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Biomedical subjects

R Silberberg

Publications and source records attributed to R Silberberg.

At least 19 recordsLinked to original sources
PubMed1996

Calculations of depth-dose distributions, cross sections and momentum loss.

The ability to know with precision the depth-dose, dose average LET, fluence and energy distributions is of great importance in many research fields, including therapeutic and diagnostic medicine when using heavy ion beams, as well as in space research. We have therefore developed a model and a computer code for calculating these distributions when using high energy proton or heavy ion beams. In this model, we use semi-empirical total reaction and partial cross section formulas developed by us and a new prescription to take into account the energy and momentum loss of the secondary nuclei. In this paper, we will also present an empirical equation for the total inelastic 4He-p cross section, as well as the partial cross sections for the production of 3He, 3H, 2H, p and n.

Algorithms↗
PubMed1990

Spallation processes and nuclear interaction products of cosmic rays.

Most cosmic-ray nuclei heavier than helium have suffered nuclear collisions in the interstellar gas, with transformation of nuclear composition. The isotopic and elemental composition at the sources has to be inferred from the observed composition near the Earth. The source composition permits tests of current ideas on sites of origin, nucleosynthesis in stars, evolution of stars, the mixing and composition of the interstellar medium and injection processes prior to acceleration. The effects of nuclear spallation, production of radioactive nuclides and the time dependence of their decay provide valuable information on the acceleration and propagation of cosmic rays, their nuclear transformations, and their confinement time in the Galaxy. The formation of spallation products that only decay by electron capture and are relatively long-lived permits an investigation of the nature and density fluctuations (like clouds) of the interstellar medium. Since nuclear collisions yield positrons, antiprotons, gamma rays and neutrinos, we shall discuss these topics briefly.

Astronomical Phenomena↗
PubMed1989

Satellite observation of atmospheric nuclear gamma radiation.

We present a satellite observation of the spectrum of gamma radiation from the Earth's atmosphere in the energy interval from 300 keV to 8.5 MeV. The data were accumulated by the gamma ray spectrometer on the Solar Maximum Mission over 3 1/2 years, from 1980 to 1983. The excellent statistical accuracy of the data allows 20 atmospheric line features to be identified. The features are superimposed on a continuum background which is modeled using a power law with index -1.16. Many of these features contain a blend of more than one nuclear line. All of these lines (with the exception of the 511-keV annihilation line) are Doppler broadened. Line energies and intensities are consistent with production by secondary neutrons interacting with atmospheric 14N and 16O. Although we find no evidence for other production mechanisms, we cannot rule out significant contributions from direct excitation or spallation by primary cosmic ray protons. The relative intensities of the observed line features are in fair agreement with theoretical models; however, existing models are limited by the availability of neutron cross sections, especially at high energies. The intensity and spectrum of photons at energies below the 511-keV line, in excess of a power law continuum, can be explained by Compton scattering of the annihilation line photons in traversing an average of approximately 21 g cm-2 of atmosphere.

Astronomy↗
PubMed1989

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.

Cosmic Radiation↗
PubMed1989

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.

Aluminum↗
PubMed1989

Fibronectin content of the annulus fibrosus in diabetic and non-diabetic sand rats.

Annulus fibrosus of intervertebral discs from diabetic and non-diabetic sand rats were examined by microspectrophotometry for fibronectin content. This was higher in the diabetic animals both in the dorsal and ventral parts and in the outer and inner lamellae of the annulus. It is suggested that diabetes-related changes in fibronectin are similar to changes in annular collagen observed in species other than sand rats.

Animals↗
PubMed1989

Age-related changes in fibronectin in annulus fibrosus of the sand rat (Psammomys obesus).

Fibronectin stains were carried out and evaluated by microspectrophotometric techniques in annuli fibrosi of vertebral discs of sand rats (Psammomys obesus) of both sexes and two age groups, 13-18 months and over 2 years of age. The distribution of fibronectin in the annulus shows a centripetal gradient from the outer to the inner laminae. Fibronectin was significantly more abundant in the annuli of old than of young animals of corresponding sex. Sex differences were not significant. The dorsal segment contained more fibronectin than the ventral, but the difference was statistically significant only in the aged females. The outer laminae of the annuli appeared consistently higher in fibronectin content than the inner laminae.

