Electron energy-loss distributions in solid, dry DNA.

Experimentally derived optical constants and X-ray attenuation cross sections were used to construct the complete dipole oscillator strength distribution for solid, dry DNA. Monte Carlo simulations of the energy loss by electrons of initial energy 5 keV to 1 MeV in DNA were performed using cumulative inelastic cross sections obtained from a formulation incorporating the constructed dipole oscillator strength distribution. The energy-loss distribution, the most probable energy loss and the mean energy loss for electrons in DNA are compared to those for liquid water, gaseous water and gaseous hexane. For the most part, the calculations show that electron energy loss in DNA is very similar to that in liquid water; however, it is quite different from both gaseous water and gaseous hexane. The mean energy losses for a 1 MeV incident electron in DNA, liquid water, gaseous water and gaseous hexane are 57.9, 56.8, 50.9 and 38.4 eV, respectively. The large differences found between the predictions for DNA and for the gaseous media bring into serious question calculations of radiation-induced damage in DNA which make use of cross sections for gaseous media. Stopping powers and continuous-slowing-down approximation ranges for the media for electrons are also presented.