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Improving the track structure simulation of alpha particles in nitrogen and propane

09.07.2014

In nanodosimetry, track structure simulations are used to determine characteristic parameters of a particle track. For example, the ionization cluster size serves to estimate the biological effectiveness of ionizing radiation on DNA segments. The simulations allow an extrapolation of track structure parameters which are measured with PTB's nanodosimeter in the gas phase of the medium, towards those of condensed matter. On the other hand, the simulations for the gas phase can be validated on the basis of the measurements.

The most important input parameters of a track structure simulation are interaction cross sections, since they describe the interaction between the projectiles and the molecules of the medium. Therefore, the accuracy of the simulation results significantly depends on the precision of the interaction cross sections. The track structure code PTra [1, 2], used at PTB for the transport of alpha particles in nitrogen and propane, previously took charge exchange processes of slow projectiles (below an energy of 1 MeV) into account only by using an empirical correction of ionization cross sections. At first sight, this type of correction provides a good approximation; however, it does not fulfill the requirements placed on a track structure simulation with the objective of providing a simulation of individual interaction processes that is as accurate as possible.

Recently, interaction cross sections for the charge exchange of alpha particles have been integrated into the PTra code. This required a thorough review of the experimental literature data of the cross sections for ionization, electron capture and electron loss of helium particles of all possible charge states with nitrogen and propane molecules. The data basis for nitrogen is reasonably consistent. For propane, however, no data exist. A corresponding cross section data set for propane was obtained by scaling the respective cross sections of methane, taking the ionization cross sections of propane for electrodes into account [2].

Simulations of the ionization cluster size distribution of alpha particles in nitrogen and propane, using the cross sections for charge exchange processes, are in satisfying agreement with experimental data (see Fig.).

Figure: Experimental and computed mean ionization cluster sizes for alpha particles in propane and nitrogen

Literature:

  1. B. Grosswendt, Radiat. Environ. Biophys. 41 (2002) 103.
  2. M. U. Bug et al., Phys. Rev. E 88 (2013) 043308.