Aging↗
PubMed1988

Histologic and morphometric observations on vertebral bone of aging sand rats.

The trabecular bone of the vertebrae of 30 male and 30 female sand rats (Psammomys obesus) aged 13 to 33 months was examined histologically and morphometrically. The usual age-linked decline of bone mass failed to occur in females and was statistically not significant in males. A few changes with age were noted at the cellular level. Sex differences were statistically significant only in animals living into the third year of life, males having a smaller bone mass than females. This abnormality of the vertebral spongiosa is attributed to pathologic local stresses caused by the numerous instances of disc degeneration and herniation. Differences in size, location, and age of the herniations are thought to account for the wide fluctuations in the individual bone parameters examined. Whereas the spine of the sand rat provides an excellent model for the study of spondylosis, it is unsuited as a model for age-linked osteoporosis.

Aging↗
PubMed1988

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.

Arsenic↗
PubMed1988

The vertebral column of diabetic sand rats (Psammomys obesus).

Intervertebral discs and vertebral spongiosa of diabetic and nondiabetic sand rats were investigated histologically and morphometrically. In diabetic animals, degeneration of the intervertebral discs was accelerated. Morphometrically, there was a consistent trend in the diabetics toward an attenuated bone turnover, which was, however, statistically significant only in regard to the length of the trabecular surface covered by osteoblasts. In the vertebral column of the sand rat, the tendency to osteoporosis - age-linked or diabetes-related - is reduced apparently due to the influence of local forces which promote osteogenesis.

Aging↗
PubMed1987

Distributed reacceleration of cosmic rays.

We develop a model in which cosmic rays, in addition to their initial acceleration by a strong shock, are continuously reaccelerated (e.g., by weak shocks) while propagating through the galaxy. The equations describing this acceleration scheme are solved analytically (approximating ionization losses by a cutoff) and numerically. Solutions for the spectra of primary and secondary cosmic rays are given in a closed analytic form, and they allow a rapid search in parameter space for viable propagation models with distributed reacceleration included. The observed boron-to-carbon ratio can be reproduced by the reacceleration theory over a range of escape parameters, some of them quite different from the standard "leaky box" model. It is also shown that even a very modest amount of reacceleration by strong shocks causes the boron-to carbon ratio to level off at sufficiently high energies, and this effect may be observed in the CRNE data. Several other curiosities in the data may be explained naturally if a modest amount of distributed reacceleration is invoked, including (a) the apparent truncation at low energy in the otherwise exponential pathlength distribution associated with the leaky box model, (b) the sub-iron isotopic anomalies and other effects noted by Silberberg et al., and (c) the discrepancy between the reported 10Be lifetime and the lifetime of cosmic rays in the dense strata of the galactic disk.

Acceleration↗
PubMed1987

Morphometric analysis of cancellous bone of young diabetic and non-diabetic sand rats (Psammomys obesus).

Cancellous vertebral bone of young diabetic sand rats (psammomys obesus) was compared with that of non-diabetic sand rats of the same sex and age. Of 5 morphometric diameters, 3--trabecular area, trabecular surface and osteoblastic surface--showed no statistically significant differences between test and control animals. By contrast, the 2 other parameters determined, namely lacunae containing osteoclasts as well as lacunae devoid of osteoclasts were significantly more numerous in diabetics than in controls.

Animals↗
PubMed1986

Effects of hyperinsulinism and of diabetes on proteoglycans of the intervertebral disc in weanling sand rats.

Weanling sand rats (Psammomys obesus) develop hyperinsulinemia or diabetes or both, if fed a standard laboratory diet without a supplement of fiber rich salt bush. The annuli fibrosi of hyperinsulinemic or diabetic animals, which are still hyperinsulinemic, show a slight but statistically significant increase in chondroitin sulfate and a lesser, statistically nonsignificant increase in keratan sulfate. Possible causes of these changes are discussed and the likelihood of a role of hyperinsulinism in their production is pointed out.

Animal Feed↗