Physical and biophysical properties of proton tracks of energies 1 keV to 300 MeV in water.

PURPOSE: To investigate physical and biophysical properties of proton tracks 1 keV-300 MeV using Monte Carlo track structure methods.
MATERIALS AND METHODS: We present model calculations for cross sections and methods for simulations of full-slowing-down proton tracks. Protons and electrons were followed interaction-by-interaction to cut-off energies, considering elastic scattering, ionisation, excitation, and charge-transfer.
RESULTS: Model calculations are presented for singly differential and total cross sections, and path lengths and stopping powers as a measure of the code evaluation. Depth-dose distributions for 160 MeV protons are compared with experimental data. Frequencies of energy loss by electron interactions increase from ∼ 3% for 10 keV to ∼ 77% for 300 MeV protons, and electrons deposit  >70% of the dose in 160 MeV tracks. From microdosimetry calculations, 1 MeV protons were found to be more effective than 5-300 MeV in energy depositions greater than 25, 50, and 500 eV in cylinders of diameters and lengths 2, 10, and 100 nm, respectively. For lower-energy depositions, higher-energy protons are more effective. Decreasing the target size leads to the reduction of frequency- and dose-mean lineal energies for protons <1 MeV, and conversely for higher-energy protons.
CONCLUSIONS: Descriptions of proton tracks at molecular levels facilitate investigations of track properties, energy loss, and microdosimetric parameters for radiation biophysics, radiation therapy, and space radiation research.