A study on the electronic stopping of protons in soft biological matter

The electronic stopping of protons in liquid water has been investigated by means of the dielectric function properties. Based on a modified Drude dielectric model, an accurate analytic parametrization of the two sets of available optical data for liquid water has been accomplished. The resultant optical loss functions predict an I-value of 80-85eV. Extension to finite momentum is included by means of simple dispersion algorithms. The influence of different dielectric function approximations and of the higher-order Z1-corrections to the stopping power (SP) of liquid water for protons in the 50-1000keV range is explored. This includes the Bragg peak which, among other things, is of great interest in radiation dosimetry and in predicting radiation damage. The model calculations are within 10-12% of ICRU values with the larger deviations being observed below the SP maximum. The higher-order Z1-corrections, associated with the Barkas and Bloch effects, contribute minimally (<2%) down to the Bragg peak region (∼100keV). At the low end examined (50-100keV) the Z14-term increases more rapidly than the Z13-term and their net contribution increases