Stability of germanium resistance thermometers at 20 K

Thirty germanium resistance thermometers have been thermally cycled 100 times between 20 and 300 K, and their stability at 20 K has been evaluated. The results reveal a wide range of stabilities, ranging from 0.1 to 20 mK. Five different modes of behavior have been provisionally classified as stable, drifting, jumping, bimodal, and irregular.     

Experimental setup for low-energy laser-based angle resolved photoemission spectroscopy

A laser-based angle resolved photoemission (ARPES) system utilizing 6 eV photons from the fourth harmonic of a mode-locked Ti:sapphire oscillator is described. This light source greatly increases the momentum resolution and photoelectron count rate, while reducing extrinsic background and surface sensitivity relative to higher energy light sources. In this review, the optical system is described, and special experimental considerations for low-energy ARPES are discussed. The calibration of the hemispherical electron analyzer for good low-energy angle-mode performance is also described. Finally, data from the heavily studied high T(c) superconductor Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) is compared to the results from higher photon energies.     

Assessing biological indicators for remediated anthropogenic urban soils

Anthropogenic urban soils, including brownfield soils, are currently characterised and evaluated using mainly physico-chemical properties. Our objective was to determine if biological indicators could provide a more comprehensive soil quality assessment relative to sustainability, identifying contamination issues, and effectiveness of remediation strategies. Plant, invertebrate and microbial assays and functional processes were evaluated at 10 brownfield/anthropogenic urban locations at different stages of remediation in northwest England. Extreme sites were discriminated on the basis of earthworm counts and a small number of indicators likely to be related to their activity. It was concluded that identifying a universally-applicable benchmark suite of biological indicators is very unlikely without considerable advancement of knowledge and technology.     

Fire detection, location and heat release rate through inverse problem solution. Part I: Theory

A proposed method of detecting, locating and sizing accidental fires, based on the solution of an inverse heat transfer problem, is described. The inverse heat transfer problem to be solved is that of the convective heating of a compartment ceiling by the hot plume of combustion gases rising from an accidental fire. The inverse problem solution algorithm employs transient temperature data gathered at the ceiling of the compartment to determine the location and heat release rate of the fire. An evaluation of the proposed fire detection system, demonstrating the limits on the accuracy of the inverse problem solution algorithm, is presented. The evaluation involves operating the inverse problem solution algorithm on transient temperature data from computer simulated compartment fires. The simulated fire data are generated assuming fires with quadratic growth rates, burning in a 20 m wide by 20 m deep by 3 m high enclosure with a smooth, adiabatic ceiling. The accuracy of the inverse problem solution algorithm in determining the location of a fire is shown to be insensitive to the errors in the fire model used in the forward problem solution, but sensitive to errors in the measured temperature data. The accuracy of the heat release rate of the fire is sensitive to both errors in the fire model and errors in the temperature data. The validity of the use of computer simulated data in the evaluation is verified with a second evaluation using fire data interpolated from published measurements taken in large-scale compartment fire burns.     

An FDTD/MoM hybrid technique for modeling complex antennas in thepresence of heterogeneous grounds

Calculating the current distribution and radiation patterns for ground-penetrating radar antennas is a challenging problem because of the complex interaction between the antenna, the ground, and any buried scatterer. Typically, numerical techniques that are well suited for modeling the antennas themselves are not well suited for modeling the heterogeneous grounds, and visa versa. For example the finite-difference time-domain (FDTD) technique is well suited for modeling fields in heterogeneous media, whereas the method of moments (MoM) is well suited for modeling complex antennas in free space. This paper describes a hybrid technique, based upon the equivalence principle, for calculating an antenna's current distribution radiation pattern when the antenna is located near an air-ground interface. The original problem is decomposed into two coupled equivalent problems: one for the antenna geometry and the other for the ground geometry, with field information passing between them via a rapidly converging iterative procedure. The fields in each region may be modeled using numerical techniques best suited to them. Results for several test cases are presented, using FDTD to model the ground problem and MoM for the antenna problem, that demonstrate the accuracy of this hybrid technique     

