4,319 research outputs found
Optical Properties of the DIRC Fused Silica Cherenkov Radiator
The DIRC is a new type of Cherenkov detector that is successfully operating
as the hadronic particle identification system for the BABAR experiment at
SLAC. The fused silica bars that serve as the DIRC's Cherenkov radiators must
transmit the light over long optical pathlengths with a large number of
internal reflections. This imposes a number of stringent and novel requirements
on the bar properties. This note summarizes a large amount of R&D that was
performed both to develop specifications and production methods and to
determine whether commercially produced bars could meet the requirements. One
of the major outcomes of this R&D work is an understanding of methods to select
radiation hard and optically uniform fused silica material. Others include
measurement of the wavelength dependency of the internal reflection
coefficient, and its sensitivity to surface contaminants, development of
radiator support methods, and selection of good optical glue.Comment: 36 pages, submitted to Nuclear Instruments and Methods
Temperature-dependent properties of the magnetic order in single-crystal BiFeO3
We report neutron diffraction and magnetization studies of the magnetic order
in multiferroic BiFeO3. In ferroelectric monodomain single crystals, there are
three magnetic cycloidal domains with propagation vectors equivalent by
crystallographic symmetry. The cycloid period slowly grows with increasing
temperature. The magnetic domain populations do not change with temperature
except in the close vicinity of the N{\P}eel temperature, at which, in
addition, a small jump in magneti- zation is observed. No evidence for the
spin-reorientation transitions proposed in previous Raman and dielectric
studies is found. The magnetic cycloid is slightly anharmonic for T=5 K. The
an- harmonicity is much smaller than previously reported in NMR studies. At
room temperature, a circular cycloid is observed, within errors. We argue that
the observed anharmonicity provides important clues for understanding
electromagnons in BiFeO3.Comment: In Press at PR
Diffusion versus linear ballistic accumulation: different models but the same conclusions about psychological processes?
Quantitative models for response time and accuracy are increasingly used as tools to draw conclusions about psychological processes. Here we investigate the extent to which these substantive conclusions depend on whether researchers use the Ratcliff diffusion model or the Linear Ballistic Accumulator model. Simulations show that the models agree on the effects of changes in the rate of information accumulation and changes in non-decision time, but that they disagree on the effects of changes in response caution. In fits to empirical data, however, the models tend to agree closely on the effects of an experimental manipulation of response caution. We discuss the implications of these conflicting results, concluding that real manipulations of caution map closely, but not perfectly to response caution in either model. Importantly, we conclude that inferences about psychological processes made from real data are unlikely to depend on the model that is used
Short-range incommensurate magnetic order near the superconducting phase boundary in Fe(1+d)Te(1-x)Se(x)
We performed elastic neutron scattering and magnetization measurements on
Fe(1.07)Te(0.75)Se(0.25) and FeTe(0.7)Se(0.3). Short-range incommensurate
magnetic order is observed in both samples. In the former sample with higher Fe
content, a broad magnetic peak appears around (0.46,0,0.5) at low temperature,
while in FeTe(0.7)Se(0.3) the broad magnetic peak is found to be closer to the
antiferromagnetic (AFM) wave-vector (0.5,0,0.5). The incommensurate peaks are
only observed on one side of the AFM wave-vector for both samples, which can be
modeled in terms of an imbalance of ferromagnetic/antiferromagnetic
correlations between nearest-neighbor spins. We also find that with higher Se
(and lower Fe) concentration, the magnetic order becomes weaker while the
superconducting temperature and volume increase.Comment: Version as appeared in PR
Structural Anomalies at the Magnetic and Ferroelectric Transitions in (R=Tb, Dy, Ho)
Strong anomalies of the thermal expansion coefficients at the magnetic and
ferroelectric transitions have been detected in multiferroic . Their
correlation with anomalies of the specific heat and the dielectric constant is
discussed. The results provide evidence for the magnetic origin of the
ferroelectricity mediated by strong spin-lattice coupling in the compounds.
Neutron scattering data for indicate a spin reorientation at the
two low-temperature phase transitions
Legacy Vehicle Fuel System Testing with Intermediate Ethanol Blends
The effects of E10 and E17 on legacy fuel system components from three common mid-1990s vintage vehicle models (Ford, GM, and Toyota) were studied. The fuel systems comprised a fuel sending unit with pump, a fuel rail and integrated pressure regulator, and the fuel injectors. The fuel system components were characterized and then installed and tested in sample aging test rigs to simulate the exposure and operation of the fuel system components in an operating vehicle. The fuel injectors were cycled with varying pulse widths during pump operation. Operational performance, such as fuel flow and pressure, was monitored during the aging tests. Both of the Toyota fuel pumps demonstrated some degradation in performance during testing. Six injectors were tested in each aging rig. The Ford and GM injectors showed little change over the aging tests. Overall, based on the results of both the fuel pump testing and the fuel injector testing, no major failures were observed that could be attributed to E17 exposure. The unknown fuel component histories add a large uncertainty to the aging tests. Acquiring fuel system components from operational legacy vehicles would reduce the uncertainty
Neel to Spin-Glass-like Phase Transition versus Dilution in Geometrically Frustrated ZnCr_{2-2x}Ga_{2x}O_4
ZnCr2O4 undergoes a first order spin-Peierls-like phase transition at 12.5 K
from a cubic spin liquid phase to a tetragonal Neel state. Using powder
diffraction and single crystal polarized neutron scattering, we determined the
complex spin structure of the Neel phase. This phase consisted of several
magnetic domains with different characteristic wave vectors. This indicates
that the tetragonal phase of ZnCr2O4 is very close to a critical point
surrounded by many different Neel states. We have also studied, using elastic
and inelastic neutron scattering techniques, the effect of nonmagnetic dilution
on magnetic correlations in ZnCr_{2-2x}Ga_{2x}O_4 (x=0.05 and 0.3). For x=0.05,
the magnetic correlations do not change qualitatively from those in the pure
material, except that the phase transition becomes second order. For x= 0.3,
the spin-spin correlations become short range. Interestingly, the spatial
correlations of the frozen spins in the x=0.3 material are the same as those of
the fluctuating moments in the pure and the weakly diluted materials
Pseudorehearsal in value function approximation
Catastrophic forgetting is of special importance in reinforcement learning,
as the data distribution is generally non-stationary over time. We study and
compare several pseudorehearsal approaches for Q-learning with function
approximation in a pole balancing task. We have found that pseudorehearsal
seems to assist learning even in such very simple problems, given proper
initialization of the rehearsal parameters
The spatial distribution of coronae on Venus
Coronae on Venus are large, generally circular surface features that have distinctive tectonic, volcanic, and topographic expressions. They range in diameter from less than 200 km to at least 1000 km. Data from the Magellan spacecraft have now allowed complete global mapping of the spatial distribution of coronae on the planet. Unlike impact craters, which show a random (i.e., Poisson) spatial distribution, the distribution of coronae appears to be nonrandom. We investigate the distribution here in detail, and explore its implications in terms of mantle convection and surface modification processes
- …