A Monte Carlo track structure code for low energy protons

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Abstract A code is described for simulation of protons (100 eV to 10 MeV) track structure in water vapor. The code simulates molecular interaction by interaction for the transport of primary ions and secondary electrons in the form of ionizations and excitations. When a low velocity ion collides with the atoms or molecules of a target, the ion may also capture or lose electrons. The probabilities for these processes are described by the quantity cross-section. Although proton track simulation at energies above Bragg peak (>0.3 MeV) has been achieved to a high degree of precision, simulations at energies near or below the Bragg peak have only been attempted recently because of the lack of relevant cross-section data. As the hydrogen atom has a different ionization cross-section from that of a proton, charge exchange processes need to be considered in order to calculate stopping power for low energy protons. In this paper, we have used state-of-the-art Monte Carlo track simulation techniques, in conjunction with the published experimental and established theoretical data, to develop a model for the extension of the proton track simulation to the low energy region. Data are presented on charge-state-fraction, proton stopping power, range and averaged energy producing an ion pair (W-values) in a mixture of hydrogen (H 2 ) and Oxygen (O 2 /2) gas. The results are compared with the published experimental data.