174,872 research outputs found
Trends in Black-White Test-Score Differentials
Until the 1970s, there were few signs of change in the historic difference of one standard deviation between average ability or achievement test scores of blacks and whites in the United States. From about 1970 to the mid- to late 1980s, there was a substantial convergence of the average achievement test scores of black and white youth; however, from the mid- to late 1980s to 1992, test scores began to diverge again. Although we place the greatest weight on data from the National Assessment of Educational Progress (NAEP), the convergence also appeared in other test series. Herrnstein and Murray's highly visible work, The Bell Curve, stands almost alone in minimizing the importance of the convergent trend. We also find a longer-term trend of convergence between the verbal abilities of blacks and whites in data from the General Social Survey (GSS), which covers adult cohorts born since 1909.
Systematic study of the symmetry energy coefficient in finite nuclei
The symmetry energy coefficients in finite nuclei have been studied
systematically with a covariant density functional theory (DFT) and compared
with the values calculated using several available mass tables. Due to the
contamination of shell effect, the nuclear symmetry energy coefficients
extracted from the binding energies have large fluctuations around the nuclei
with double magic numbers. The size of this contamination is shown to be
smaller for the nuclei with larger isospin value. After subtracting the shell
effect with the Strutinsky method, the obtained nuclear symmetry energy
coefficients with different isospin values are shown to decrease smoothly with
the mass number and are subsequently fitted to the relation . The resultant volume and
surface coefficients from axially deformed covariant DFT calculations are
and MeV respectively. The ratio is in good
agreement with the value derived from the previous calculations with the
non-relativistic Skyrme energy functionals. The coefficients and
corresponding to several available mass tables are also extracted. It is shown
that there is a strong linear correlation between the volume and surface
coefficients and the ratios are in between for all
the cases.Comment: 16 pages, 6 figure
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A New Layer Casting System for Ceramic Laser Rapid Prototyping Apparatus
In the existing Ceramic Laser Fusion system, slurry is fed on the high
temperature surface of the green part; therefore, a part of water infiltrates into the
green block and vaporizes before the process of layer casting. As a result, the slurry
viscosity rises gradually; the quality of the layer surface is not uniform, and the green
part density is uneven. The aim of present study is to develop a new layer casting
system which can solve the problems mentioned above to obtain a green part with
uniform surface quality and density, and to shorten the time-taken of part fabrication.
The first part of the paper illustrates the major requirements and parameters of a slurry
distributor; the second part describes the integration of the slurry feeding device and
layer casting system. The integrated system can feed slurry and cast thin layer
simultaneously; consequently, the drawbacks of the existing system can be eliminated
and the time-taken of the layer casting can be shortened. A variable-frequency drive
(inverter) is used to control the motor speed. The relation between the frequency and
the slurry delivery can be included in the process control program to adjust the
quantity in accordance with the layer thickness; hence, the waste of the slurry can be
reduced.Mechanical Engineerin
Evaluation of Target Date Funds
Target date funds are an emerging class of investment products, designed for retirement savings. The project considered methodologies for ranking such funds
A Study of Anyon Statistics by Breit Hamiltonian Formalism
We study the anyon statistics of a dimensional Maxwell-Chern-Simons
(MCS) gauge theory by using a systemmetic metheod, the Breit Hamiltonian
formalism.Comment: 25 pages, LATE
Two-dimensional electron-gas actuation and transduction for GaAs nanoelectromechanical systems
We have fabricated doubly clamped beams from GaAs/AlGaAs quantum-well heterostructures containing a high-mobility two-dimensional electron gas (2DEG). Applying an rf drive to in-plane side gates excites the beam's mechanical resonance through a dipole–dipole mechanism. Sensitive high-frequency displacement transduction is achieved by measuring the ac emf developed across the 2DEG in the presence of a constant dc sense current. The high mobility of the incorporated 2DEG provides low-noise, low-power, and high-gain electromechanical displacement sensing through combined piezoelectric and piezoresistive mechanisms
Decay Modes of the Hoyle State in
Recent experimental results give an upper limit less than 0.043\% (95\% C.L.)
to the direct decay of the Hoyle state into 3 respect to the sequential
decay into {Be}+. We performed one and two-dimensional tunneling
calculations to estimate such a ratio and found it to be more than one order of
magnitude smaller than experiment depending on the range of the nuclear force.
This is within high statistics experimental capabilities. Our results can also
be tested by measuring the decay modes of high excitation energy states of
C where the ratio of direct to sequential decay might reach 10\% at
(C)=10.3 MeV. The link between a Bose Einstein Condensate (BEC) and
the direct decay of the Hoyle state is also addressed. We discuss a
hypothetical `Efimov state' at (C)=7.458 MeV, which would mainly
{\it sequentially} decay with 3 of {\it equal energies}: a
counterintuitive result of tunneling. Such a state, if it would exist, is at
least 8 orders of magnitude less probable than the Hoyle's, thus below the
sensitivity of recent and past experiments.Comment: 6 pages, 2 figures, accepted by Phys. Lett.
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