title: Physical and biophysical properties of proton tracks of energies 1 keV to 300 MeV in water.
abstract: OBJECTIVE
  To investigate physical and biophysical properties of proton tracks 1 keV-300 MeV using Monte Carlo track structure methods.
  
  
  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.
authors.affiliation: Radiation Biophysics Group, Department of Oncology Pathology, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Thiansin
authors.initials: T
authors.lastname: Liamsuwan
authors.forename: Shuzo
authors.initials: S
authors.lastname: Uehara
authors.forename: Dimitris
authors.initials: D
authors.lastname: Emfietzoglou
authors.forename: Hooshang
authors.initials: H
authors.lastname: Nikjoo
mesh_terms: D000465:Algorithms; D001703:Biophysics; D003198:Computer Simulation; D004583:Electrons; D008962:Models, Theoretical; D009010:Monte Carlo Method; D011522:Protons; D011874:Radiometry; D012542:Scattering, Radiation; D014867:Water
chemical_list: D011522:Protons; D014867:Water