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L W Townsend

Publications and source records attributed to L W Townsend.

At least 37 records · Page 2Linked to original sources
PubMed2002

Absorbed fraction sensitivity to changes in size of the ICRP nose model.

The Human Respiratory Tract Model of ICRP Publication 66 is used for calculations of dose in the extrathorathic (ET1) airways. Scaling for age and body size is included in determining the mass of the target tissue (basal cells) in ET1 but is not included in deriving the absorbed fraction for particulate radiation. For dose calculations, it has been assumed that all absorbed fractions for particulate radiation published in ICRP Publication 66 are independent of age and body size. Therefore, these absorbed fractions are applied to calculate specific effective energy values not only for the Reference Worker but also for non-adults with noses of different sizes. In this note changes to the size of the cylinder model of the anterior nose in ICRP 66 are made by varying the surface area, the cylinder radius, and the thickness of tissue beyond the target region (basal cell). The energy deposition (absorbed fractions) in the target region (basal cells) is calculated using the MCNP4B (Monte Carlo) code to study the effects of these changes on the predicted absorbed fractions within the cylinder model.

Electrons↗
PubMed2001

Estimation of electron absorbed fractions in the extrathoracic airways.

Dose calculations using the respiratory tract model presented in Publication 66 of the International Commission on Radiological Protection (ICRP) frequently predict that the basal cells of the anterior portion of the nose, the extrathoracic region ET1 of the model, are the most highly irradiated tissue of the body. The dose to the basal cells is averaged over a layer of tissue 10 microm thick located at a depth of 40 microm into the airway. Reported here are the results of a series of absorbed fraction calculations undertaken to compare with values tabulated in ICRP Publication 66. The Monte Carlo code MCNP4B and the geometric model of the ET1 region specified in Publication 66 were used in the calculations. Although some calculated differences are evident between the two sets of absorbed fractions, typically less than 20%, the calculations confirm that the electron absorbed fractions tabulated in Publication 66 are not responsible for the high estimates of the ET1 dose.

Electrons↗
PubMed2001

Predicting dose-time profiles of solar energetic particle events using Bayesian forecasting methods.

Bayesian inference techniques, coupled with Markov chain Monte Carlo sampling methods, are used to predict dose-time profiles for energetic solar particle events. Inputs into the predictive methodology are dose and dose-rate measurements obtained early in the event. Surrogate dose values are grouped in hierarchical models to express relationships among similar solar particle events. Models assume nonlinear, sigmoidal growth for dose throughout an event. Markov chain Monte Carlo methods are used to sample from Bayesian posterior predictive distributions for dose and dose rate. Example predictions are provided for the November 8, 2000, and August 12, 1989, solar particle events.

Bayes Theorem↗
PubMed2001

Overview of active methods for shielding spacecraft from energetic space radiation.

During the 1960's and into the early 1970's, investigations were conducted related to the feasibility of using active radiation shielding methods, such as afforded by electromagnetic fields, as alternatives to passive, bulk material shielding to attenuate space radiations. These active concepts fall into four categories: (1) electrostatic fields; (2) plasma shields; (3) confined magnetic fields; and (4) unconfined magnetic fields. In nearly all of these investigations, consideration was given only to shielding against protons or electrons, or both. During the 1980's and 1990's there were additional studies related to proton shielding and some new studies regarding the efficacy of using active methods to shield from the high energy heavy ion (HZE particle) component of the galactic cosmic ray spectrum. In this overview, each concept category is reviewed and its applicability and limitations for the various types of space radiations are described. Recommendations for future research on this topic are made.

Cosmic Radiation↗
PubMed2000

Interplanetary crew dose rates for the August 1972 solar particle event.

Parsons, J. L. and Townsend, L. W. Interplanetary Crew Dose Rates for the August 1972 Solar Particle Event. Using the coupled neutron-proton space radiation transport computer code (BRYNTRN), estimates of dose rates of protons in the skin, ocular lens and bone marrow, behind various thicknesses of aluminum shielding, for crews on space missions outside the Earth's magnetosphere, are made for the large solar particle event (SPE) of August 1972. Overall, the August 1972 dose rates are significantly higher than those estimated for any of the events that occurred in August-December 1989. The dose rates in the August 1972 SPE are not low dose rates as specified by the major national and international advisory bodies and committees.

Astronauts↗
PubMed1999

Optical model methods of predicting nuclide production cross sections from heavy ion fragmentation.

