Low-energy electron interactions with liquid water and energy depositions in nanometric volumes

Interactions of low-energy electrons with liquid water and DNA were investigated. Elastic and inelastic interaction cross sections were calculated using appropriate models for the response of atomic nuclei, valence band and inner shells. These models included an extended Drude dielectric model for valence-band excitations, a sum-rule-constrained binary-encounter model for inner-shell ionizations, and a phase-shift analysis model for elastic interactions. Applying calculated cross sections, an event-by-event Monte Carlo (MC) program was developed to simulate electron transport in liquid water and energy depositions in nanometric volumes. This simulation provided an estimate of strand breaks of DNA due to direct and indirect actions by low-energy electrons in a simplified DNA model. This model consisted of two parallel cylinders of 0.5 nm in diameter, 16 nm in height and 1 nm in separation. Any energy deposition greater than 17.6 eV in the cylinder was assumed to cause a single strand break (ssb) by direct action. An energy deposition of 12.6 eV or greater in liquid water within 0.5 nm of the cylinder surface was assumed to induce an OH radical which had a probability of 0.13 to produce a ssb by indirect action. When two ssbs occurred on opposite strands separated by 10 or fewer base pairs, a double strand break (dsb) was assumed. The number of deposition events for ssbs or dsbs per electron emission was calculated and analyzed for different electron energies and electron emission positions. The simplified model provided useful information on the energy depositions for DNA strand breaks and on the effectiveness of low-energy electrons in various geometric and irradiation conditions.