R. M. Walker

Nuclear tracks in solids

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Nuclear tracks in solids. Principles and Applications

An extensive discussion of particle tracks in solids, quantitative methods for particle identification from them, and applications of the techniques in a number of fields are presented. Applications in geochronology, cosmic ray physics, meteoritic and lunar science, nuclear physics, chemical analysis, micro-chemical mapping, and radiation dosimetry are included. Each chapter contains numerous photographs and a substantial bibliography.

Identification of Complex Aromatic Molecules in Individual Interplanetary Dust Particles

Seventeen stratospherically collected particles—eight of which are classified as interplanetary dust particles (IDPs), seven of which are classified as probable terrestrial contaminants, and two of which have uncertain origins—were studied with a microprobe two-step laser mass spectrometer. Many polycyclic aromatic hydrocarbons(PAHs) and their alkylated derivatives were identified in two of the eight IDPs. The PAHs observed include a high-mass envelope not found in meteorites or terrestrial contaminants and prominent odd-mass peaks suggestive of nitrogen-containing functional groups attached to aromatic chromophores. In addition, the complexity of the IDP mass spectra has no precedence in previous studies of meteorite samples or their acid residues. Extensive checks were performed to demonstrate that the PAH signals are not caused by terrestrial contaminants.

Laboratory infrared transmission spectra of individual interplanetary dust particles from 2.5 to 25 microns

A vulcanizable polyacrylate elastomer composition having improved resistance to hydrocarbon oils is provided which comprises one or more lower alkyl acrylic acid ester monomers polymerized with a comonomer containing either a halogen or an epoxy group, in the presence of from about 0.25 to 10 percent, by weight of total monomer, of a low molecular weight, unsaturated polymer of butadiene or isoprene or copolymers thereof with styrene or acrylonitrile. In addition, a process for preparing such elastomer compositions is also provided, together with the vulcanization of such elastomers and the resulting vulcanized polyacrylate.

Ion microprobe isotopic measurements of individual interplanetary dust particles

Ion microprobe measurements of D/H ratios in individual fragments of eight stratospheric dust particles give δD values ranging from −386 to +2534‰ relative to SMOW. The δD values in five particles far exceed those in terrestrial samples and prove that the samples are interplanetary dust particles (IDPs). The hydrogen isotopic composition is heterogeneous on a scale of a few microns demonstrating that the dust is unequilibrated. Measurements of D/H ratios in conjunction with elemental and molecular ion signals in different fragments of individual IDPs show that a carbonaceous phase, not water, is the carrier of the D enrichments. Previous infrared transmission measurements have shown that IDPs fall into three main spectral classes. Particles from two of those three IR classes show large D/H ratios. Two particles studied from the third class do not. However, one of these contains solar flare tracks and is extraterrestrial. Thus, most, but not all, IDPs contain hydrogen with a non-terrestrial isotopic composition. Carbon isotopic measurements on fragments of three IDPs give ratios similar to terrestrial values and show a largely uniform isotopic composition for a given particle. Small, but significant, differences in δ13C of ~40‰ between particles are seen. No correlations between the hydrogen and carbon isotopic compositions are observed. The magnesium and silicon isotopic compositions of fragments of three IDPs are found to be normal within measurement errors.

Spin Resonance in Electron Irradiated Silicon

The spin resonance behavior in room temperature irradiated n‐type silicon is observed to be significantly different for silicon grown in quartz crucibles from that grown by the floating zone method. The dominant spectrum in each is discussed. The defects giving rise to the spectra are interpreted as containing impurity atoms and as having formed when the impurities trap mobile interstitials and/or vacancies. In quartz crucible grown silicon, the impurity may be oxygen. In the floating zone material, the impurity appears to be the phosphorus used in the doping. A 20°K irradiation and anneal is described which suggests the temperatures at which this defect motion is occurring. Features of the spin resonance spectra suggest that the vacancy may be the mobile effect.

Silicon Nitride from Supernovae

Seven presolar silicon nitride (Si3N4) dust grains have been identified (five unambiguously and two probably) in separates of the Tieschitz (H3.6) and Murchison (CM2) meteorites, confirming previous tentative identifications of this mineral as a presolar component. These rare (2 ppb in Murchison) grains have isotopic compositions similar to those of the uncommon class of meteoritic SiC known as grains X (~60 ppb in Murchison), namely 28Si and 15N excesses relative to solar, both 13C excesses and deficits, and extremely high inferred 26Al/27Al ratios. These isotopic compositions coupled with Ca and Ti isotopic anomalies seen in some SiC grains X point to an origin in Type II supernova ejecta for SiC grains X, and by analogy for the Si3N4 grains as well. However, substantial discrepancies exist between the isotopic characteristics of the grains and the compositions predicted by supernova models.

First Documented Case of Anadromy in a Population of Introduced Rainbow Trout in Patagonia, Argentina

Abstract The examination of population-specific adaptations of introduced salmonids to the wide range of environments found in Patagonia (southern South America) can help unveil some of the genetic and environmental contributions to life history variation. The rainbow trout Oncorhynchus mykiss introduced into Argentina originated from a few parental stocks. Although some of these stocks were anadromous, all of the dozens of established populations described until now have been freshwater resident. In this paper we provide the first documentation of the presence of an anadromous run of rainbow trout in the Santa Cruz River, the second largest river of Argentinean Patagonia. Microsatellite analysis revealed that anadromous and resident fish from the Santa Cruz River are genetically indistinguishable, probably representing alternative life histories within the same population. Both wild types are very different from the fish of Danish origin that were reared in a local hatchery, suggesting that they are desc...

Observations of Charged‐Particle Tracks in Solids

The characteristics of tracks produced by fission fragments have been studied by transmission electron microscopy in the following layer structure materials: molybdenite (MoS2), talc, clinochlore, jefferisite, and various micas including biotite, phlogopite, muscovite, and synthetic fluor‐phlogopite. The measured track widths showed little correlation with the decomposition temperatures and it was concluded that the track formation cannot be described by a simple thermal spike model. The absence of any new rings or spots in the electron diffraction pattern of a heavily irradiated sample led to the conclusion that the tracks themselves consisted of a disordered region and not of a definable new crystalline phase. In many of the materials the tracks faded during observation in the beam. Although this fading could be eliminated by using a cold stage, it was not caused simply by the thermal instability of tracks; some materials which showed rapid track fading retained tracks to higher temperatures in external...

Indigenous Polycyclic Aromatic Hydrocarbons in Circumstellar Graphite Grains from Primitive Meteorites

We report the measurement of polycyclic aromatic hydrocarbons (PAHs) in individual circumstellar graphite grains extracted from two primitive meteorites, Murchison and Acfer 094. The 12C/13C isotope ratios of the grains in this study range from 2.4 to 1700. Roughly 70% of the grains have an appreciable concentration of PAHs (500-5000 parts per million [ppm]). Independent molecule-specific isotopic analyses show that most of the PAHs appear isotopically normal, but in several cases correlated isotopic anomalies are observed between one or more molecules and their parent grains. These correlations are most evident for 13C-depleted grains. Possible origins of the PAHs in the graphite grains are discussed.