Challenges in Monte Carlo track structure modelling.

PURPOSE: Although great progress has been made, numerous challenges remain in the development of Monte Carlo (MC) charged-particle track structure simulation models. Such models have evolved from the simple gas phase target models to those using condensed phase interaction data coupled with complex targets representing cellular and molecular constituents of mammalian tissue. A wide choice of MC models is now available ranging from those based on the physics of continuous slowing down, to simulations following each interaction on an event-by-event basis. The choice of code depends largely on requirements for computational speed, and the degree of detail required; however, one must be continuously vigilant to recognise the inherent limitations of the model chosen. CONCLUSIONS: There remain numerous questions of the accuracy and completeness of the interaction physics that present challenges to MC modellers. Recent evidence suggests that the yields of electrons with energies less than a few hundred eV might be substantially overestimated by the elastic and inelastic cross-sections used in many codes. Densely ionising heavy ions present modelling challenges when the rate of energy loss is sufficient to ionise essentially 'every' atom along the ion path. Effects of electron capture and loss by moving heavy ions present significant challenges for modellers particularly for accurate simulation for ions heavier than protons and helium ions? The average effective-charge provides an inadequate description for estimating differential cross-sections for energy loss. These and other questions are considered.

Electron Emission from Foils and Biological Materials after Proton Impact.

Electron emission spectra from thin metal foils with thin layers of water frozen on them (amorphous solid water) after fast proton impact have been measured and have been simulated in liquid water using the event-by-event track structure code PARTRAC. The electron transport model of PARTRAC has been extended to simulate electron transport down to 1 eV by including low-energy phonon, vibrational and electronic excitations as measured by Michaud et al. (Radiat. Res. 159, 3-22, 2003) for amorphous ice. Simulated liquid water yields follow in general the amorphous solid water measurements at higher energies, but overestimate them significantly at energies below 50 eV.

Modeling energy deposition in trabecular spongiosa using the Monte Carlo code PENELOPE.

The inhomogeneity of the target tissue can play an important role in assessing the radiation dose to critical biological targets. This is particularly relevant for calculations of energy deposition by ionizing radiation within regions of radiosensitive trabecular spongiosa. The main focus of this project is the creation of simple quadric-based geometric models of trabecular spongiosa designed specifically for implementation into the general-purpose Monte Carlo radiation transport code PENELOPE. Validation of these models is determined through comparisons of internal dimensions of the model with those of actual bone sites as well as through comparisons of absorbed fractions to the regions within the trabecular spongiosa following the monoenergetic emission of electrons with initial energies ranging from 10 keV to 4 MeV from sources contained within the trabecular spongiosa. On average, absorbed fraction values determined in the current study fall within 2% of values determined by previously published methods. The results of this work suggest that the use of quadric-based geometric models of trabecular spongiosa is a simple and adequate method for implementation into Monte Carlo radiation transport codes.