Biomedical subjects
Publications and source records attributed to L W Townsend.
The disappearance of collective flow effects in heavy ion collisions is investigated using a microscopic optical model formalism for estimating collision momentum transfers. Phenomenological expressions for the balance energy are obtained which agree very well with measurements for various experimental collision pairs and with results obtained from Boltzmann-Uehling-Uhlenbeck simulations.
This is the final report of a detailed study of the interaction of 670A MeV neon ions with water, used as a presumed tissue-equivalent target. A first comparison of the data with theoretical fluence spectra predicted by the one-generation heavy-ion transport code HZESEC was reported previously. In the present article, subsequent nuclear interactions of the fragment are taken into account, using the LBLBEAM multigeneration heavy-ion transport code, which incorporated new features and modifications intended to address some of the approximations made in the previous calculation. The LBLBEAM code uses the method of characteristics and an iterative procedure to solve a one-dimensional Boltzmann transport equation for the first through third successive generations of nuclear reaction products; it includes a recent version of the semiempirical model used to derive nuclear interaction cross sections. The stopping power used for the theory was calculated in the same way that experimental time-of-flight and energy-loss data are converted to obtain a comparison independent of stopping power; accordingly, good agreement was found between calculated and measured neon fluence spectra in the Bragg peak region. Multiple scattering effects were considered separately for each isotope in the present work. Acceptance factors were calculated as previously, assuming that all projectile fragments originate from the first nuclear interaction. The results show that lower-mass isotopes can account for the high-LET portions of the spectrum in measured fluence spectra. Third-generation products become increasingly important as a source of lighter fragments for depths comparable with the primary particle mean free path, accounting for between one-third and one-half of carbon and lighter particles near the Bragg peak; higher-order interactions were negligible for the detector geometry and material thicknesses examined. Agreement between measured and calculated fluence spectra is 30% (20% for integral fluences). Inclusion of hydrogen, helium, and lithium fragments improves agreement between calculated and measured RBE values for spermatogonial cell survival, but tertiary particle acceptance and track structure effects need to be understood in greater detail to predict RBE accurately.
Calculations of the longitudinal and transverse momentum distributions for 3H production at large momentum in alpha-12C collisions near 2 A GeV are compared to experiment. Triton exchange and final-state interactions are shown to represent large corrections to the impulse approximation for proton knockout. A method for calculating interference effects for inelastic fragmentation is discussed. Good agreement with experiment is found using a phenomenological overlap function for 3H-p successful in describing 3H production in pion-induced reactions. Comparisons to momentum distributions obtained through (p, 2p) and (e, ep) reactions on 4He are made.
Whenever energetic solar protons produced by solar particle events traverse bulk matter, they undergo various nuclear and atomic collision processes which significantly alter the physical characteristics and biologically important properties of their transported radiation fields. These physical interactions and their effect on the resulting radiation field within matter are described within the context of a recently developed deterministic, coupled neutron-proton space radiation transport computer code (BRYNTRN). Using this computer code, estimates of human exposure in interplanetary space, behind nominal (2 g/cm2) and storm shelter (20 g/cm2) thicknesses of aluminum shielding, are made for the large solar proton event of August 1972. Included in these calculations are estimates of cumulative exposures to the skin, ocular lens, and bone marrow as a function of time during the event. Risk assessment in terms of absorbed dose and dose equivalent is discussed for these organs. Also presented are estimates of organ exposures for hypothetical, worst-case flare scenarios. The rate of dose equivalent accumulation places this situation in an interesting region of dose rate between the very low values of usual concern in terrestrial radiation environments and the high dose rate values prevalent in radiation therapy.
The risk of radiation-induced cancer to space travelers outside the earth's magnetosphere will be of concern on missions to the Moon and beyond to Mars. High energy galactic cosmic rays with high charge (HZE particles) will penetrate the spacecraft and the bodies of the astronauts, sometimes fragmenting into nuclear secondary species of lower charge but always ionizing densely, thus causing cellular damage which may lead to malignant transformation. To quantitate this risk, the concept of dose equivalent (in which a quality factor Q as a function of LET is assumed) may not be adequate, since different particles of the same LET may have different efficiencies for tumor induction. Also, RBE values on which quality factors are based depend on response to low-LET radiation at low doses, a very difficult region for which to obtain reliable experimental data. Thus, we introduce a new concept, a fluence-related risk coefficient (F), which is the risk of a cancer per unit particle fluence and which we call the risk cross section. The total risk is the sum of the risk from each particle type: sigma i integral Fi(Li) phi i(Li) dLi, where Li is the LET and phi i(Li) is the fluence-LET spectrum of the ith particle type. As an example, tumor prevalence data in mice are used to estimate the probability of mouse Harderian gland tumor induction per year on an extra-magnetospheric mission inside an idealized shielding configuration of a spherical aluminum shell 1 g/cm2 thick. The combined shielding code BRYNTRN/GCR is used to generate the LET spectra at the center of the sphere. Results indicate a yearly prevalence at solar minimum conditions of 0.06, with 60% of this arising from charge components with Z between 10 and 28, and two-thirds of the contribution arising from LET components between 10 and 200 keV/micrometers.
An overview of galactic cosmic ray (GCR) interaction and transport methods, as implemented in the Langley Research Center GCR transport code, is presented. Representative results for solar minimum, exo-magnetospheric GCR dose equivalents in water are presented on a component by component basis for various thicknesses of aluminum shielding. The impact of proposed changes to the currently used quality factors on exposure estimates and shielding requirements are quantified. Using the cellular track model of Katz, estimates of relative biological effectiveness (RBE) for the mixed GCR radiation fields are also made.
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.
