Delta Electron Database for the Calculation of Astronaut

Delta Electron Database for the Calculation of Astronaut Dose Rate and Cell Damage from Space Radiation Stephen Emerson and Jane H. Mac. Gibbon, University of North Florida Possible Biological Effects of Radiation The Space Radiation Problem Effects of Space Radiation on the Body The most likely event starts with space radiation impacting the shielding of the space shuttle or station. As the radiation travels through the spacecraft shielding it may undergo nuclear fragmentation producing a different distribution of particle species and energy. The radiation may then travel through the astronaut causing ionization and/or other energy deposition. When ionizing radiation interacts with the body, DNA damage can occur. Ionizing radiation cause DNA damage by directly striking the DNA molecule or it can create free radicals in the cell which then attack the DNA molecule. Space radiation, including the radiation on other planets and moons, is significantly more dangerous than radiation on Earth. Space radiation has three main components: galactic cosmic radiation; solar particles; and the Van Allen radiation belts. Space radiation particles are mainly highly energetic ions (hydrogen to iron), photons and electrons. Space radiation research focuses on predicting the amount of radiation The Van Allen belts are just one of the radiation hazards astronauts must be protected from during space travel. on the body are fairly well When understood. ionizing radiation interacts with a target atom, the amount of energy transmitted unit length to the target by the radiation is known as the However science has still yet toperfind linear energy transfer (LET). LET can be useful in determining the accurate methods and models to predict the potential biological effects due to radiation dose and subsequent In generalthe biological effects are greater for high LET radiation because amount and type of radiation body will more energy is deposited in each cell. receive when exposed to radiation. The • Heavy Ions are high LET radiation focus of this research is to help further • Photons and X-Rays are low LET radiation provide accurate modeling of radiation energy transfer. Once the DNA has been damaged, the cell will do one of three things. The DNA damage may cause the cell to die, (apoptosis); the cell may repair the DNA correctly and the cell will continue to operate normally; or the cell may incorrectly repair the DNA damage leaving the cell with a permanent error, potentially causing cancer and other permanent long term effects. Several of these effects include: • Chromosomal Damage Fluorescent markers indicate the presence of double-strand breaks in DNA caused by low levels of x-ray radiation. • Radiation Carcinogenesis Delta Electron Ionization Processes astronauts will receive during space travel and the long-term consequences of exposure to space radiation. The Problem of Bio-Radiation The long and short term effects of radiation Effects of Radiation on Cells Linear Energy Transfer (LET) • Radiation Mutagenesis DNA being damaged by radiation • Radiation Induced Cataracts The Bethe-Bloch Equation is a good approximation for the electron energy loss (stopping power) due to ionization. Delta Electron Ionization Processes Total Cross Section The total cross-section is the probability of an incoming projectile ionizing an electron from a target atom or molecule. Singly Differential Cross Section (SDCS) The singly differential cross section is the differential ionization probability with respect to the outgoing electron energy. Total cross-sections for the ionization of molecular oxygen by protons. Singly differential cross sections for the ionization of several molecular species by 1 Me. V protons. Double Differential Cross Section (DDCS) The doubly differential cross section (DDCS) is the differential probability of an incoming projectile ionizing a target electron expressed with respect to the angle and energy of the outgoing electron. The DDCS gives the most accurate 3 -D look at the ionization process. The DDCS is more complicated that the total cross-section and SDCS and cannot be modeled theoretically. Direct Coulomb Ionization The binary encounter peak occurs at the electron angle and energy given by the kinematics for a classical collision between the projectile ion and an orbital electron of the target atom. When an incoming ion interacts with an atom, the most significant energy exchange is the Coulomb interaction between the charged projectile and the outer electrons of the target atom. If the impact parameter (the closest distance of approach between the ion and the target atom) is small, the energy transferred to the target electron is large and the electron can be treated as quasi-free. If the impact parameter is large, the energy transfer is smaller and the influence of the target nucleus must be considered. DDCS for the ionization of molecular oxygen by a 30 Me. V O 5+ oxygen ion. The electron energy at the binary encounter peak scale with Double differential cross sections for the ionization of electrons at 20° from molecular hydrogen by protons of various impact energies. the electron angle as Loss Electrons Binary Encounter Peak . Loss electrons are classified as two types depending on where they originate. The two types are electron loss (of a projectile electron) to the continuum (ELC) and electron capture (of a target electron by the projectile) to the continuum (ECC). Both types are emitted with nearly zero momentum in the frame of the moving projectile. DDCS for ionization of water vapour by 0. 3 Me. V u-1 protons and helium ions. The loss electron contribution is noticeable on the 15° He+ spectrum around 160 e. V. Auger Electron Emission Spontaneous decay of excited states or inner-shell vacancies created in the iontarget collision can produce ionized electrons with discrete energy levels. Excited outer-shell target electrons produce Auger electrons isotropically with low energy. Auger electrons from the projectile have high energy in the lab frame. DDCS at 160º for ionization of argon gas by 100 ke. V protons and argon ions. Auger electrons from the target can be seen at low energies Methods, Measurements and Parameterization Techniques The Delta Electron Database Normalized Data Sets VBA Code Microsoft Visual Basic for Applications (VBA) is used to write the code to manipulate the database and datasets. Several different VBA modules were used to normalize the datasets. Below is a snapshot of just one of the modules of VBA code. Over 1200 data sets of experimentally measured DDCS have been compiled into our database. The data is essentially decades worth of ionization research results. The data covers many different projectile species (hydrogen to uranium) and target species (including hydrocarbons and water) and a wide range of projectile energies. Information on each dataset is stored in the Master Spreadsheet. A large part of the database task was normalizing the individual data sets to one common set of units so the data sets could be accurately compared to one and another. This process included unit conversion, mathematical conversion and in some cases energy conversions. Above is one set of normalized data. Goals and Further Research The main long term goal of this research project is to use this database in the modeling of astronaut exposure to ionizing radiation. Currently there is no accurate method to predict the short and long term effects of ionizing radiation astronauts will receive during future space endeavors including a return trip to the Moon and to Mars. Current and future tasks include creating a user friendly interface that allows researchers to select individual targets and ionizing projectiles over various energy distributions and by other projectile and target characteristics. The selected data sets can be used directly in Monte Carlo radiation codes. We are also developing methods to parameterize the data sets. The parameterized mathematical fits will be used in analytical and numerical modeling applications. Using data sets obtained from experiments and parameterizations directly derived from the experimental data should significantly improve the accuracy of the modeling of radiation exposure. Currently the modeling relies on inaccurate theoretical approximations to the DDCS or data from a limited range of projectile species. Multi-variable parameterization will also allow the user predict radiation exposure for projectiles, targets and energy ranges for which the cross-sections have not yet been measured. This work was supported by funding from UNF Department of Chemistry and Physics and UNF Summer Research Awards