Quantum mechanical optical potential methods for calculating inclusive isotope and element production cross sections from the fragmenting of heavy nuclei by intermediate- and high-energy protons and heavy ions are presented based upon a modified abrasion-ablation-FSI (frictional spectator interaction) collision model. The abrasion stage is treated as a quantum mechanical knockout process that leaves the residual prefragment in an excited state. Prefragment excitation energies are estimated using a combined liquid drop and FSI method. In ablation the prefragment deexcites by particle and photon emission to produce the final fragment. Contributions from electromagnetic dissociation to single nucleon removal cross sections are incorporated using a Weiszacker-Williams theory that includes electric dipole and electric quadrupole interactions. Estimates of elemental and isotopic production cross sections are in good agreement with published cross section measurements for a variety of projectile-target-beam energy combinations.

Argon↗
PubMed1999

Dose uncertainties for large solar particle events: input spectra variability and human geometry approximations.

The true uncertainties in estimates of body organ absorbed dose and dose equivalent, from exposures of interplanetary astronauts to large solar particle events (SPEs), are essentially unknown. Variations in models used to parameterize SPE proton spectra for input into space radiation transport and shielding computer codes can result in uncertainty about the reliability of dose predictions for these events. Also, different radiation transport codes and their input databases can yield significant differences in dose predictions, even for the same input spectra. Different results may also be obtained for the same input spectra and transport codes if different spacecraft and body self-shielding distributions are assumed. Heretofore there have been no systematic investigations of the variations in dose and dose equivalent resulting from these assumptions and models. In this work we present a study of the variability in predictions of organ dose and dose equivalent arising from the use of different parameters to represent the same incident SPE proton data and from the use of equivalent sphere approximations to represent human body geometry. The study uses the BRYNTRN space radiation transport code to calculate dose and dose equivalent for the skin, ocular lens and bone marrow using the October 1989 SPE as a model event. Comparisons of organ dose and dose equivalent, obtained with a realistic human geometry model and with the oft-used equivalent sphere approximation, are also made. It is demonstrated that variations of 30-40% in organ dose and dose equivalent are obtained for slight variations in spectral fitting parameters obtained when various data points are included or excluded from the fitting procedure. It is further demonstrated that extrapolating spectra from low energy (< or = 30 MeV) proton fluence measurements, rather than using fluence data extending out to 100 MeV results in dose and dose equivalent predictions that are underestimated by factors as large as 2-3. Finally, it is also demonstrated that the use of equivalent sphere approximations to represent body organ self-shielding distributions results in organ doses and dose equivalent predictions that are 2-3 times larger than values obtained with anthropomorphic shielding configurations.

Aluminum↗
PubMed1999

Solar particle event dose and dose-rate distributions: parameterization of dose-time profiles, with subsequent dose-rate analysis.

Calculations of total dose and equivalent dose as functions of time, as well as dose-rate and equivalent dose rate since event start are presented for fifteen of the larger solar particle events that occurred during the period between November 1987 and August 1991. The doses, dose-equivalents, and rates presented are for exposures to the skin, ocular lens, and bone marrow behind a thickness of aluminum shielding which provides protection comparable to that of a thin spacecraft. The calculated dose vs time profiles are parameterized using a Weibull cumulative distribution as the fitting function. Parameters are determined using least-squares techniques. Fitted curves are then differentiated to produce smoothed dose-rate curves for each of the events. These results provide a useful starting point for the development of methods to predict the cumulative doses and times to reach various dose limits from a limited number of dosimeter measurements early in the evolution of a solar particle event.

Algorithms↗
PubMed1998

Liquid drop model considerations in HZE particle fragmentation by hydrogen.

The fragmenting of high energy, heavy ions by hydrogen targets is an important physical process in several areas of space radiation protection research. Quantum mechanical, optical model methods for calculating cross sections for particle fragmentation by hydrogen have been developed from a modified abrasion-ablation collision formalism. The abrasion stage is treated as a knockout process which leaves the residual prefragment in an excited state. In the ablation stage the prefragment excitation to produce the final fragment. The prefragment excitation energies are estimated from a combination of liquid drop model considerations and frictional-spectator interaction processes. Estimates of elemental and isotopic production cross sections are in good agreement with published cross section measurements.

Cosmic Radiation↗
PubMed1998

New equivalent sphere approximation for BFO dose estimation: solar particle events.

The use of a 5 cm tissue equivalent sphere model to obtain dose estimates for the blood-forming organs from energetic space radiations has been widespread for some time. Recent studies have noted that calculated doses obtained using the 5 cm equivalent sphere model were very conservatively overestimated when compared to those obtained with a detailed body geometry. Such conservatism may introduce significant shield weight penalties if used in spacecraft design studies. The use of detailed human geometry models will yield more accurate estimates of blood-forming organ doses and dose equivalents, but with a concomitant reduction in computational ease. In this work we propose a preliminary, yet new blood-forming organ equivalent sphere approximation for use in estimating SPE exposure and in shield design studies that is more realistic than the existing 5-cm approximation.

Aluminum↗
PubMed1998

Microscopic fragmentation model for galactic cosmic ray studies.

