5,938 research outputs found
Investigation of the use of microwave image line integrated circuits for use in radiometers and other microwave devices in X-band and above
Program results are described in which the use of a/high permittivity rectangular dielectric image waveguide has been investigated for use in microwave and millimeter wavelength circuits. Launchers from rectangular metal waveguide to image waveguide are described. Theoretical and experimental evaluations of the radiation from curved image waveguides are given. Measurements of attenuation due to conductor and dielectric losses, adhesives, and gaps between the dielectric waveguide and the image plane are included. Various passive components are described and evaluations given. Investigations of various techniques for fabrication of image waveguide circuits using ceramic waveguides are also presented. Program results support the evaluation of the image line approach as an advantageous method for realizing low loss integrated electronic circuits for X-band and above
Assessment of Neuropsychological Trajectories in Longitudinal Population-Based Studies of Children
This paper provides a strategy for the assessment of brain function in longitudinal cohort studies of children. The proposed strategy invokes both domain-specific and omnibus intelligence test approaches. In order to minimise testing burden and practice effects, the cohort is divided into four groups with one-quarter tested at 6-monthly intervals in the 0–2-year age range (at ages 6 months, 1.0, 1.5 and 2.0 years) and at annual intervals from ages 3–20 (one-quarter of the children at age 3, another at age 4, etc). This strategy allows investigation of cognitive development and of the relationship between environmental influences and development at each age. It also allows introduction of new domains of function when age-appropriate. As far as possible, tests are used that will provide a rich source of both longitudinal and cross-sectional data. The testing strategy allows the introduction of novel tests and new domains as well as piloting of tests when the test burden is relatively light. In addition to the recommended tests for each age and domain, alternative tests are described. Assessment methodology and knowledge about child cognitive development will change over the next 20 years, and strategies are suggested for altering the proposed test schedule as appropriate
Property rights, collective action and technologies for natural resource management: a conceptual framework
Environmental management, Gender, Capacity,
CMB Likelihood Functions for Beginners and Experts
Although the broad outlines of the appropriate pipeline for cosmological
likelihood analysis with CMB data has been known for several years, only
recently have we had to contend with the full, large-scale, computationally
challenging problem involving both highly-correlated noise and extremely large
datasets (). In this talk we concentrate on the beginning and end of
this process. First, we discuss estimating the noise covariance from the data
itself in a rigorous and unbiased way; this is essentially an iterated
minimum-variance mapmaking approach. We also discuss the unbiased determination
of cosmological parameters from estimates of the power spectrum or experimental
bandpowers.Comment: Long-delayed submission. In AIP Conference Proceedings "3K Cosmology"
held in Rome, Oct 5-10, 1998, edited by Luciano Maiani, Francesco Melchiorri
and Nicola Vittorio, 343-347, New York, American Institute of Physics 199
Variational discrete variable representation for excitons on a lattice
We construct numerical basis function sets on a lattice, whose spatial
extension is scalable from single lattice sites to the continuum limit. They
allow us to compute small and large bound states with comparable, moderate
effort. Adopting concepts of discrete variable representations, a diagonal form
of the potential term is achieved through a unitary transformation to Gaussian
quadrature points. Thereby the computational effort in three dimensions scales
as the fourth instead of the sixth power of the number of basis functions along
each axis, such that it is reduced by two orders of magnitude in realistic
examples. As an improvement over standard discrete variable representations,
our construction preserves the variational principle. It allows for the
calculation of binding energies, wave functions, and excitation spectra. We use
this technique to study central-cell corrections for excitons beyond the
continuum approximation. A discussion of the mass and spectrum of the yellow
exciton series in the cuprous oxide, which does not follow the hydrogenic
Rydberg series of Mott-Wannier excitons, is given on the basis of a simple
lattice model.Comment: 12 pages, 7 figures. Final version as publishe
Theory of exciton fine structure in semiconductor quantum dots: quantum dot anisotropy and lateral electric field
Theory of exciton fine structure in semiconductor quantum dots and its
dependence on quantum dot anisotropy and external lateral electric field is
presented. The effective exciton Hamiltonian including long range electron-hole
exchange interaction is derived within the k*p effective mass approximation
(EMA). The exchange matrix elements of the Hamiltonian are expressed explicitly
in terms of electron and hole envelope functions. The matrix element
responsible for the "bright" exciton splitting is identified and analyzed. An
excitonic fine structure for a model quantum dot with quasi- two-dimensional
anisotropic harmonic oscillator (2DLAHO) confining potential is analyzed as a
function of the shape anisotropy, size and applied lateral electric field
Design of a flight qualified long-life cryocooler
A second generation Stirling cycle cryogenic refrigerator with a linear drive, magnetic bearings, and clearance seals; designed to produce 5 watts of cooling at 65 Kelvin and to meet Space Shuttle mission requirements is described. The first generation refrigerator met all performance specifications, and operated with no failure for over 12,030 hours. Meeting launch requirements necessitated improvements in the electromagnetic bearings, the radial position sensors, and in the structural design of the moving elements. Organic contamination was eliminated by the use of all metal and ceramic construction. Reductions in system input power are attained by an integral magnetic spring/motor for the displacer and by more efficient linear motors and drive electronics
The Distance to the Cygnus Loop from Hubble Space Telescope Imaging of the Primary Shock Front
We present a Hubble Space Telescope/WFPC2 narrow-band H-alpha image of a
region on the northeastern limb of the Cygnus Loop supernova remnant. This
location provides a detailed example of where the primary blast wave first
encounters the surrounding interstellar medium. The filament structure is seen
in exquisite detail in this image, which was obtained primarily as an EARLY
ACQuisition image for a follow-up spectroscopic program. We compare the HST
image to a digitized version of the POSS-I red plate to measure the proper
motion of this filament. By combining this value for the proper motion with
previous measurements of the shock velocity at this position we find that the
distance to the Cygnus Loop is 440 (+130, -100) pc, considerably smaller than
the canonical value of 770 pc. We briefly discuss the ramifications of this new
distance estimate for our understanding of this prototypical supernova remnant.Comment: 18 pages, 3 Figures (2 JPEG and one Postscript
A Second UV "Light Bulb" behind the SN 1006 Remnant
A point X-ray source located 9 arcmin northeast of the center of SN~1006 has
been spectroscopically identified as a background QSO, with a redshift of
0.335. The object is moderately bright, with magnitude V=18.3. If its
ultraviolet spectrum is typical of low-z quasars, this object will be a second
(after the Schweizer-Middleditch star) source to use for absorption
spectroscopy of material within SN 1006. Absorption spectra provide a unique
probe for unshocked ejecta within this supernova remnant, and can possibly
solve the long-standing problem of "missing" iron in the remnants of Type Ia
supernovae.Comment: 7 pages plus 3 figures: Fig 1 is postscript, Fig 2 and Fig 3 are
jpeg. Latex, uses aaspp4.st
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