Track Structure and Radiation Transport Model for Space Radiobiology Studies

Radiobiology experiments performed in space are deemed necessary for validation of risk-assessment methods. The understanding of space radiobiology experiments must combine knowledge of the space radiation environment, radiation transport, and models of biological response. The heavy ion transport code HZETRN has recently been combined with improved models of the galactic cosmic rays (GCR) and extensive comparisons made to measurements on the space shuttle with a tissue equivalent, proportional counter. HZETRN was also coupled with track-structure models of biological damage &om heavy ions. 2ack-structure calculations using improved models of the radial d m distribution around the path of heavy ions provide a good description of ground-based experiments for inactivation cross sections. Therefore, we use these models to predict inactivation of Bacillus Subtilk spores in space. Calculations consider single-particle effects, as well as the background from law linear energy transfer ions of the GCR and trapped radiations on the radial distributions of effects measured in plastic detectors. 1996. Published by Elsevier Science Ltd on behalf of COSPAR INTRODUCTION Risk assessment for future manned space Q h t requires validation of methods of predicting the expected harmful biological effects. Ground-based facilities are necessary to develop and verify physical models for radiation transport, interaction cross sections for the transport model, and track structure effects in energy deposition. Animal and cell experiments may provide understanding of biological effects such as carcinogenesis, mutation, and damage to the central nervous system. Ground-based radiation biology experiments with monoenergetic ion beams may aid the development of models of heavy ion effects and serve as basic information to estimate space exposure of astronauts. These ground-based models are combined with dynamic space environment data and spacecraft shielding properties to provide the final prediction of risk to astronauts on space missions. The validation of these risk evaluation methods required for spaceflight also must consider the expected radiation environment to be encountered after modification by spacecraft and body shielding, and eventually of radiobiological response of test biological systems, preferably those used in the ground-based models. The outcome of the validation process would include considerations on the modification of the response due to microgravity and the physiological stresses of spaceflight. The assignment of the error in the risk estimates is required to complete the didation process for space radiation risk assessment /I/. The scientific development and validation process for space radiations will continue for many years. The emphasis of the present work will be on the spacecraft validation of both radiation transport modela and models of biophysical damage. The Langley Research Center has a vigorous program in the development of laboratory and space radiation transport codes for HZE particles. The (12)183 JASR 18:12-G (12)184 F. A. Cucinotta ct al. transport codes as developed by Wilson et al. 12-41 are built on the premise that the required transport methods must be amendable to validation with laboratory HZE beams. Badhwar and O'Neill/5/ have developed a model of the galactic cosmic ray (GCR) environment which is included in the HZETRN code 141 and represents a substantial improvement in the representation of the GCR environment over earlier models 161. Recent measurements onboard the space shuttle have been made with an active tissue equivalent proportional counter (TEPC). Ongoing comparisons of dose and dose equivalent between the TEPC and transport models are being made along with comparisons between measured lined energy (y) spectrum and calculated linear energy transfer (LET) spectrum 171. The validity of such comparisons are limited by the large width of the ion track, energy loss fluctuations in crossing the gas volume, leakage of electrons generated in the counter walls into the sensitive volume, and nuclear fragmentation effects within the device 181. Future measurements are expected with particle telescopes, allowing for identification of particle type and energy for direct comparisons to calculated particle energy spectra. S p d g h t radiobiology experiments have been recently reviewed by Horneck 191 and Nelson /lo/. An experiment measuring the inactivation of spores of Bacillus Subtilus (B. Spores) as a function of the impact parameter from single HZE particles has been made in the Biostack experiments on Apollo 16 and 17 and on the ApolleSoyuz Test Project (ASTP). In these experiments the spores are held in contact with a plastic detector sheet (cellulose nitrate). Etching techniques allow for identification of the inactivation versus impact parameter to an accuracy of about f 0.2 pm. Measurements suggest that the inactivation p r o b a b i i extends to much larger distances than expected from ground-based measwements 1111. Cellulose nitrate has a threshold response for particle identification which prohibits identification of particles below threshold values in energy deposition. We use the action cross section model of Katz 1121 combined with models of the radiation environment and transport to predict the damage rate from the