline. Finally, the method is applied to in-air screen measurements at the Pulsed Energetic Electrons for Research (PEER) end station of the Australian Synchrotron. After fitting the correction factors, the mean and standard deviation of the estimation errors for the final emittance, diameter, divergence, and mean kinetic energy were $(-0.06 \pm 3.17)\%$, $(0.15 \pm 0.66)\%$, $(0.18 \pm 2.18)\%$, and $(-0.03 \pm 0.06)\%$, respectively. Simulated quadrupole scans successfully recovered the initial beam parameters, with errors of at most $13\%$. PEER measurements yielded emittance estimates ranging from $0.238$ to $1.753~\mathrm{mm\,mrad}$ across scans, which were within the expected range. Following an initial quadrupole scan, the method allows the in-air beam size to be estimated for other quadrupole configurations. Further work is required to improve the consistency and reliability of the experimental estimates.