R. Silberberg

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.

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.

Calculation of the vavilov distribution: A correction

Abstract The probability distribution for energy deposition in absorbers, allowing for electron escape, has been recalculated. This calculated distribution is compared with an experimental one generated by muons in a pilot Y plastic scintillator. A computer program to calculate the distribution is given.

Model analysis of space shuttle dosimetry data

An extensive model analysis of plastic track detector measurements of high-LET particles on the Space Shuttle has been performed. Three shuttle flights: STS-51F (low-altitude, high-inclination), STS-51J (high-altitude, low-inclination), and STS-61C (low-altitude, low-inclination) are considered. The model includes contributions from trapped protons and galactic cosmic radiation, as well as target secondary particles. Target secondaries, expected to be of importance in thickly shielded space environments, are found to be a significant component of the measured LET (linear energy transfer) spectra.

Let-Distributions and Radiation Doses Due to Cosmic Rays

A radiation transport calculation of all nuclides in cosmic rays has been carried out for elements H to Ni. (The unattenuated flux at solan min, outside the magnetosphere is 108/cm2 year.) The computer code treats nuclear spallation reactions, ionization losses and geomagnetic effects. LET-distributions and radiation doses due to cosmic rays can be calculated at any point in spacecraft and at various depths in any material. Results of sample calculations, including single event upsets, are presented. Hydrogenous material s are shown to be more effective than aluminum for shielding against the high-LET components that generate soft upsets.

High‐energy radiation environment during manned space flights

Radiation doses from cosmic rays on long‐duration space missions, at solar minimum, with 4 g/cm2 Al shielding and 5 g/cm2 tissue self‐shielding exceed by a factor of about 60 the allowable dose of 0.5 rem/year to the general public and by a factor of about 6 the dose of 5 rem/year allowable to radiation workers. The annual dose of 30 rem nearly equals the allowable dose to a few volunteer astronauts which is 40 rem/year. Rare solar flare particle events would deliver doses of about 200 to 300 rem, in a period of about one day (like the 1956 and 1972 flaresl) for the above conditions of shielding. These doses are lethal to about 20 percent and 40 percent of the recipients respectively.

Fast heavy ions in the heliosphere

Abstract The study of heavy ions in space can provide a greater understanding of contemporary nucleosynthesis in our galaxy, as well as the acceleration and propagation of energetic particles in the interplanetary medium and the magnetosphere. We describe here a satellite experiment that uses plastic track detectors to study the age of cosmic-ray source material and the distribution of pathlengths over which the heaviest cosmic rays travel to earth. The experiment will also search for the singly charged particles, thought to make up the anomalous component, and for low-energy heavy ions deep in the magnetosphere.

Origin, Propagation and Distributed Acceleration of Cosmic Rays

The source composition of cosmic rays is presented. An origin in supernova shock-wave interactions with boundary regions of the stellar wind, or acceleration of stellar flare particles accounts for most cosmic rays, though some contribution of Wolf-Rayet star material to 12C, 160 and 22Ne improves the fit. After the principal acceleration of stellar particles to cosmic-ray energies by strong shocks of young supernova remnants, a moderate reacceleration takes place by shocks of old supernova remnants that occupy a large part of the total volume of the galactic disk. This distributed acceleration is interspersed with periods of cosmic ray traversal of dense interstellar clouds, when most of the cosmic ray spallation reactions occur, accompanied also by gamma rays from π0 - decay. The effects of distributed acceleration on the ratios of secondary-to-primary cosmic rays are presented. The cosmic rays we observe near the earth probably diffuse in the Galactic disk and the inner part of the Galactic halo, since their confinement time of 107 years deduced from the abundance of 10Be exceeds that of the disk confinement model. The gamma-ray data acquired on COS-B imply a large halo as a long-duration confinement region from which cosmic rays ultimately escape into the inter-galactic space.

Nuclear Cross Sections, Cosmic Ray Propagation and Source Composition

Most cosmic rays with atomic number Z ≥ 6 suffer nuclear collisions in the interstellar gas, with transformation of nuclear composition. The isotopic and elemental source composition has to be inferred from the observed composition near the earth. The uncertainty in the inferred source composition is largely due to uncertainties in cross sections—especially for nuclides that have large secondary components. These uncertainties will be explored by comparing the calculated and observed abundances of the secondary components. A precise knowledge of nuclear cross sections is essential for unraveling the parameters of cosmic ray propagation: the mean path length traversed and its energy dependence, the distribution function of path lengths, the confinement time of cosmic rays in the galaxy, and acceleration after fragmentation. Various models of cosmic ray propagation will be critically explored.