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Biomedical subjects

L W Townsend

Publications and source records attributed to L W Townsend.

At least 55 records · Page 3Linked to original sources
PubMed1996

The fragmentation of 510 MeV/nucleon iron-56 in polyethylene. II. Comparisons between data and a model.

The results of a Monte Carlo model for calculating fragment fluences and LET spectra are compared to data taken with 600 MeV/nucleon iron ions incident on an accelerator beamline configured for irradiation of biological samples, with no target and with 2, 5 and 8 cm of polyethylene. The model uses a multi-generation nuclear fragmentation code, coupled with a formulation of ionization energy loss based on the Bethe-Bloch equation. In the region where the data are reliable and the experimental acceptance is well understood, many of the features of the experimental spectra are well replicated by the model. To obtain good agreement with the experimental data, the model must allow for at least two generations of fragment production in the target.

Algorithms↗
PubMed1995

Theoretical nuclear database for high-energy, heavy-ion (HZE) transport.

Theoretical methods for estimating high-energy, heavy-ion (HZE) particle absorption and fragmentation cross-sections are described and compared with available experimental data. Differences between theory and experiment range from several percent for absorption cross-sections up to about 25%-50% for fragmentation cross-sections.

Cosmic Radiation↗
PubMed1994

NUCFRG2: a semiempirical nuclear fragmentation model.

The semiempirical abrasion/ablation model has been successful in generating a large nuclear data base for use in the study of high charge and energy (HZE) ion beams, radiation physics and galactic cosmic ray shielding. The cross sections generated agree with the measured HZE fragmentation data to the degree that different experimental groups agree among themselves. Several improvements in the model have been made including a Coulomb trajectory correction, an improved treatment of nuclear attenuation factors, an improved second order correction to the spectator fragment excitation spectrum, a pre-equilibrium emission process, and competitive equilibrium emission of additional hydrogen and helium isotope fragments.

Carbon↗
PubMed1994

Analysis of radiation risk from alpha particle component of solar particle events.

The solar particle events (SPE) will contain a primary alpha particle component, representing a possible increase in the potential risk to astronauts during an SPE over the often studied proton component. We discuss the physical interactions of alpha particles important in describing the transport of these particles through spacecraft and body shielding. Models of light ion reactions are presented and their effects on energy and linear energy transfer (LET) spectra in shielding discussed. We present predictions of particle spectra, dose, and dose equivalent in organs of interest for SPE spectra typical of those occurring in recent solar cycles. The large events of solar cycle 19 are found to have substantial increase in biological risk from alpha particles, including a large increase in secondary neutron production from alpha particle breakup.

Alpha Particles↗
PubMed1994

Estimates of HZE particle contributions to SPE radiation exposures on interplanetary missions.

Estimates of radiation doses resulting from possible HZE (high energy heavy ion) components of solar particle events (SPEs) are presented for crews of manned interplanetary missions. The calculations assume a model spectrum obtained by folding measured solar flare HZE particle abundances with the measured energy spectra of SPE alpha particles. These hypothetical spectra are then transported through aluminum spacecraft shielding. The results, presented as estimates of absorbed dose and dose equivalent, indicate that HZE components by themselves are not a major concern for crew protection but should be included in any overall risk assessment. The predictions are found to be sensitive to the assumed spectral hardness parameters.

Alpha Particles↗
PubMed1994

Galactic cosmic ray transport methods: past, present, and future.

The development of the theory of high charge and energy (HZE) ion transport is reviewed. The basic solution behavior and approximation techniques will be described. An overview of the HZE transport codes currently available at the Langley Research Center will be given. The near term goal of the Langley program is to produce a complete set of one-dimensional transport codes. The ultimate goal is to produce a set of complete three-dimensional codes which have been validated in the laboratory and can be applied in the engineering design environment. Recent progress toward completing these goals is discussed.

Computer Simulation↗
PubMed1994

Solar modulation and nuclear fragmentation effects in galactic cosmic ray transport through shielding.

Crews of manned interplanetary missions may accumulate significant radiation exposures from the galactic cosmic ray (GCR) environment in space. Estimates of how these dose levels are affected by the assumed temporal and spatial variations in the composition of the GCR environment, and by the effects of the spacecraft and body self-shielding on the transported radiation fields are presented. In this work, the physical processes through which shielding alters the transported radiation fields are described. We then present estimates of the effects on model calculations of (1) nuclear fragmentation model uncertainties, (2) solar modulation, (3) variations between solar cycles, and (4) proposed changes to the quality factors which relate dose equivalent to absorbed dose.

Aluminum↗
PubMed1994

Galactic cosmic ray radiation levels in spacecraft on interplanetary missions.

Using the Langley Research Center galactic cosmic ray (GCR) transport computer code (HZETRN) and the computerized anatomical man (CAM) model, crew radiation levels inside manned spacecraft on interplanetary missions are estimated. These radiation-level estimates include particle fluxes, LET (linear energy transfer) spectra, absorbed dose, and dose equivalent within various organs of interest in GCR protection studies. Changes in these radiation levels resulting from the use of various different types of shield materials are presented.