We consider the inclusive inelastic scattering of heavy ions using the Glauber model and the independent particle approximation. Inclusive inelastic distributions for projectile excitation of the target and total inelastic scattering, where all projectile and target excited states are summed, are discussed using closure. The total inelastic distribution when integrated is shown to be equivalent to the absorption cross section found from applying the optical theorem to the elastic scattering amplitude in the coherent approximation. Calculations are presented for several heavy-ion pairs using realistic nuclear densities in a large mass number approximation.
A multiple-scattering series for describing the quasielastic peak in nucleus-nucleus collisions is derived using the high-energy optical model. The effects of multiple knockout of target nucleons and internal excitation of the projectile are studied and found to be important for large energy loss and momentum transfers in inclusive alpha-4He scattering at 7 GeV/c. An approximate evaluation of higher-order inelastic collision terms is considered for forward-peaked wave functions and is demonstrated to be accurate.
The solar particle events of August through December 1989, among the largest ever recorded, are analyzed to assess the potential hazards to humans on interplanetary missions from events of these types. Using the coupled neutron-proton space radiation transport computer code, BRYNTRN, risk estimates for the effects of exposures to the skin, ocular lens, and bone marrow are made for nominal thicknesses of the spacecraft aluminum shielding. Risk assessment in terms of absorbed dose is made for each event. Also presented are estimates of organ absorbed dose and dose equivalent for pairs of events which occurred within 30-day periods, and for the cumulative August through December 1989 period.
Using the Langley Research Center galactic cosmic-ray transport computer code and the Computerized Anatomical Man model, initial estimates of interplanetary exposure of astronauts to galactic cosmic rays, during periods of solar minimum activity, are made for a realistic human geometry shielded by various thickness of spacecraft aluminum shielding. Conventional dose assessment in terms of total absorbed dose and dose equivalent is made for the skin, ocular lens, and bone marrow. Included in the analyses are separate evaluations of the contributions from the incident primary ions, from subsequent-generation fragmentation products, and from target fragments. In all cases considered, the equivalent sphere approximation yielded conservative overestimates for the actual organ exposures.
An optical model description of momentum transfer in relativistic heavy ion collisions, based upon composite particle multiple-scattering theory, is presented. The imaginary component of the complex momentum transfer, which comes from the absorptive part of the optical potential, is shown to be the main contributor to the momentum loss of the projectile. Within the context of the Goldhaber formalism, predictions of fragment momentum distribution observables are made and compared with experimental data. Use of the model as a tool for estimating collision impact parameters is also discussed.
An optical potential abrasion-ablation collision model is used to calculate hadronic dissociation cross sections for one, two, and three nucleon removal for the first time for a 14.6A GeV 28Si beam fragmenting in aluminum, tin, and lead targets. These estimates are compared with recent semi-inclusive measurements. Significant differences between some calculated and measured semi-inclusive cross sections exist which cannot be resolved without measurements of the exclusive channel hadronic cross sections. Calculations for each exclusive reaction channel contributing to the semi-inclusive cross sections are presented and discussed.
Relativistic heavy ion charge-exchange reactions yield fragments (delta Z = +1) whose longitudinal momentum distributions are downshifted by larger values than those associated with the remaining fragments (delta Z = -1, -2, ...). Kinematics alone cannot account for the observed downshifts; therefore, an additional contribution from collision dynamics must be included. In this work, an optical model description of collision momentum transfer is used to estimate the additional dynamical momentum downshift. Good agreement between theoretical estimates and experimental data is obtained.
High-energy protons traversing tissue produce local sources of high-linear-energy-transfer (LET) ions through nuclear fragmentation. We examine the contribution of these target fragments to the biological effectiveness of high-energy protons using the cellular track model. The effects of secondary ions are treated in terms of the production collision density using energy-dependent parameters from a high-energy fragmentation model. Calculations for mammalian cell cultures show that at high dose, at which intertrack effects become important, protons deliver damage similar to that produced by gamma rays, and with fragmentation the relative biological effectiveness (RBE) of protons increases moderately from unity. At low dose, where sublethal damage is unimportant, the contribution from target fragments dominates, causing the proton effectiveness to be very different from that of gamma rays with a strongly fluence-dependent RBE. At high energies, the nuclear fragmentation cross sections become independent of energy. This leads to a plateau in the proton single-particle-action cross section, below 1 keV/micron, since the target fragments dominate.
Using the coupled neutron-proton space radiation transport computer code (BRYNTRN), estimates of human exposure in interplanetary space, behind various thicknesses of aluminum shielding, are made for the large solar proton events of August 1972 and October 1989. A comparison of risk assessment in terms of total absorbed dose for each event is made for the skin, ocular lens, and bone marrow. Overall, the doses associated with the August 1972 event were higher than those with the October 1989 event and appear to be more limiting when compared with current guidelines for dose limits for missions in low Earth orbit and more hazardous with regard to potential acute effects on these organs. Both events could be life-threatening if adequate shielding is not provided.
Explore the source record for details and available documents.
Significant discrepancies between theory and experiment have previously been noted for nucleon emission via electromagnetic processes in relativistic nucleus-nucleus collisions. The present work investigates the hypothesis that these discrepancies have arisen due to uncertainties about how to deduce the experimental electromagnetic cross section from the total measured cross section. An optical-model calculation of single neutron removal is added to electromagnetic cross sections and compared to the total experimental cross sections. Good agreement is found thereby resolving some of the earlier noted discrepancies. A detailed comparison to the recent work of Benesh, Cook, and Vary is made for both the impact parameter and the nuclear cross section. Good agreement is obtained giving an independent confirmation of the parametrized formulas developed by those authors.