John R. Letaw

Guidelines for predicting single-event upsets in neutron environments (RAM devices)

A simple, graphical technique for estimating SEU rates in most neutron environments is provided. The technique is based on recently improved burst generation rate estimates for silicon. The method considers the point energy deposition from elastic, inelastic, and spallation reactions of neutrons in silicon over a wide range of incident energies (0.1 MeV to 1000 MeV). Alpha-particle energy deposition, which may cause errors in some devices, varies nonlinearly with increasing sensitive volume and is not considered. >

Comparison of distributed reacceleration and leaky-box models of cosmic-ray abundances (Z = 3-28)

A large collection of elemental and isotopic cosmic-ray data has been analyzed using the leaky-box transport model with and without reacceleration in the interstellar medium. Abundances of isotopes and elements with charges Z = 3-28 and energies E = 10 MeV/nucleon-1 TeV/nucleon were explored. Our results demonstrate that reacceleration models make detailed and accurate predictions with the same number of parameters or fewer as standard leaky-box models. Ad hoc fitting parameters in the standard model are replaced by astrophysically significant reacceleration parameters. Distributed reacceleration models explain the peak in secondary-to-primary ratios around 1 GeV/nucleon. They diminish the discrepancy between rigidity-dependent leakage and energy-independent anisotropy. They also offer the possibility of understanding isotopic anomalies at low energy.

Galactic Cosmic Radiation Doses to Astronauts Outside the Magnetosphere

The dose and dose equivalent from galactic cosmic radiation outside the magnetosphere have been computed. Each of the principal radiation components were considered. These include primary cosmic rays, spallation fragments of the heavy ions, and secondary products (protons, neutrons, alphas, and recoil nuclei) from interactions in tissue. COnventional quality factors were used in converting from dose to dose equivalent.

On the abundances of ultraheavy cosmic rays

Recent data from the HEAO 3 and Ariel 6 satellites on elemental abundances of ultraheavy cosmic rays (33< or =Z< or =83) are analyzed using a new propagation code. General agreement with earlier analyses is observed. Evidence for a breakdown of the correlation between ionization potentials and the solar system/cosmic-ray source abundance ratio is presented. We find that the best fit to experimental data (approx.5 GeV per nucleon) occurs when propagation is calculated at lower energies (approx.1 GeV per nucleon). This is interpreted as evidence for distributed acceleration of cosmic rays. Additional effects, including ionization loss, altered path-length distributions, and r-process enhancement, are considered.

A comparison of neutron-induced SEU rates in Si and GaAs devices

The single-event-upset rates due to neutron-induced nuclear recoils have been calculated for Si and GaAs components using the HETC and MCNP codes and the ENDF data base for (n, p) and (n, alpha) reactions. For the same critical charge and sensitive volume, the upset rate in Si exceeds that of GaAs by a factor of about 1.7, mainly because more energy is transferred in neutron interactions with lighter Si nuclei. The upset rates due to neutrons are presented as functions of critical charge and atmospheric altitude. Upsets induced by cosmic-ray nuclei, secondary protons and neutrons are compared.