THE EFFECT OF HETEROGENEITY ON WOOD DRYING,PART I: MODEL DEVELOPMENT AND PREDICTIONS

ABSTRACT

A two dimensional model which can predict the effects of the anisotropy and heterogeneity on the transport phenomena which occur in wood during drying is developed. It is shown that the appropriate driving potential for moisture transport is the ratio of the moisture content to the driving potential. In its one dimensional form, die model results compare favorably with experimental measurements for drying in the radial direction. In its two dimensional form the model is used to predict drying in a direction midway between the radial and the tangential. In this case free water moves in a diagonal direction because the low density earlywood dries faster than the latewood during the early stages of drying. The result is significant gradients in moisture content, not only in the drying direction, but also in the direction parallel to the drying surface.     

Some convergence considerations in space-domain moment-methodanalysis of a class of wide-band microstrip antennas

The method of moments (MoM) analysis of probe-fed rectangular microstrip patches requires the inclusion of a probe-to-patch attachment mode-expansion function when the substrate thickness d⩾0.02λ, where λ is the free-space wavelength. The results for the input impedance showed increased divergence with measurements when the attachment mode was omitted from the full-wave analysis. The attachment mode can be expressed as an infinite eigenfunction series that increases the fill time of the impedance matrix in an MoM analysis. In an earlier investigation, the infinite eigenfunction series was reduced to a residue series that required one or two terms compared to about 55 terms for the eigenfunction series. In this paper, the convergence properties of the eigenfunction and residue series are investigated in view of rigorous MoM analysis. The relative errors resulting from replacing the eigenfunction by the residue series for the attachment mode, are compared by numerically evaluating a class of two-dimensional (2-D) spatial integrals shown to be closely related to the elements of an MoM impedance matrix. Additionally, the computation times for the evaluation of these integrals for the two forms of the attachment mode-expansion function are also included. Based on the superior convergence properties of the residue series for the attachment mode-expansion function, it is mathematically justified that this form can readily be used for analytic reduction of the spatial, reaction integrals from four to 2-D forms. This feature allows further reduction of the fill time of the MoM impedance matrix, suggesting the possibility of developing an efficient space-domain MoM technique for modeling of wide-band microstrip antennas     

THE EFFECT OF HETEROGENEITY ON WOOD DRYING,PART II: EXPERIMENTAL RESULTS

ABSTRACT

Experimental measurements of drying rate, moisture distribution, surface moisture content, and temperature distribution are reported for softwood dried in the radial, tangential, and mixed (between radial and tangential) directions. The effects of both the heterogeneous and the anisotropic structure of wood are observed. The drying curves for tangential drying exhibit two distinct transition points - one when the surface reaches the fiber saturation point and one when the surface becomes completely dry. These transitions are not observed consistently for drying in the radial and mixed directions. For mixed drying, as a result of anisotropy, the drying rate is always higher at the side of the sample to which the growth rings point at die surface. Measurements of the surface mass transfer coefficient indicate that the theoretical value which is analogous to the convective heat transfer coefficient agrees well with that measured experimentally at both very high and very low values of the surface moisture content. At intermediate values of the moisture content the ratio of the experimental to the theoretical convective mass transfer coefficient can be as low as 0.20. The model discussed in Part I predicts results that are in good qualitative agreement with the experimental results presented in this paper.     

A matched-filter-based reverse-time migration algorithm forground-penetrating radar data

Ground-penetrating radar (GPR) is a remote sensing technique used to obtain information on subsurface features from data collected over the surface. The process of collecting data may be viewed as mapping from the object space to an image space. Since most GPRs use broad beam width antennas, the energy reflected from a buried structure is recorded over a large lateral aperture in the image spare, migration algorithms are used to reconstruct an accurate scattering map by refocusing the recorded scattering events to their true spatial locations through a backpropagation process. The goal of this paper is to present a pair of finite-difference time-domain (FDTD) reverse-time migration algorithms for GPR data processing. Linear inverse scattering theory is used to develop a matched-filter response for the GPR problem. The reverse-time migration algorithms, developed for both bistatic and monostatic antenna configurations, are implemented via FDTD in the object space. Several examples are presented