John Tweed

A simple model for straggling evaluation

Some straggling models had largely been abandoned in favor of Monte Carlo simulations of straggling which are accurate but time consuming, limiting their application in practice. The difficulty of simple analytic models is the failure to give accurate values past 85% of the particle range. A simple model is derived herein based on a second order approximation upon which rapid analysis tools are developed for improved understanding of material charged particle transmission properties.

Reduced discretization error in HZETRN

The deterministic particle transport code HZETRN is an efficient analysis tool for studying the effects of space radiation on humans, electronics, and shielding materials. In a previous work, numerical methods in the code were reviewed, and new methods were developed that further improved efficiency and reduced overall discretization error. It was also shown that the remaining discretization error could be attributed to low energy light ions (A<4) with residual ranges smaller than the physical step-size taken by the code. Accurately resolving the spectrum of low energy light particles is important in assessing risk associated with astronaut radiation exposure. In this work, modifications to the light particle transport formalism are presented that accurately resolve the spectrum of low energy light ion target fragments. The modified formalism is shown to significantly reduce overall discretization error and allows a physical approximation to be removed. For typical step-sizes and energy grids used in HZETRN, discretization errors for the revised light particle transport algorithms are shown to be less than 4% for aluminum and water shielding thicknesses as large as 100g/cm^2 exposed to both solar particle event and galactic cosmic ray environments.

Design and Testing of Improved Spacesuit Shielding Components

In prior studies of the current Shuttle Spacesuit (SSA), where basic fabric lay-ups were tested for shielding capabilities, it was found that the fabric portions of the suit give far less protection than previously estimated due to porosity and non-uniformity of fabric and LCVG components. In addition, overall material transmission properties were less than optimum. A number of alternate approaches are being tested to provide more uniform coverage and to use more efficient materials. We will discuss in this paper, recent testing of new material lay-ups/configurations for possible use in future spacesuit designs.

Recent Progress in the Development of a Multi-Layer Green's Function Code for Ion Beam Transport

To meet the challenge of future deep space programs, an accurate and efficient engineering code for analyzing the shielding requirements against high‐energy galactic heavy radiation is needed. To address this need, a new Greens function code capable of simulating high charge and energy ions with either laboratory or space boundary conditions is currently under development. The computational model consists of combinations of physical perturbation expansions based on the scales of atomic interaction, multiple scattering, and nuclear reactive processes with use of the Neumann‐asymptotic expansions with non‐perturbative corrections. The code contains energy loss due to straggling, nuclear attenuation, nuclear fragmentation with energy dispersion and downshifts. Previous reports show that the new code accurately models the transport of ion beams through a single slab of material. Current research efforts are focused on enabling the code to handle multiple layers of material and the present paper reports on pr...

A Multi-Layer Green's Function Code for Ion Beam Transport

*† ‡ § A new version of the HZETRN code capable of simulating HZE ions with either laboratory or space boundary conditions is currently under development. The new code GRNTRN is based on recent improvements in the Green’s function approach to ion beam transport. Recent publications show that the new code accurately models the transport of ion beams through a single slab of material. Current research efforts are focused on enabling the code to handle multiple layers of material and the present paper reports on progress made towards that end.

Recent Advances in the Green's Function Technique for Ion Beam Transport

*† ‡ § ** †† ‡‡ §§ The Green’s function approach to ion transport greatly facilitates the modeling of laboratory radiation environments and allows for the direct testing of transport approximations of material transmission properties. This approach has been used successfully by radiation investigators at the NASA, Langley Research Center to construct simple solutions that were supported by experimental evidence with HZE ion beams. Recent additions to the Green’s function solution include energy straggling, fragmentation energy widths and downshifts. Accurate analytical approximations for the first three terms in the perturbation series are presented here and it is shown that the remaining terms may be incorporated by making use of a non-perturbative technique. The solution is validated by comparison with recent experimental results.

VALIDATION OF A TRANSPORT MODEL FOR A SHIELD AND SOLID STATE DETECTOR ARRANGEMENT

A critical enabling technology for the Human Exploration and Development of Space is the provision of adequate radiation protection to the astronauts and their equipment. In consequence, there is considerable interest in the development of new shielding materials for protection against the hazards of galactic cosmic rays. Since it is impractical to verify the shielding properties of every candidate material and configuration in space, it is desirable to develop a protocol for the rapid assessment of shielding properties. Solid-state detectors are often used to measure the charge and energy of particles in ion beam experiments. The direct measurement is energy deposited in the detector. As a means of separating the charge components in typical shield transmission studies with observation, a stack of many such detectors is used followed by analysis to determine charge and energy. The development of a transport model for the shield and detector arrangement and evaluation of prediction of the energy loss spectrum for direct comparison with the experimentally derived data allows a rapid assessment of the shield transmission characteristics. Such a model has recently been developed and is validated herein by comparison with results from particle beam experiments.

Towards a 3D Space Radiation Transport Code

High-speed computational procedures for space radiation shielding have relied on asymptotic expansions in terms of the off-axis scatter and replacement of the general geometry problem by a collection of flat plates. This type of solution was derived for application to human rated systems in which the radius of the shielded volume is large compared to the off-axis diffusion limiting leakage at lateral boundaries. Over the decades these computational codes are relatively complete and lateral diffusion effects are now being added. The analysis for developing a practical full 3D space shielding code is presented.