Production of light pseudoscalars in external electromagnetic fields by the Schwinger mechanism

We generalize the Schwinger mechanism and calculate the probability of the decay of intense electromagnetic fields to pseudoscalar particles. We also point out that our estimate for axion emission in a previous paper was incorrect.Comment: 25 pages including 9 figures. Version that matches published versio

Should the Pomeron and imaginary parts be modelled by two gluons and real quarks?

We illustrate that solution of the Schwinger-Dyson equation for the gluon propagator in QCD does not support an infrared softened behaviour, but only an infrared enhancement. This has consequences for the modelling of the Pomeron in terms of dressed gluon exchange. It highlights that an understanding of the Pomeron within QCD must take account of the bound state nature of hadrons.Comment: 7 pages, latex, 2 figures, replaced ~\epsfig... by \mbox{\epsfig...

Radiation Damage of F8 Lead Glass with 20 MeV Electrons

Using a 20 MeV linear accelerator, we investigate the effects of electromagnetic radiation on the optical transparency of F8 lead glass. Specifically, we measure the change in attenuation length as a function of radiation dose. Comparing our results to similar work that utilized a proton beam, we conclude that F8 lead glass is more susceptible to proton damage than electron damage.Comment: 5 pages, 6 figure

Vacuum polarization in two-dimensional static spacetimes and dimensional reduction

We obtain an analytic approximation for the effective action of a quantum scalar field in a general static two-dimensional spacetime. We apply this to the dilaton gravity model resulting from the spherical reduction of a massive, non-minimally coupled scalar field in the four-dimensional Schwarzschild geometry. Careful analysis near the event horizon shows the resulting two-dimensional system to be regular in the Hartle-Hawking state for general values of the field mass, coupling, and angular momentum, while at spatial infinity it reduces to a thermal gas at the black-hole temperature.Comment: REVTeX 4, 23 pages. Accepted by PRD. Minor modifications from original versio

Analytical approximation of the stress-energy tensor of a quantized scalar field in static spherically symmetric spacetimes

Analytical approximations for ${}$ and ${}$ of a quantized scalar field in static spherically symmetric spacetimes are obtained. The field is assumed to be both massive and massless, with an arbitrary coupling $\xi$ to the scalar curvature, and in a zero temperature vacuum state. The expressions for ${}$ and ${}$ are divided into low- and high-frequency parts. The contributions of the high-frequency modes to these quantities are calculated for an arbitrary quantum state. As an example, the low-frequency contributions to ${}$ and ${}$ are calculated in asymptotically flat spacetimes in a quantum state corresponding to the Minkowski vacuum (Boulware quantum state). The limits of the applicability of these approximations are discussed.Comment: revtex4, 17 pages; v2: three references adde

Fast Algorithm for Partial Covers in Words

A factor $u$ of a word $w$ is a cover of $w$ if every position in $w$ lies within some occurrence of $u$ in $w$. A word $w$ covered by $u$ thus generalizes the idea of a repetition, that is, a word composed of exact concatenations of $u$. In this article we introduce a new notion of $\alpha$-partial cover, which can be viewed as a relaxed variant of cover, that is, a factor covering at least $\alpha$ positions in $w$. We develop a data structure of $O(n)$ size (where $n=|w|$) that can be constructed in $O(n\log n)$ time which we apply to compute all shortest $\alpha$-partial covers for a given $\alpha$. We also employ it for an $O(n\log n)$-time algorithm computing a shortest $\alpha$-partial cover for each $\alpha=1,2,\ldots,n$

Polishing of CVD-Diamond Substrates Using Reactive Ion Etching

Multichip modules (MCM)have proved to be a viable packaging technology for achieving small size and high performance. By their nature, MCMs typically integrate multiple bare die into a module that can be the plastic or ceramic package. As a result, the MCMrequires an efficient mechanism for removing excess heat. Diamond with its excellent thermal conductivity, is the ideal choice as a substrate material for these applications. Chemical vapor deposited (CVD) diamond substrates makes possible the practical realization of a novel diamond based 3-D MCM. However, the diamond films grown by CVD technique are polycrystalline and have non-uniform filmroughness and randomly faceted crystals. These non-planar surfaces reduce the diamond\u27s thermal management efficiency. Therefore, itbecomes imperative that the asdeposited diamond films be polished for use inMCMs. Chemical assisted mechanical polishing (CAMP) technique has been developed at HiDEC,University of Arkansas. In this technique diamond is lapped against an alumina plate under a load in the presence of certain chemicals. Although CAMP technique reduces the lapping time considerably, stillnewer techniques must be developed to reduce polishing cost further. We are currently using reactive ion etching (RIE) to substantially reduce the polishing time. Preliminary studies using reactive ion etching showed etch rates of 500 - lOOOA/min at low pressures. These etched films showed a considerably higher polishing rate (using CAMP technique) than the nonetched films. Changes in the morphology and structure of the diamond films due to etching and polishing were characterized by scanning electron microscopy (SEM), Dektak profilometer and Raman spectroscopy. This paper presents a systematic study ofRIEand CAMP of CVD-diamond substrates

