Francis A. Cucinotta

Effects of track structure and cell inactivation on the calculation of heavy ion mutation rates in mammalian cells

It has long been suggested that inactivation severely effects the probability of mutation by heavy ions in mammalian cells. Heavy ions have observed cross sections of inactivation that approach and sometimes exceed the geometric size of the cell nucleus in mammalian cells. In the track structure model of Katz the inactivation cross section is found by summing an inactivation probability over all impact parameters from the ion to the sensitive sites within the cell nucleus. The inactivation probability is evaluated using the dose-response of the system to gamma-rays and the radial dose of the ions and may be equal to unity at small impact parameters for some ions. We show how the effects of inactivation may be taken into account in the evaluation of the mutation cross sections from heavy ions in the track structure model through correlation of sites for gene mutation and cell inactivation. The model is fit to available data for HPRT mutations in Chinese hamster cells and good agreement is found. The resulting calculations qualitatively show that mutation cross sections for heavy ions display minima at velocities where inactivation cross sections display maxima. Also, calculations show the high probability of mutation by relativistic heavy ions due to the radial extension of ions track from delta-rays in agreement with the microlesion concept. The effects of inactivation on mutations rates make it very unlikely that a single parameter such as LET or Z*2/beta(2) can be used to specify radiation quality for heavy ion bombardment.

Viable Transfer of Microorganisms in the Solar System and Beyond

It is now generally accepted that at the beginning of our solar system a considerable amount of organic molecules and water were imported to the early Earth as well as to the other terrestrial planets via asteroids and comets [1-3]. The period of heavy bombardment lasted until approximately 3.8 billion years (Ga) ago. These impactors would on the one hand have been delivering the volatiles as precursors of life, on the other hand, if sufficiently large and fast, would have eroded the atmosphere and perhaps sterilized the Earth and/or Mars, if life existed there [4, 5].

Monte-Carlo Simulation of Ionizing Radiation Tracks

Ionizing radiation comprises several types of energetic particles such as electrons, ions, and neutrons and energetic photons (X-rays or γ-rays). They are widely used in medicine for diagnosis and for cancer radiotherapy either as a curative or adjunctive treatments. Ionizing radiation is also known to increase cancer risk and other late risks include cataracts, heart disease and central nervous system effects. While the biological pathways involved in the effects of radiation are numerous and complex, they are initiated by physical, physicochemical and chemical interactions of the radiation with the medium. Because of the stochastic nature of radiation interactions, Monte-Carlo simulations techniques are very convenient not only to help our understanding of the mechanisms of interaction of ionizing radiation with matter, but they are also used in practical applications such as in microdosimetry, accelerator design and radiotherapy treatment planning. Therefore, several Monte-Carlo simulation codes of radiation tracks have been developed with different purposes (reviewed in Nikjoo et al., 2006). In this chapter, the Monte-Carlo techniques used in particle transport related to energy deposition will be reviewed first. In the second part, the concept of cross section will be discussed. The cross sections are of particular importance for radiation transport, because they represent the probability of interaction of radiation with the medium. The differential cross sections are used to find the energy of the interaction and the direction of the particles afterward. The total cross sections are needed to calculate the mean free path between two interactions of the radiation. Therefore most of the discussion will be on the interaction cross sections for electrons, ions and photons, their sampling and their use in radiation transport codes.

International Space Station Radiation Shield Augmentation Optimization

NASA has made a commitment to reduce astronaut radiation exposures to levels As Low As Reasonably Achievable (ALARA). Inherent in this commitment is the need to predict and optimize crew radiation exposures during a mission. A number of software tools have been developed at NASA Langley Research Center to estimate International Space Station radiation levels by combining computational models of the radiation shielding the station provides and computational models of the highly anisotropic low Earth orbit radiation environment. The radiation shielding models used in these analyses are typically based upon CAD models of International Space Station modules, equipment racks, and stores. These models are coupled with software that allows a user to interactively relocate equipment racks and immediately recalculate local radiation levels in an immersive environment. New software tools have been developed which extend this capability so that different strategies of shield augmentation may be evaluated and compared in an automated environment. These tools produce tailored geometries for shield augmentation designs that attempt to meet specified constraints on volume, mass, thickness, and resulting radiation levels. Results presented focus on reducing radiation levels within the crew quarters where crewmembers spend several hours each day.

Track structure model for damage to mammalian cell cultures during solar proton events

Solar proton events (SPEs) occur infrequently and unpredictably, thus representing a potential hazard to interplanetary space missions. Biological damage from SPEs will be produced principally through secondary electron production in tissue, including important contributions due to delta rays from nuclear reaction products. We review methods for estimating the biological effectiveness of SPEs using a high energy proton model and the parametric cellular track model. Results of the model are presented for several of the historically largest flares using typical levels and body shielding.