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141.
V.I. Zoran M. Ganciu A.M. Pointu C.B. Collins I.-I. Popescu 《Hyperfine Interactions》1997,107(1-4):415-430
An inexpensive, tunable, filamentary X-ray generator, based on the bremsstrahlung of a pulsed, high-power-density electron
beam cruising along a dielectric fiber is presented. At small scale, it encodes some of the features of an X-ray machine for
triggering the energy stored in the long-lived 178Hfm2 nuclear isomer, in particular an optimum coupling to the active medium
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
142.
MacMullan S.J. Collins O.M. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》1998,44(1):197-214
This paper analyzes the performance of concatenated coding systems operating over the binary-symmetric channel (BSC) by examining the loss of capacity resulting from each of the processing steps. The techniques described in this paper allow the separate evaluation of codes and decoders and thus the identification of where loss of capacity occurs. They are, moreover, very useful for the overall design of a communications system, e.g., for evaluating the benefits of inner decoders that produce side information. The first two sections of this paper provide a general technique (based on the coset weight distribution of a binary linear code) for calculating the composite capacity of the code and a BSC in isolation. The later sections examine the composite capacities of binary linear codes, the BSC, and various decoders. The composite capacities of the (8,4) extended Hamming, (24, 12) extended Golay, and (48, 24) quadratic residue codes appear as examples throughout the paper. The calculations in these examples show that, in a concatenated coding system, having an inner decoder provide more information than the maximum-likelihood (ML) estimate to an outer decoder is not a computationally efficient technique, unless generalized minimum-distance decoding of an outer code is extremely easy. Specifically, for the (8,4) extended Hamming and (24, 12) extended Golay inner codes, the gains from using any inner decoder providing side information, instead of a strictly ML inner decoder, are shown to be no greater than 0.77 and 0.34 dB, respectively, for a BSC crossover probability of 0.1 or less, However, if computationally efficient generalized minimum distance decoders for powerful outer codes, e.g., Reed-Solomon codes, become available, they will allow the use of simple inner codes, since both simple and complex inner codes have very similar capacity losses 相似文献
143.
Christopher M. Collins Shizhe Li Qing X. Yang Michael B. Smith 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,125(2):233-241
A method for calculatingB1field strength and homogeneity as functions of radiofrequency shield geometry is presented. The method requires use of three-dimensional finite-element analysis, birdcage-coil theory, and antenna-array theory. Calculations were performed for a 12-element birdcage coil (19 cm diameter, 21 cm length) at 125 MHz. CalculatedB1field strengths and homogeneities for the coil in 25 different shields and in no shield are given. For configurations where the shield is longer than the coil, bothB1field strength and homogeneity decrease as shield diameter decreases or as shield length increases. In configurations where the shield is shorter than the coil and has a diameter of 25.6 cm,B1homogeneity is greater than in an unshielded coil.B1field strength was measured experimentally at 125 MHz in a birdcage coil of the same geometry as the model within shields of four different diameters. Calculated results very closely matched experimental measurement. 相似文献
144.
Beck S.E. Collins S.M. O'Hanlon J.F. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1994,22(2):128-135
Methods for moving charged particles in RF processing plasmas are investigated. These methods include varying RF power, varying chamber pressure, attraction and repulsion by an electrostatic probe, and movement with magnetic fields. Varying RF power changes the depth of the potential wells where particles are trapped. The RF power affects shape and location of the traps and the bulk plasma potential. Increasing the chamber pressure moves the sheath edge closer to the wafer being processed. Since particle traps are found at the plasma sheath edge increasing the chamber pressure will move the particle traps (and any trapped particles) closer to the wafer being processed. The Langmuir probe can repel particles when under negative bias and attract them when positively biased. This probe can also distort the sheath edge when the tip resides within the sheath. Applying a magnetic field can change the characteristics of the particle traps and produce a force on the charged dust particles 相似文献
145.
