Recent advances in the study of solid surfaces and thin films using variable-energy positron beams are reviewed. In first part authors discuss process moderation technical aspects beam production application. The second is (roughly) organized sections that apply to increasing time scales appropriate positron-solid interaction. These (a) encounter scattering effects, (b) energy loss stopping profiles, (c) diffusion thermalized positrons, (d) positron-surface interactions, (e) studies defects...

The decay rate has been determined for positrons annihilating with electrons while localized in the image-induced potential well at (110) surface of a clean well-annealed single crystal Al 300 K. lifetime associated this state was found to be 580 \ifmmode\pm\else\textpm\fi{} 10 psec. This value, which never before directly measured, is disagreement present theories. Changes spectra were also from ion sputtering and exposure various amounts oxygen.

Positrons implanted with varying energies (0-20 keV) have been used to study silicon epilayers grown by molecular-beam epitaxy on Si(100) substrates. Defects at the initial growth interface and throughout overlayer observed depth profiled. In addition, field-driven positron drift in some of is shown be consistent estimated concentrations (active) interfacial impurities. The demonstrates that positrons can nondestructively profile structural defects electric fileds thin films interfaces.

The behavior of positrons in crystalline and amorphous ice has been studied with a beam monoenergetic incident energies 0--4.5 keV. Positronium (Ps) is formed the bulk diffuses until it annihilates or escapes from surface. Measurements were carried out on fraction ortho-Ps leaving surface Doppler broadening 511-keV \ensuremath{\gamma} annihilation line. For 0--60 eV Ps formation probability shows large variations. These variations are associated so-called Ore gaps reflect electronic...

This study provides estimates of climate change impacts on U.S. agricultural yields and the economy through end 21st century, utilizing multiple scenarios. Results from a process-based crop model project future increases in wheat, grassland, soybean yield due to atmospheric CO2 change; corn sorghum show more muted responses. using econometric models less positive results. Both tend by century than several other similar studies. Using provide an integrated sector model, welfare gain is...

Silicon (100) crystals are implanted with 1-MeV ${\mathrm{Si}}^{2+}$ ions to a fixed fluence of 1\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ions/${\mathrm{cm}}^{2}$ at systematically different incident-ion dose rates, R, and substrate temperatures, T. A critical dependence on T is found for ${\mathit{R}}_{\mathit{t}}$, the flux formation continuous amorphous layer in material. The activation energy characterizing process be 0.9\ifmmode\pm\else\textpm\fi{}0.05 eV, which attributed collapse...

Positron diffusion in Si(100) and Si(111) has been studied using a variable-energy positron beam. The coefficient is found to be ${D}_{+}$=2.7\ifmmode\pm\else\textpm\fi{}0.3 ${\mathrm{cm}}^{2}$/sec Makhov-type implantation profile, which demonstrated fit the data more reliably than commonly applied exponential profile. diffusion-related parameter, ${E}_{0}$, results from 4.2\ifmmode\pm\else\textpm\fi{}0.2 keV, significantly longer previously reported values. A drastic reduction ${E}_{0}$...

Silicon (100) wafers have been irradiated at 300 K with silicon ions or helium energies between 0.2 and 5.0 MeV. Fluences ranged from ${10}^{11}$ to ${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. The associated defect profiles analyzed using variable-energy positron-beam methods. Displaced-atom distributions extracted Rutherford-backscattering-channeling (RBSC) measurements supplemented by infrared (1.8 \ensuremath{\mu}m) absorption yield divacancy concentrations. Defect annealing...

Positron emission from a W(110) single crystal has been studied as function of copper coverage utilizing variable-energy positron beam in conjunction with low-energy electron diffraction and Auger-electron spectroscopy. Evidence is presented that indicates significant localization occurs at defects associated the Cu/W(110) interface, which can be removed by high-temperature annealing. Our data also reveal new information about islanding on tungsten, providing reliable means identifying...

Crystalline and amorphous ice were studied with monoenergetic positrons incident energies $0<E<5$ keV. Positronium (Ps) forms in the diffuses as a neutral particle until annihilation or escape from surface. Measurement of fraction $o$-Ps leaving surface reveals two distinct contributions to total Ps-formation probability (both Ore- spur-type) well Ps diffusion coefficient, 0.17\ifmmode\pm\else\textpm\fi{}0.09 ${\mathrm{cm}}^{2}$/sec crystalline ice. In addition, positronium trapping is...