We describe theoretical considerations for developing models of heavy ion fragmentation appropriate for galactic cosmic ray studies. Previous models have been based on parametric fits to limited experimental data or models that ignored some aspects of the reaction dynamics, including nuclear cluster effects. The abrasion-ablation description of the fragmentation process is re-formulated to describe the excitation spectrum of pre-fragment nuclei. The resulting spectrum is shown to be related to the many-body response of the nuclear ground-state and excited states, and an approach to simplify this function is discussed. An analytic solution to the nuclear de-excitation process is described which includes a realistic level spectrum of the GCR nuclei (A < 60). Comparisons are made to experiments for fragmentation of 24Mg, 32S, and 56Fe beams on several targets and results are discussed.

Cosmic Radiation↗
PubMed1998

Optical model methods of predicting nuclide production from spallation reactions.

Quantum mechanical optical model methods for calculating isotope production cross sections from the spallation of heavy nuclei by high-energy protons are developed from a modified abrasion-ablation collision formalism. The abrasion step is treated quantum-mechanically as a knockout process which leaves the residual prefragment nucleus in an excited state. In ablation the prefragment deexcites to produce the final fragment. The excitation energies of the prefragments are estimated from a combination of liquid drop and frictional-spectator interaction considerations. Estimates of elemental and isotopic production cross sections are in good agreement with recently published cross section measurements.

Cosmic Radiation↗
PubMed1997

Abrasion-ablation model for neutron production in heavy ion collisions.

In intermediate energy nucleus-nucleus collisions, neutron production at forward angles is observed to occur with a Gaussian shape that is centered near the beam energy and extends to energies well above that of the beam. This paper presents an abrasion-ablation model for making quantitative predictions of the neutron spectrum. To describe neutrons produced from the abrasion step of the reaction where the projectile and target overlap, we use the Glauber model and include effects of final-state interactions. We then use the prefragment mass distribution from abrasion with a statistical evaporation model to estimate the neutron spectrum resulting from ablation. Measurements of neutron production from Ne and Nb beams are compared with calculations, and good agreement is found.

Cosmic Radiation↗
PubMed1996

Overview of nuclear fragmentation models and needs.

It has been known for some time that adequate assessment of spacecraft shield requirements and concomitant estimates of astronauts radiation exposures from galactic cosmic radiation requires accurate, quantitative methods for characterizing these radiation fields as they pass through thick absorbers. The main nuclear interaction processes involved are (1) nuclear elastic and inelastic collisions, and (2) nuclear breakup (fragmentation) and electromagnetic dissociation (EMD). Nuclear fragmentation and EMD are important because they alter the elemental and isotopic composition of the transported radiation fields. At present, there is no suitably accurate theory for predicting nuclear fragmentation cross sections for all collision pairs and energies of interest in space radiation protection. Typical cross-section differences between theory and experiment range from about 25 percent to a factor of two. The resulting errors in transported flux, for high linear energy transfer (LET) particles, are comparble to these cross-section errors. In this overview, theoretical models of heavy ion fragmentation currently used to generate input data bases for cosmic-ray transport and shielding codes are reviewed. Their shortcomings are discussed. Further actions needed to improve their accuracy and generality are presented.

Cosmic Radiation↗
PubMed1996

Light ion components of the galactic cosmic rays: nuclear interactions and transport theory.

Light nuclei are present in the primary galactic cosmic rays (GCR) and are produced in thick targets due to projectile or target fragmentation from both nucleon and heavy ion induced reactions. In the primary GCR, 4He is the most abundant nucleus after 1H. However, there are also a substantial fluxes of 2H and 3He. In this paper we describe theoretical models based on quantum multiple scattering theory for the description of light ion nuclear interactions. The energy dependence of the light ion fragmentation cross section is considered with comparisons of inclusive yields and secondary momentum distributions to experiments described. We also analyze the importance of a fast component of lights ions from proton and neutron induced target fragmentation. These theoretical models have been incorporated into the cosmic ray transport code HZETRN and will be used to analyze the role of shielding materials in modulating the production and the energy spectrum of light ions.

Cosmic Radiation↗
PubMed1996

Theoretical model of HZE particle fragmentation by hydrogen targets.

The fragmenting of high energy, heavy ions (HZE particles) by hydrogen targets is an important, physical process in several areas of space radiation research. In this work quantum mechanical optical model methods for estimating cross sections for HZE particle fragmentation by hydrogen targets are presented. The cross sections are calculated using a modified abrasion-ablation collision formalism adapted from a nucleus-nucleus collision model. Elemental and isotopic production cross sections are estimated and compared with report measurements for the breakup of neon, sulphur, and iron, nuclei at incident energies between 400 and 910 MeV/nucleon. Good agreement between theory and experiment is obtained.

Cosmic Radiation↗