Karen L. Collins 《Order》1991,8(4):375-381
The special properties of planar posets have been studied, particularly in the 1970's by I. Rival and others. More recently, the connection between posets, their corresponding polynomial rings and corresponding simplicial complexes has been studied by Stanley and others. This paper, using work of Björner, provides a connection between the two bodies of work, by characterizing when planar posets are Cohen-Macaulay. Planar posets are lattices when they contain a greatest and a least element. We show that a finite planar lattice is lexicographically shellable and therefore Cohen-Macaulay iff it is rank-connected. 相似文献
146.
Bradley C.J. Collins P.J. Fortuny-Guasch J. Hastriter M.L. Nesti G. Terzuoli A.J. Jr. Wilson K.S. 《Geoscience and Remote Sensing, IEEE Transactions on》2005,43(10):2185-2191
Several popular bistatic calibration techniques are investigated and comparisons made between the relative merits of the various techniques. The analysis addresses sensitivity to object alignment error, sensitivity to polarization impurity, and ease of implementation. Both theoretical concepts and practical considerations are discussed based on measurements accomplished at the European Microwave Signature Laboratory of the European Commission Joint Research Center, Ispra, Italy. This facility has the capability to produce far-field fully polarimetric precision bistatic measurements in a 30-cm-diameter quiet zone, suitable for comparing different calibration methods. 相似文献
147.
This paper develops a hybrid moment method (MM) based numerical model for electromagnetic scattering from large finite-by-infinite planar slot arrays. The model incorporates the novel concept of a physical basis function (PBF) to reduce dramatically the number of required unknowns. The model can represent a finite number of slot columns with slots oriented along the infinite axis, surrounded by an arbitrary number of coplanar dielectric slabs. Each slot column can be loaded with a complex impedance to tailor the array's edge currents. An individual slot column is represented by equivalent magnetic scattering currents on an unbroken perfectly conducting plane. Floquet theory reduces the currents to a single reference element. In the array's central portion, where the edge perturbations are negligible, the slot column reference elements are combined into a single basis function. Thus, one PBF can represent an arbitrarily large number of slot columns. A newly developed one-sided Poisson sum formula is used to calculate the mutual coupling between the PBF and the slot columns in the presence of a stratified dielectric media. The array scanning method (ASM) gives the mutual coupling between the individual slot columns. The hybrid method is validated using both numerical and experimental reference data. The results demonstrate the method's accuracy as well as its ability to handle array problems too large for traditional MM solutions 相似文献
148.
Telemedicine is a wide ranging subject which encompasses teleconsultation, electronic patient records, continuing medical education and public health information systems. Telemedicine will impact every aspect of health care delivery from remote clinical analysis and consultation between experts based in specialist hospitals, to teleconsultation as a substitute for physical referral between the GP and a hospital, and ultimately the remote delivery of medical monitoring and care into the h ome. Telepresence, where remote human expertise is transported by communications networks to a remote location, has its own unique place within the spectrum of telemedicine applications. This paper describes some of those applications of medical telepresence into which BT have been researching with various clinical partners. 相似文献
149.
Collis JM Siegmann WL Collins MD Simpson HJ Soukup RJ 《The Journal of the Acoustical Society of America》2007,122(4):1987-1993
A tank experiment was carried out to investigate underwater sound propagation over an elastic bottom in flat and sloping configurations. The purpose of the experiment was to evaluate range-dependent propagation models with high-quality experimental data. The sea floor was modeled as an elastic medium by a polyvinyl chloride slab. The relatively high rigidity of the slab requires accounting for shear waves in this environment. Acoustic measurements were obtained along virtual arrays in the water column using a robotic apparatus. Elastic parabolic equation solutions are in excellent agreement with data. 相似文献
150.
We present a new closure for the mean rate of stretching of a dissolved polymer by homogeneous isotropic turbulence. The polymer is modeled by a bead-spring-type model (e.g., Oldroyd B, FENE-P, Giesekus) and the analytical closure is obtained assuming the Lagrangian velocity gradient can be modeled as a Gaussian, white-noise stochastic process. The resulting closure for the mean stretching depends upon the ratio of the correlation time for strain and rotation. Additionally, we derived a second-order expression for circumstances when strain and rotation have a finite correlation time. Finally, the base level closure is shown to reproduce results from direct numerical simulations by simply modifying the coefficients. 相似文献