In a study of (100) Si growth by molecular-beam epitaxy, we have observed an interesting phenomenon associated with epitaxial at low temperatures. Electron-microscope imaging reveals that the surface no longer remains planar but develops series cusps {111}-oriented facets generate linear arrays spherical defects. Both electron microscopy and variable-energy positron-annihilation spectroscopy been used to determine defects are in fact microvoids.

We have investigated the reemission of positrons from a 1500-\AA{}-thick unsupported Ni(100) foil irradiated with collimated beam variable energy 0 to 6 keV. At 5 keV, (19 \ifmmode\pm\else\textpm\fi{} 3)% emerge low transmission side foil; \ensuremath{\cong} 55% slow-positron spectrum is concentrated in 0.2-eV-wide peak at 1.1 eV. The remoderator as efficient W(100) foils by Chen et al. [Phys. Rev. B 31, 4123 (1985)] and had only one-third spread energy. Foils this type will thus be very...

The temperature dependence of the effective positron diffusion length ${L}_{+}$ has been studied in various metallic glasses with variable-energy positrons. All initial measurements show a very short (${L}_{+}\ensuremath{\approx}10$ \AA{}) that is alloy dependent. This more than 2 orders magnitude smaller annealed crystalline metals, and ascribed to trapping at high concentration intrinsic defects. Larger values are found metal-metalloid metal-metal glasses. suggested be caused by boron...

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTProton Magnetic Resonance Spectrum of 1,1,2,2,4,4,5,5-OctadeuteriocyclohexaneNorbert Muller and Peter J. SchultzCite this: Phys. Chem. 1964, 68, 7, 2026–2028Publication Date (Print):July 1, 1964Publication History Published online1 May 2002Published inissue 1 July 1964https://pubs.acs.org/doi/10.1021/j100789a515https://doi.org/10.1021/j100789a515research-articleACS PublicationsRequest reuse permissionsArticle Views19Altmetric-Citations25LEARN ABOUT...

For neutron-irradiated Mo containing voids, a temperature dependence of the positron trapping intensity is observed between 9 and 400 K. Shallow traps are responsible have an activation energy 0.1\ifmmode\pm\else\textpm\fi{}0.05 eV. Deep microstructural identified as small loops dislocation tangles presumed to be shallow traps. Evidence positronium formation in voids at high presented.

Experimental data for positron and electron backscattering from thick solids are presented as a function of incident outgoing angles energies, target atomic number. A full description the experimental features limitations is presented. The compared with Monte Carlo calculated distributions, in all cases agreement extremely good. Specularlike scattering positrons at oblique observed explained terms strength elastic-scattering interaction.

Direct measurements of doubly differential (angle and energy) distributions backscattered positrons are reported using an electro- statically guided slow-positron beam. Backscattering yields for 35-keV incident normally on the target were measured as a function both scattering angle \ensuremath{\theta} atomic number Z(4\ensuremath{\le}Z\ensuremath{\le}82). Absolute backscattering coefficients compared to previous corresponding values electrons. Some apparent inconsistencies among various...

Proton magnetic resonance studies of the second moment S absorption spectrum and spin—lattice relaxation time T1 have been made over a wide temperature range in solid trimethylacetonitrile, (CH3)3CCN. The results indicate that dominant molecular motion occurring this compound is t-butyl group about C–CN axis coupled with rotation methyl groups their C3 axes. ``one-step'' nature change dependence which characterized by single minimum two types involved must occur such rapid succession as to...

We present new experimental results for the angle and energy resolved backscattering of positrons electrons from Al Au at oblique incident angles. For ${\mathit{e}}^{+}$ on Au, both ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ Al, evidence specular scattering is observed. we find excellent agreement between these data Monte Carlo simulations based Penn dielectric function.

The near-surface structure of low-energy- (0.5--1.5 keV) Ar-bombarded Si(100) has been studied using high-resolution x-ray-absorption near-edge spectroscopy and extended fine-structure with synchrotron radiation, medium-energy ion scattering, variable-energy positron-annihilation spectroscopy, angle-resolved x-ray photoemission spectroscopy. ion-induced defect for silicon, the distribution incorporated Ar silicon carbide formed during dynamic mixing process are directly nondestructively...