A theoretical analysis of laser flowmeters

This paper gives a unified theoretical analysis of Laser Doppler systems covering both the optical and electronic aspects, and applicable to all systems which have either a real or virtual fringe pattern. Previous experimental results by Greated for the Doppler spectrum are confirmed and in addition a fuller treatment of the low frequency part of the spectrum is presented.     

Radiation damage and decay characteristics of zinc selenide emission bands and their equivalence to zinc sulphide emission bands

Irradiation of zinc selenide at ～ 20°K with electrons capable of displacing zinc atoms results in the production of cathodoluminescence emission bands at 610 and 630 nm. Enhancement of the 630 nm emission band by electron damage requires the presence of copper in the samples. These emissions have been shown to give a peak energy shift to lower energies after excitation, as is characteristic of donor-acceptor pair recombination. It is concluded that the 610 and 630 nm emission bands of zinc selenide are equivalent to the self-activated and copper green emission bands of zinc sulphide. The 530 nm emission band of zinc selenide is not sensitive to electron damage, shows no time shift and is thought to be equivalent to the copper blue emission of zinc sulphide.     

Development of [j] in young, midwestern, American children

Beginning at the age of about 14 months, eight children who lived in a rhotic dialect region of the United States were recorded approximately every 2 months interacting with their parents. All were recorded until at least the age of 26 months, and some until the age of 31 months. Acoustic analyses of speech samples indicated that these young children acquired [inverted r] production ability at different ages for [inverted r]'s in different syllable positions. The children, as a group, had started to produce postvocalic and syllabic [inverted r] in an adult-like manner by the end of the recording sessions, but were not yet showing evidence of having acquired prevocalic [inverted r]. Articulatory limitations of young children are posited as a cause for the difference in development of [inverted r] according to syllable position. Specifically, it is speculated that adult-like prevocalic [inverted r] production requires two lingual constrictions: one in the mouth, and the other in the pharynx, while postvocalic and syllabic [inverted r] requires only one oral constriction. Two lingual constrictions could be difficult for young children to produce.     

Molality as a unit of measure for expressing 1H MRS brain metabolite concentrations in vivo

Absolute concentrations of cerebral metabolite in in vivo 1H magnetic resonance spectroscopy studies (1H-MRS) are widely reported in molar units as moles per liter of tissue, or in molal units as moles per kilogram of tissue. Such measurements require external referencing or assumptions as to local water content. To reduce the scan time, avoid assumptions that may be invalid under specific pathologies, and provide a universally accessible referencing procedure, we suggest that metabolite concentrations from 1H-MRS measurements in vivo be reported in molal units as moles per kilogram of tissue water. Using internal water referencing, a two-compartment water model, a simulated brain spectrum for peak identification, and a spectroscopic bi-exponential spin-spin relaxation segmentation technique, we measured the absolute concentrations for the four common 1H brain metabolites: choline (Cho), myo-inositol (mIno), phosphocreatine + creatine (Cr), and N-acetyl-aspartate (NAA), in the hippocampal region (n = 26) and along the Sylvian fissure (n = 61) of 35 healthy adults. A stimulated echo localization method (20 ms echo time, 10 ms mixing time, 4 s repetition time) yielded metabolite concentrations, uncorrected for metabolite relaxation or contributions from macromolecule resonances, that were expectantly higher than with molar literature values. Along the Sylvian fissure the average concentrations (coefficient of variation (CV)) in mmoles/kg of tissue water were 17.6 (12%) for NAA, 14.2 (9%) for Cr, 3.6 (13%) for Cho, and 13.2 (15%) for mIno. Respective values for the hippocampal region were 15.7 (20%), 14.7 (16%), 4.6 (19%), and 17.7 (26%). The concentrations of the two regions were significantly different (p 相似文献   

Dehn filling in relatively hyperbolic groups

We introduce a number of new tools for the study of relatively hyperbolic groups. First, given a relatively hyperbolic group G, we construct a nice combinatorial Gromov hyperbolic model space acted on properly by G, which reflects the relative hyperbolicity of G in many natural ways. Second, we construct two useful bicombings on this space. The first of these, preferred paths, is combinatorial in nature and allows us to define the second, a relatively hyperbolic version of a construction of Mineyev. As an application, we prove a group-theoretic analog of the Gromov-Thurston 2π Theorem in the context of relatively hyperbolic groups. The first author was supported in part by NSF Grant DMS-0504251. The second author was supported in part by an NSF Mathematical Sciences Post-doctoral Research Fellowship. Both authors thank the NSF for their support. Most of this work was done while both authors were Taussky-Todd Fellows at Caltech.     

Fast analysis with Zeeman graphite furnace AAS

Stabilized Temperature Platform Furnace (STPF) methods have been adapted and altered to reduce the analytical time to less than 1 min per sample with no loss of analytical precision or accuracy. It is shown that this could be further reduced to about 30 s per sample if certain changes in instrumentation are implemented, especially in the software and firmware that control the autosampler. The sample uptake rate for the autosampler should overlap the cooldown of the tube from the prior determination. Also, the sample should be deposited onto a heated platform. In this work the pyrolysis step and, in most cases, the use of a matrix modifier has been omitted. Since backgrounds were therefore larger, the use of Zeeman correction was usually required, but continuum background correction was not tried. To confirm that these fast analytical methods might be practical, more than 10 standard reference materials were analyzed for several elements including Pb, Cd, Cu, Ni, As and Cr. The paper is not primarily intended to provide routine and reliable methods; it is intended to test the feasibility of these fast methods.     

A counterion condensation theory for the relaxation,rise, and frequency dependence of the parallel polarization of rodlike polyelectrolytes

We calculate the relaxation, rise, and frequency dependence of the parallel polarization of counterions condensed on rodlike polyelectrolytes and charged colloids. Emphasis is placed on an effective counterion mobility along the rod as strongly influenced by electrostatic interactions.