background of particles not identifiable in the BiOBtack experiment. More recently radiobiology experiments have been undertaken on the International Microgravity Laboratory (IML) flown on the space shuttle and it is expected that new space radiobiology data will be available in the future. In the present report we consider the space validation process for a limited measurement of the physical fields within a spacecraft in comparison to the HZETRN code. We further consider the use of a ground-based biological model for B. Spores studied in heavy ion accelerators using the track structure model of Katz 1121. F i y , we try to explain inactivation effects measured in the Biwtack experiments using the radiation transport and track structure model in an attempt to validate these models for spaceflight. Our ability to predict the response of such simple systems must support our confidence in predicting risks for astronauts in future space missions. GALACTIC HEAVY ION TRANSPORT MODEL The propagation of the GCR and their secondaries through bulk matter is described by the Boltzman equation which in the straightahead approximation is of the form /2,13/: where u, denotes the range scaling parameter which is equal to Z j 2 / ~ j where Aj and Zj are the charge and mass numbers of ion j, respectively. In equation (I), E represents energy (MeVIamu), S(E) is the proton stopping power, cr(E) is the total cross section, +j(z, E) is the differential flux spectrum, and fjk(E, E') is the differential energy cross section for redistribution of particle type and energy through elastic scattering or nuclear reactions such as fkagmentation. The numerical solution to equation (1) has been developed by Wilson et al. 1131 using the method of characteristics with the production terms separated into projectile fkagmentation and target fragmentation terms. The HZETRN code usenergy dependent nuclear interaction cross sections and assumes realistic energy spectra for light mass particles (A < 4). For heavy ions, secondaries are assumed to be produced at the velocity of the projectile nucleus. Further details on the transport methods and data base are found in 12-41. Models for Radiobiological Studies (12)185 The free space GCR energy spectrum has been calculated by Badhwar and OINeill by fitting measured differential energy spectra from 1954-1989 to the stationary Fokker-Plank equation to eetimate the diffuaion coefficient or equivalently the deceleration parameter 151. It has been shown by Badhwar and OINeill /5/ that this description fits the existing data to a root mean square error of about 10% nearly independent of the energy. Values for the deceleration parameter +(MV) are obtained using the linear correlation of the derived +(MV) and the measured Climax neutron monitor rate and the polarity of the interplanetary magnetic field. Further details of this model are given in references /5,7/. The free space GCR spectrum are modified using the orbit averaged geomagnetic transmission functions from the CREME code /6/ including earth shadowing and then inputted into HZETRN as a boundary value for transport inside spacecraft shieldii. Low LET particles from the splash albedo are calculated in /7/ using the splash proton energy spectrum of Armstrong and Colburn 1141. In Tbble 1 are comparisons of TEPC measurements and calculated dose and dose equivalent from the GCR on recent shuttle missions. For the lightly shielded payload bay and dosimeter location 2 (DLOC2) on the shuttle the agreement is generally within 15%. In Figure 1, a comparison of calculations with the TEPC measurements for the integral LET spectrum on STS-56 is shown. We note that the measurements are for the lineal energy transfer y in a 2 pm right circular cylinder, while the calculations are for the unrestricted LET. The agreement between calculations and the TEPC is quite good betweeu values of y or LET of 50-200 keV/pm. TABLE 1 Comparison of measured and calculated doses in spaceflight The agreement found in Figure 1 requirea clarification of the relationship between calculations and detector response, especially for high energy ions. Three basic assumptions are required in order for a direct comparison of lineal energy spectra to LET spectra to be meaningful. The ion trackwidth must be emall compared to the gas volume dimensions, the LET across the volume must be constant, and nuclear fragments produced in the gas must be in equilibrium with the wall source terme. lhwkwidth effects for ions passing through the gas lose energy to the wall thus reducing the energy deposit and effectively shifting to lower lineal energy values. HZE ions passing through the wall leak electrons into the gas volume forming energy pulses at relatively low lineal energies. STS-56 (57' x 290 km) Quality Fact or 3.74 4.27 2.43 4.00 0.94 2.99 3.51 1.50 3.36 0.89 Dose Equivalent ~ S v / d a ~ 422.0 400.4 34.0 434.4 0.97 414.0 358.0 12.0 370.0 1.12 Location Payload (TEPC) GCR Model Albedo Total (Model) (TEPC/Model) Dloc2 (TEPC) GCR Model Albedo Total (Model) (TEPC/Model) Remarks ICRP-60 ICRP-60 ICW-60 ICRP-60 ICRPSO ICRP-26 ICRP-26 ICRP-26 ICRP-26 ICRP-26 Dose P G Y / ~ ~ Y 112.7 94.6 14.0 108.6 1.04 138.3 102.0 8.0 110.0 1.26 STS-51 (28.5' x 290 km) ICRP-60 ICRP-60 ICRP-60 ICRPdO ICRP-60 3.31 3.48 2.43 3.18 1.04 144.2 120.0 34.0 154.0 0.94 Payload (TEPC) GCR Model Albedo Total (Model) (TEPC/Model) 43.5 34.5 14.0 48.5