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Benchmarking track structure Monte Carlo simulations

02.01.2012

A tool for verification of the secondary electron transport in a track structure simulation has been suggested by Toburen et al. [1], who have measured angle-dependent secondary electron spectra induced by 6 MeV protons in a thin layer of amorphous solid water (ASW).

In this work, simulations have been performed for the set-up of this experiment, using the PTB Track structure code (PTra) [2] and Geant4-DNA Version 9.4 p.02 [3]. To enable electron transport below the ionization threshold, additional excitation and dissociative attachment anion states measured by Michaud et al. [4] were included in PTra and activated in Geant4. Additionally, a surface potential was introduced in both simulations, such that the escape probability for an electron depends on its energy and impact angle at the ASW/vacuum interface.

For vanishing surface potential, the simulated spectra are in good agreement with the measured spectra for electron energies above 50 eV (Figure). Below this energy, the simulations overestimate the yield of electrons by a factor up to 4 (PTra) or 7 (Geant4), which is still a better agreement than obtained in previous simulations using the Monte Carlo code PARTRAC [1]. The agreement of our simulations with experimental data is significantly improved by using a work function, while a positive potential barrier seems inappropriate.

Figure : Total yield of secondary electrons as a function of their energy obtained by integrating measured and simulated angle-resolved spectra over the emission angle. Shown are the simulations spectra obtained by PTra using different assumptions for the surface barrier.

Literature

  1. L. H. Toburen, S. L. McLawhorn, R. A. McLawhorn, K. D. Carnes, M. Dingfelder, J. L. Shinpaugh:
    Electron emission from amorphous solid water induced by passage of energetic protons and fluorine ions.
    Radiat. Res. 174, 107-118 (2010).
  2. B. Grosswendt:
    Formation of ionisation clusters in nanometric structures of propane-based tissue-equivalent gas or liquid water by electrons and α-particles.
    Radiat. Environ. Biophys. 41, 103-112 (2002).
  3. S. Incerti, A. Ivanchenko, M. Karamitros, A. Mantero, P. Moretto, H. N. Tran, B. Mascialino, C. Champion, V. N. Ivanchenko, M. A. Bernal, Z. Francis, C. Villagrasa, G. Baldacchino, P. Gueye, R. Capra, P. Nieminen, C. Zacharatou:
    Comparison of GEANT4 very low energy cross section models with experimental data in water.
    Med. Phys. 37 4692-4708 (2010).
  4. M. Michaud, A. Wen, L. Sanche:
    Cross sections for low-energy (1-100 eV) electron elastic and inelastic scattering in amorphous ice.
    Radiat. Res. 159, 3-22 (2003).