Comparative Dosimetric Estimates of a 25 keV Electron Microbeam with Three Monte Carlo Codes The calculations presented compare the different performances of the three Monte Carlo codes: PENetration and Energy LOss of Positrons and Electrons code (PENELOPE-1999), Monte Carlo N-Particle transport code system (MCNP-4C), Positive Ion Track Structure code (PITS), used for the evaluation of dose profiles from a 25 keV electron micro-beam traversing individual cells. The overall model of a cell is a

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... ater cylinder equivalent for the three codes but with a different internal scoring geometry: water filled cylindrical shells for PENELOPE and MCNP, whereas spheres are used for the PITS code. A cylindrical cell geometry with scoring volumes with the shape of water filled cylindrical shells was initially selected for PENELOPE and MCNP because of its superior simulation of the actual shape and dimensions of a cell and for its improved computer-time efficiency if compared to spherical internal volumes. Some of the transfer points and energy transfer that constitute a radiation track may actually fall in the space between spheres, that would be outside the spherical scoring volume. This internal geometry, along with the PENELOPE algorithm and along with the combination of numerical and analytical differential cross sections, drastically reduced the computer time when using this code if comparing with event-by-event Monte Carlo codes like PITS. This preliminary work has been important to address dosimetric estimates at low electron energies. It demonstrates that codes like PENELOPE can be used for dose evaluation, even with such small geometries and energies involved, which are far below the normal use for which the code was created. Further work (initiated in Summer 2002) is still needed, to create a user-code for PENELOPE that allows uniform comparison of exact cell geometries, integral volumes and also microdosimetric scoring quantities, a field where track-structure codes like PITS, written for this purpose, are believed to be superior. ii 11. 3-D plot of the track simulation using MCNP Abstract The calculations presented compare the different performances of the three Monte Carlo codes: PENetration and Energy LOss of Positrons and Electrons code (PENELOPE-1999), Monte Carlo N-Particle transport code system (MCNP-4C), Positive Ion Track Structure code (PITS), used for the evaluation of dose profiles from a 25 keV electron micro-beam traversing individual cells. The overall model of a cell is a water cylinder equivalent for the three codes but with a different internal scoring geometry: water filled cylindrical shells for PENELOPE and MCNP, whereas spheres are used for the PITS code. A cylindrical cell geometry with scoring volumes with the shape of water filled cylindrical shells was initially selected for PENELOPE and MCNP because of its superior simulation of the actual shape and dimensions of a cell and for its improved computer-time efficiency if compared to spherical internal volumes. Some of the transfer points and energy transfer that constitute a radiation track may actually fall in the space between spheres, that would be outside the spherical scoring volume. This internal geometry, along with the PENELOPE algorithm and along with the combination of numerical and analytical differential cross sections, drastically reduced the computer time when using this code if comparing with event-by-event Monte Carlo codes like PITS. This preliminary work has been important to address dosimetric estimates at low electron energies. It demonstrates that codes like PENELOPE can be used for dose evaluation, even with such small geometries and energies involved, which are far below the normal use for which the code was created. Further work (initiated in Summer 2002) is still needed, to create a user-code for PENELOPE that allows uniform comparison of exact cell geometries, integral volumes and also microdosimetric scoring quantities, a field where track-structure codes like PITS, written for this purpose, are believed to be superior.