The Christiansen Effect in Saturn's narrow dusty rings and the spectral identification of clumps in the F ring

Stellar occultations by Saturn's rings observed with the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft reveal that dusty features such as the F ring and the ringlets in the Encke and the Laplace Gaps have distinctive infrared transmission spectra. These spectra show a narrow optical depth minimum at wavelengths around 2.87 microns. This minimum is likely due to the Christiansen Effect, a reduction in the extinction of small particles when their (complex) refractive index is close to that of the surrounding medium. Simple Mie-scattering models demonstrate that the strength of this opacity dip is sensitive to the size distribution of particles between 1 and 100 microns across. Furthermore, the spatial resolution of the occultation data is sufficient to reveal variations in the transmission spectra within and among these rings. For example, in both the Encke Gap ringlets and F ring, the opacity dip weakens with increasing local optical depth, which is consistent with the larger particles being concentrated near the cores of these rings. The strength of the opacity dip varies most dramatically within the F ring; certain compact regions of enhanced optical depth lack an opacity dip and therefore appear to have a greatly reduced fraction of grains in the few-micron size range.Such spectrally-identifiable structures probably represent a subset of the compact optically-thick clumps observed by other Cassini instruments. These variations in the ring's particle size distribution can provide new insights into the processes of grain aggregation, disruption and transport within dusty rings. For example, the unusual spectral properties of the F-ring clumps could perhaps be ascribed to small grains adhering onto the surface of larger particles in regions of anomalously low velocity dispersion.Comment: 42 pages, 15 figures, accepted for publication in Icarus. A few small typographical errors fixed to match correction in proof

Bed material agglomeration during fluidized bed combustion. Technical progress report, April 1, 1995--June 30, 1995

During this quarter, agglomeration tests were conducted in a laboratory-scale fluidized bed combustor using coal and {open_quotes}model{close_quotes} components which allowed controlled amounts of clays and pyrites to be added during the test. These tests permitted a more direct evaluation of the interaction between iron compounds and aluminosilicates. With additional clay and pyrite (under simulated local reducing conditions found at coal feed locations) large agglomerates formed. The agglomerates were many times larger than those formed with a standard coal feed. When only clay was added to the fuel (no additional pyrite), agglomerates formed but they were much smaller and very friable. These tests support the hypothesis that local reducing conditions promote the interaction of iron in a +2 state and aluminosilicate material in the coal which leads to agglomeration during fluidized bed combustion. Also during this quarter, a deposit which formed in a fluidized bed boiler of a Texas-New Mexico Power Company was analyzed to determine the chemical and mineralogic mechanisms responsible for deposit formation. Mineral phases were determined by x-ray diffraction (XRD). Bulk chemical composition was determined by x-ray fluorescence spectroscopy (XRF). Polished sections of the deposit were made for optical and scanning electron microscopy

Minimum EMG burst duration in healthy controls : implications for electrodiagnosis in movement disorders

Background EMG burst duration can provide additional diagnostic information when investigating hyperkinetic movement disorders, particularly when a functional movement disorder is suspected. It is generally accepted that EMG bursts <50ms are pathological. Objective To re‐assess minimum physiological EMG burst duration. Methods Surface EMG was recorded from face, trunk and limb muscles in controls (n=60; age 19‐85). Subjects were instructed to generate the briefest possible ballistic movements involving each muscle (40 repetitions) or, in muscles spanning joints, to generate rapid rhythmic alternating movements (20‐30s), or both. Results We found no effect of age on EMG burst duration. However, EMG burst duration varied significantly between body regions. Rhythmic EMG bursts were shorter than ballistic bursts but only significantly so for lower limbs (p<0.001). EMG bursts of duration <50ms were frequently observed, particularly in appendicular muscles. Conclusion We present normal reference data for minimum EMG burst duration, which may assist clinical interpretation when investigating hyperkinetic movement disorders