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L H Toburen

Publications and source records attributed to L H Toburen.

7 recordsLinked to original sources
PubMed2001

Development of a Monte Carlo track structure code for low-energy protons in water.

PURPOSE: The development of a new generation of Monte Carlo track structure code is described, which simulates full slowing down of low-energy proton history tracks (lephist) in the range 1 keV-1 MeV in water. MATERIAL AND METHODS: All primary protons are followed down to 1 keV and all electrons to 1 eV. All primary interactions, including elastic scattering, ionization, excitation and charge exchange processes by protons and neutral hydrogen were taken into account. Cross-sections for proton and hydrogen impact were obtained from experimental data for water. Where data were lacking, the existing experimental data were fitted and extrapolated. The tracks of secondary electrons were generated using the electron track code kurbuc. The cross-sections and the energy transfer data were individually evaluated for the principal interactions induced by protons and hydrogen atoms in water. The analysis starts with the published cross-section data for water using a semi-empirical model including contributions from the neutral hydrogen atoms. For excitation cross-sections, the original Miller-Green analytical formula was used. For ionization by neutral hydrogen atoms, the same energy spectrum was assumed for secondary electrons as for protons. The total cross-sections were taken from the experiment of Blorizadeh and Rudd (1986b, c). For the stripping of charge by neutral hydrogen the data of Toburen et al. (1968) were used. RESULTS: Data are presented on total and differential elastic cross-sections as a function of energy and scattering angle respectively; single and double differential cross-sections for secondary electrons ejected by various energy proton impact; total cross-sections due to proton and hydrogen impact on water; stopping power cross-sections; and fraction of stopping power for water for protons as a functions of proton energy. CONCLUSIONS: Tracks were analysed to provide confirmation on the reliability of the code and information on physical quantities, such as range, W, restricted stopping power, radial dose profiles and some microdosimetric parameters. Model calculations show good agreement with the experimental and calculated data.

Algorithms↗
PubMed1998

Ionization and charge-transfer: basic data for track structure calculations.

It is widely accepted that an understanding of the detailed structure of charged particle tracks is essential for interpreting the mechanistic consequences of energy deposition by high linear energy transfer (LET) radiation. The spatial relationship of events along the path of a charged particle, including excitation, ionization, and charge-transfer, govern subsequent chemical, biochemical, and biological reactions that can lead to adverse biologic effects. The determination of spatial patterns of ionization and excitation relies on a broad range of cross-section data relating the interactions of charged particles to the molecular constituents of the absorbing medium. It is important that these data be absolute in magnitude, comprehensive in scope, and reliable if accurate assessment of track structure parameters is to be achieved. Great strides have been made in the development of this database, understanding the underlying theory, and developing analytic models, particularly for interactions involving electrons and protons with atoms and molecules. The database is less comprehensive for interactions involving heavier charged particles, especially those that carry bound electrons, and for interactions in condensed phase media. Although there has been considerable progress in understanding the physical mechanisms for interactions involving fast heavy ions and atomic targets during the past few years, we still lack sufficient understanding to confidently predict cross-sections for these ions with biologically relevant material. In addition, little is known of the interaction cross-sections for heavy charged particles as they near the end of their track, i.e., for low velocity ions where collision theory is less well developed and where the particle's net charge fluctuates owing to electron capture and loss processes. This presentation focuses on the current status of ionization and charge-transfer data. Compilations, reviews, Internet sources, theoretical models, and recent data applicable to track structure calculations are discussed.

Carbon↗
PubMed1997

A comparison of independently conducted dose assessments to determine compliance and resettlement options for the people of Rongelap Atoll.

Rongelap Island was the home of Marshallese people numbering less than 120 in 1954; 67 were on the island and severely exposed to radioactive fallout from an atomic weapons test in March of that year. Those resident on Rongelap were evacuated 50 h after the test, returned 3 y later, then voluntarily left their home island in 1985 due to their ongoing fear of radiation exposure from residual radioactive contamination. Following international negotiations in 1991, a Memorandum of Understanding (MOU) was signed in early 1992 between the Republic of the Marshall Islands Government, the Rongelap Atoll Local Government, the U.S. Department of Energy, and the U.S. Department of the Interior. In this MOU it was agreed that the Republic of the Marshall Islands, with the aid of the U.S. Department of Energy, would carry out independent dose assessments for the purpose of assisting and advising the Rongelap community on radiological issues related to a safe resettlement of Rongelap. The MOU enacted two action levels which were agreed to be used to establish whether mitigation should be considered as a condition for resettlement of Rongelap Island: (1) no individual should receive an annual dose in the future of 1 mSv or more, above that from natural background radiation, assuming that his/her diet consists of only locally produced foods, and (2) the total surface soil concentration of plutonium and other transuranic elements must be less than 629 Bq kg(-1) (averaged over the top 5 cm). Environmental radiological data and dietary information were collected over two years (1992-1993) for the purpose of predicting future potential doses to Rongelapese who might resettle. In 1994, four independent assessments were reported, including one from each of the following entities: Marshall Islands Nationwide Radiological Study; Lawrence Livermore National Laboratory; an independent advisor from the United Kingdom (MCT); and a committee of the National Research Council. All four assessments concluded that possibly more than 25% of the adult population could exceed the 1 mSv y(-1) dose level based on strict utilization of a local food diet. The purpose of this report is to summarize the methodology, assumptions, and findings from each of four assessments; to summarize the recommendations related to mitigation and resettlement options; to discuss unique programmatic aspects of the study; and to consider the implications of the findings to the future of the Rongelap people.

Adult↗
PubMed1991

Atomic and molecular physics in the gas phase.

The spatial and temporal distributions of energy deposition by high-linear-energy-transfer radiation play an important role in the subsequent chemical and biological processes leading to radiation damage. Because the spatial structures of energy deposition events are of the same dimensions as molecular structures in the mammalian cell, direct measurements of energy deposition distributions appropriate to radiation biology are infeasible. This circumstance has led to the development of models of energy transport based on a knowledge of atomic and molecular interactions that enable one to simulate energy transfer on an atomic scale. Such models require a detailed understanding of the interactions of ions and electrons with biologically relevant material. During the past 20 years, there has been a great deal of progress in our understanding of these interactions, much of it coming from studies in the gas phase. These studies provide information on the systematics of interaction cross sections, and lead to knowledge of the regions of energy deposition where molecular and phase effects are important-knowledge that guides development in appropriate theory. In this report, studies of the doubly differential cross sections, which are crucial to the development of stochastic energy deposition calculations and track structure simulation, are reviewed. We discuss areas of understanding and address directions for future work. Particular attention is given to experimental and theoretical findings that have changed the traditional view of secondary electron production for charged-particle interactions with atomic and molecular targets.

Animals↗