2,454 research outputs found
Growth and characterization of binary and pseudo-binary 3-5 compounds exhibiting non-linear optical behavior. Undergraduate research opportunities in microgravity science and technology
In line with the specified objectives, a Bridgman-type growth configuration in which unavoidable end effects - conventionally leading to growth interface relocation - are compensated by commensurate input-power changes is developed; the growth rate on a microscale is predictable and unaffected by changes in heat transfer conditions. To permit quantitative characterization of the growth furnace cavity (hot-zone), a 3-D thermal field mapping technique, based on the thermal image, is being tested for temperatures up to 1100 C. Computational NIR absorption analysis was modified to now permit characterization of semi-insulating single crystals. Work on growth and characterization of bismuth-silicate was initiated. Growth of BSO (B12SiO20) for seed material by the Czochralski technique is currently in progress. Undergraduate research currently in progress includes: ground based measurements of the wetting behavior (contact angles) of semiconductor melts on substrates consisting of potential confinement materials for solidification experiments in a reduced gravity environment. Hardware modifications required for execution of the wetting experiments in a KC-135 facility are developed
Galaxy Tracers and Velocity Bias
This paper examines several methods of tracing galaxies in N-body simulations
and their effects on the derived galaxy statistics, especially measurements of
velocity bias. Using two simulations with identical initial conditions, one
following dark matter only and the other following dark matter and baryons,
both collisionless and collisional methods of tracing galaxies are compared to
one another and against a set of idealized criteria. None of the collisionless
methods proves satisfactory, including an elaborate scheme developed here to
circumvent previously known problems. The main problem is that galactic
overdensities are both secularly and impulsively disrupted while orbiting in
cluster potentials. With dissipation, the baryonic tracers have much higher
density contrasts and much smaller cross sections, allowing them to remain
distinct within the cluster potential. The question remains whether the
incomplete physical model introduces systematic biases. Statistical measures
determined from simulations can vary significantly based solely on the galaxy
tracing method utilized. The two point correlation function differs most on
sub-cluster scales with generally good agreement on larger scales. Pairwise
velocity dispersions show less uniformity on all scales addressed here. All
tracing methods show a velocity bias to varying degrees, but the predictions
are not firm: either the tracing method is not robust or the statistical
significance has not been demonstrated. Though theoretical arguments suggest
that a mild velocity bias should exist, simulation results are not yet
conclusive.Comment: ApJ, in press, 23 pages, plain TeX, 8 of 13 figures included, all
PostScript figures (4.8 MB) available via anonymous ftp from
ftp://astro.princeton.edu/summers/tracers . Also available as POPe-616 on
http://astro.princeton.edu/~library/prep.htm
Mass Estimates of X-Ray Clusters
We use cosmological gas dynamic simulations to investigate the accuracy of
galaxy cluster mass estimates based on X-ray observations. The experiments
follow the formation of clusters in different cosmological models and include
the effects of gravity, pressure gradients, and hydrodynamical shocks. A subset
of our ensemble also allows for feedback of mass and energy from galactic winds
into the intracluster medium. We find that mass estimates based on the
hydrostatic, isothermal beta-model are remarkably accurate when evaluated at
radii where the cluster mean density is between 500-2500 times the critical
density. Applied to 174 artificial ROSAT images constructed from the
simulations, the distribution of the estimated-to-true mass ratio is nearly
unbiased and has a standard deviation of 14-29%. The scatter can be
considerably reduced (to 8-15%) by using an alternative mass estimator that
exploits the tightness of the mass-temperature relation found in the
simulations. The improvement over beta-model estimates is due to the
elimination of the variance contributed by the gas outer slope parameter. We
discuss these findings and their implications for recent measurements of
cluster baryon fractions.Comment: TeX, 24p; 11 Postscript figs. Submitted to the Astrophysical Journa
Galaxy tracers in N-body simulations
Using the method of smoothed particle hydrodynamics, we have modeled the formation of a compact group of galaxies with sufficient resolution to trace galaxies. Radiative cooling allows the baryons to dissipate their thermal energy and collapse to overdensities characteristic of real galaxies. With their cross section greatly reduced, these galaxy tracers remain distinct during cluster formation while their dark matter halos merge. In addition, the number density, the mass distribution function, and even the morphology of these objects are similar to those of observed galaxies. A viable population of galaxy tracers can be unambiguously defined
Vibration in Planetary Gear Systems with Unequal Planet Stiffnesses
An algorithm suitable for a minicomputer was developed for finding the natural frequencies and mode shapes of a planetary gear system which has unequal stiffnesses between the Sun/planet and planet/ring gear meshes. Mode shapes are represented in the form of graphical computer output that illustrates the lateral and rotational motion of the three coaxial gears and the planet gears. This procedure permits the analysis of gear trains utilizing nonuniform mesh conditions and user specified masses, stiffnesses, and boundary conditions. Numerical integration of the equations of motion for planetary gear systems indicates that this algorithm offers an efficient means of predicting operating speeds which may result in high dynamic tooth loads
Treatment of Obesity in Mentally Retarded Persons: The Rehabilitator\u27s Role
Obesity is a common problem for the mentally retarded and nonretarded populations. Prevalence estimates ranging from 40 to 80 million obese Americans have been reported. The relationship between obesity and cardiovascular disease, diabetes mellitus, and other health related problems is strong. Also, the greater the degree of obesity, the higher the risk of medical problems. In addition to the health problems associated with obesity, the obese mentally retarded person is likely to be the object of increased social prejudice and nonacceptance as a result of being mentally retarded and obese.
Fortunately, this solution does not need to be an intractable one. Van Itallie cited studies reporting a positive influence for weight reduction on health. Another treatment goal has been enhanced self-esteem. Given these promising outcomes for weight reduction, the field of obesity has witnessed an explosion of diet programs and exercise regimes to promote weight loss. These programs have varied in their initial success but nearly all have failed to produce long-term maintenance of weight loss. The application of behavioral procedures to the problem of obesity has produced more promising results. This approach has also been successfully extended to the mentally retarded population.
This article describes the treatment rationale and procedures for a behavioral self-control package that has been developed for the obese retarded population. Implications of this approach for professionals concerned with rehabilitation efforts for mentally retarded persons will be delineated
Escape from stimuli correlated with transitions across lean and rich schedules of reinforcement
On a multiple-schedule of reinforcement, when a rich component is followed by a relatively lean component, responding is interrupted by extended pausing. The present experiment examined whether the discriminative stimulus correlated with this rich to lean transition acquired aversive properties. Four pigeons were exposed to a compound schedule with two fixed-ratio components programmed on the center key. One component ended with the delivery of a rich reinforcer (7-s access to grain) and the second with a lean reinforcer (1-s access). Each component was correlated with a distinctive key color. At the beginning of some components, a side key was activated. A single peck on this stimulus termination key replaced the discriminative stimulus on the center key with a white light regardless of whether the ongoing component was rich or lean. As in previous research, pausing was a joint function of the past and upcoming conditions of reinforcement and the FR requirement. At relatively large ratios, if the upcoming reinforcer was rich, pauses were short regardless of the past reinforcer. If the upcoming reinforcer was lean, pauses were longer and the length was directly influenced by the past reinforcer; the longest pauses were observed in the transitions from a rich to a lean reinforcer. At larger ratios, removal of the multiple-schedule stimuli occurred most often in the presence of a stimulus signaling a lean reinforcer and rarely in the presence of a stimulus signaling a rich reinforcer. The past reinforcer affected the frequency of escape for two pigeons (P822 and P830); they most often terminated the lean stimulus when it was preceded by a rich reinforcer
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