C. H. Tsao

Neutron Generated Single-Event Upsets in the Atmosphere

Heavy cosmic ray nuclei are mostly attenuated with a shielding of 50 g/cm2 atmospheric gas. However, the shielding acts as a generator of neutrons, evaporated or knocked out of nuclei. These neutrons generate highly ionizing nuclear recoils that produce single-event upsets in microelectronic components. To attenuate the secondary neutron flux over 300 g/cm2 of atmospheric material is required. The numerous slow protons from nuclear interactions in shielding will also genetrate upsets in sensitive components, which have a low critical charge. At altitudes below 65,000 feet, most single-event upsets are due to these secondary particles. The upset rates due to neutrons and slow secondary protons from cosmic ray, solar flare particle, and trapped radiation particle interactions are presented as a function of the critical charge.

Radiation hazards on space missions outside the magnetosphere

Future space missions outside the magnetosphere will subject astronauts to a hostile and unfamiliar radiation environment. An annual dose equivalent to the blood-forming organs (BFOs) of approximately 0.5 Sv is expected, mostly from heavy ions in the galactic cosmic radiation. On long-duration missions, an anomalously-large solar energetic particle event may occur. Such an event can expose astronauts to up to approximately 25 Gy (skin dose) and up to approximately 2 Sv (BFO dose) with no shielding. The anticipated radiation exposure may necessitate spacecraft design concessions and some restriction of mission activities. In this paper we discuss our model calculations of radiation doses in several exo-magnetospheric environments. Specific radiation shielding strategies are discussed. A new calculation of aluminum equivalents of potential spacecraft shielding materials demonstrates the importance of low-atomic-mass species for protection from galactic cosmic radiation.

Semiempirical Cross Sections, and Applications to Nuclear Interactions of Cosmic Rays

We have formulated semiempirical calculations of partial cross sections (papers ST in the references) for calculating the many yields, as yet unmeasured, of high-energy reactions. Here we present parameters that are updated on the basis of recent experimental data. These calculations have been employed in the analysis of cosmic-ray data when measured values are unavailable. (About half of the cosmic rays heavier than helium have undergone spallation since their acceleration.) The calculations have yielded: (a) the elemental composition of cosmic rays at the sources, (b) the expected isotopic composition of cosmic rays at the top of the atmosphere, (c) the mean path length of material traversed by cosmic rays, and the distribution function of path lengths, and (d) predicted abundances of long-lived radioactive secondary cosmic-ray nuclides for the determination of confinement time in the galaxy. The current status of these calculations is presented, using the experimental data on cross sections and on the composition of cosmic rays reported up to September, 1975.

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.

Electron capture decay of cosmic rays

Cosmic-ray nuclei are close to fully ionized during their passage through the Galaxy. Electron capture decay is rare among these nuclides because most do not have bound electrons. Under certain conditions, specifically low energies and/or high charges, electron capture becomes an essential factor in determining cosmic-ray composition. In this paper we discuss the general nature of electron capture decay in cosmic rays and describe specific measurements which can reveal the existence of electron capture decay, and energy and density-dependent processes in the interstellar medium.

Matrix Methods of Cosmic Ray Propagation

Matrix methods of cosmic ray propagation involve reduction of the propagation equations to a set of coupled linear differential equations. This reduction results in an elegant separation of local propagation effects, such as nuclear fragmentation, from global effects such as pathlength distribution. The matrix equations are solved by simple methods. We show how the reduction to linear form is made for ionization loss and solar modulation. In addition, we show how to calculate propagation errors.

Cosmic-Ray Heavy Ions at and above 40,000 Feet

The flux and LET-spectra of heavy cosmic ray nuclei and their secondary progeny have been calculated at aircraft flight altitudes. The associated frequency of single event upsets is presented and compared with neutron-induced events.

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.

LET-distributions and doses of HZE radiation components at near-earth orbits

Among cosmic rays, the heavy nuclei ranging from carbon to iron provide the principal contribution to the dose equivalent. The LET-distributions and absorbed dose aid dose equivalent have been calculated and are presented as a function of shielding and tissue self-shielding. At solar minimum, outside the magnetosphere, the unshielded dose equivalent of nuclei with atomic number Z > or = 6 is about 47 rem/year. The contribution of the target nuclei adds 7 rem/year. With 4 g/cm2 aluminum shielding, and at a depth of 5 cm in a biological phantom of 30 cm diameter, the respective values are 11 and 10 rem/year. Corresponding dose rates for orbits with various inclinations are presented, as well as the LET distributions of various components of cosmic rays.

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.