First use of electron swarm technique on THF informs refinement of interaction cross-sections

Understanding electron transport in human tissue is important for modeling radiation damage caused by medical applications like radiotherapy. Electron scattering cross-sections have been estimated for some simple biomolecules, including gaseous water and tetrahydrofuran (THF) — a sugar that links phosphate groups in the backbone of DNA. Electron swarm experiments, which study parameters related to their motions in gas, are the only available way to test these cross-section predictions.

A pulsed-Townsend swarm technique, a standard gas-discharge method using a uniform field, provided measurements of electron transport coefficients through gaseous THF for the first time, reported in The Journal of Chemical Physics. The drift velocities and first-Townsend coefficients, assessing ionization and attachment processes, were analyzed in conjunction with a multi-term Boltzmann equation to assess theoretical and experimental cross-section accuracy. The analysis highlighted discrepancies in the theoretical physics.

Co-author Madalyn Casey explained that this led the group to examine cross-sections that are theoretically or experimentally limited in the literature, including those for neutral dissociation and dissociative electron attachment. They used iterative changes to the cross-sections to try and fit the experimental measurements, while respecting the physical processes that are likely to happen. From the results, the authors propose modifications to the THF-vapor electron scattering cross-section database to improve consistency with measured swarm data.

Casey aims to examine more biological molecules and emphasized the necessity for the field to move into condensed-phase molecular analysis as a more realistic model of human tissue. “We want to motivate more experiments testing data available in the literature, to get a common database of molecules and their interactions so that we can move towards modeling tissue,” said Casey.

Source: “Self-consistency of electron-THF cross sections using electron swarm techniques,” by M. J. E. Casey, J. de Urquijo, L. N. Serkovic Loli, D. G. Cocks, G. J. Boyle, D. B. Jones, M. J. Brunger, and R. D. White, Journal of Chemical Physics (2017). The article can be accessed at https://doi.org/10.1063/1.5004717 .