Bone Marrow↗
PubMed1994

Optical model analyses of heavy ion fragmentation in hydrogen targets.

Quantum-mechanical optical-model methods for calculating cross sections for the fragmentation of high-energy heavy ions by hydrogen targets are presented. The cross sections are calculated with a knockout-ablation collision formalism which has no arbitrary fitting parameters. Predictions of elemental production cross sections from the fragmentation of 1.2A GeV 139La nuclei and of isotope production cross sections from the fragmentation of 400A MeV 32S nuclei are in good agreement with recently reported experimental measurements.

Cosmic Radiation↗
PubMed1994

Role of intrinsic width in fragment momentum distributions in heavy ion collisions.

It is demonstrated that the intrinsic widths incorporating correlations in conjunction with dynamical contributions give better agreement with experiments for collisions in the energy range of 200A MeV to 2A GeV than using only intrinsic widths without correlations. The sensitivity of the intrinsic width decreases with increasing projectile mass. A simple recipe for calculating intrinsic width with correlations is presented.

Argon↗
PubMed1994

Bose condensation of nuclei in heavy ion collisions.

Using a fully self-consistent quantum statistical model, we demonstrate the possibility of Bose condensation of nuclei in heavy ion collisions. The most favorable conditions of high densities and low temperatures are usually associated with astrophysical processes and may be difficult to achieve in heavy ion collisions. Nonetheless, some suggestions for the possible experimental verification of the existence of this phenomenon are made.

Elementary Particles↗
PubMed1993

Cross section parameterizations for cosmic-ray nuclei. I. Single nucleon removal.

Parameterizations of single nucleon removal from the electromagnetic and strong interactions of cosmic rays with nuclei are presented. These parameterizations are based upon the theoretical models developed by Baur, Bertulani, Benesh, Cook, Vary, Norbury, and Townsend. They should be very suitable for use in cosmic-ray propagation through interstellar space, Earth's atmosphere, lunar samples, meteorites, spacecraft walls, and lunar and martian habitats.

Cosmic Radiation↗
PubMed1993

Optical model analyses of 1.65 A GeV argon fragmentation: cross sections and momentum distributions.

An optical potential fragmentation model capable of predicting fragmentation cross sections and fragment momentum distributions is used to analyze recent measurements of 1.65 A GeV argon projectiles fragmenting in carbon and potassium-chloride targets obtained with the Heavy Ion Spectrometer System (HISS) at the Lawrence Berkeley Laboratory Bevalac. The theoretical model uses an abrasion-ablation-FSI (frictional spectator interaction) collision formalism to estimate elemental and isotopic production cross sections for comparison with the measured values. The collision momentum transfer model is incorporated into a Goldhaber formalism to analyze measured transverse and longitudinal distributions of the projectile fragments. Good agreement between theory and experiment is obtained for all observables.

Argon↗
PubMed1993

Geometric model for nuclear absorption from microscopic theory.

A parameter-free geometric model for nuclear absorption is derived from microscopic theory. The expression for the absorption cross section in the eikonal approximation taken in integral form is separated into a geometric contribution, described by an energy-dependent effective radius, and two surface terms which are shown to cancel in an asymptotic series expansion. For collisions of light nuclei, an expression for the effective radius is derived using harmonic-oscillator nuclear density functions. A direct extension to heavy nuclei with Woods-Saxon densities is made by identifying the equivalent half density radius for the harmonic-oscillator functions. Coulomb corrections are incorporated and a simplified geometric form of the Bradt-Peters type obtained. Results spanning the energy range of 1 MeV/nucleon to 1 GeV/nucleon are presented. Good agreement with experimental results are obtained.

Elementary Particles↗
PubMed1993

Universal characteristics of transverse momentum transfer in intermediate energy heavy ion collisions.

A microscopic optical model formalism for estimating momentum transfer in intermediate energy heavy ion collisions predicts universal behavior of the transverse component. In particular, for symmetric systems (Ap = AT) heavier than niobium, it appears that values of P perpendicular/A are independent of the mass and charge of the colliding nuclei and vary only with impact parameter and incident beam energy. This suggests that momentum transfer per nucleon saturates to some limiting value with increasing mass.

Calcium↗
PubMed1993

Widths of transverse momentum distributions in intermediate-energy heavy-ion collisions.

The need to include dynamical collision momentum transfer contributions, arising from interacting nuclear and Coulomb fields, to estimates of fragment momentum distributions is discussed. Methods based upon an optical potential model are presented. Comparisons with recent experimental data of the Siegen group for variances of transverse momentum distributions for gold nuclei at 980 A MeV fragmenting on silver foil and plastic nuclear track detector targets are made. The agreement between theory and experiment is good.

Gold↗