12,519 research outputs found
Method and apparatus for precision control of radiometer
A radiometer controller of a radiation detector is provided with a calibration method and apparatus comprised of mounting all temperature sensitive elements of the controller in thermostatically controlled ovens during calibration and measurements, using a selected temperature that is above any which might be reached in the field. The instrument is calibrated in situ by adjusting heater power (EI) to the receptor cavity in the radiometer detector to a predetermined full scale level and is displayed by a meter
Evolution of the bilayer nu = 1 quantum Hall state under charge imbalance
We use high-mobility bilayer hole systems with negligible tunneling to
examine how the bilayer nu = 1 quantum Hall state evolves as charge is
transferred from one layer to the other at constant total density. We map
bilayer nu = 1 state stability versus imbalance for five total densities
spanning the range from strongly interlayer coherent to incoherent. We observe
competition between single-layer correlations and interlayer coherence. Most
significantly, we find that bilayer systems that are incoherent at balance can
develop spontaneous interlayer coherence with imbalance, in agreement with
recent theoretical predictions.Comment: 4 pages, 4 figure
Association of molecules using a resonantly modulated magnetic field
We study the process of associating molecules from atomic gases using a
magnetic field modulation that is resonant with the molecular binding energy.
We show that maximal conversion is obtained by optimising the amplitude and
frequency of the modulation for the particular temperature and density of the
gas. For small modulation amplitudes, resonant coupling of an unbound atom pair
to a molecule occurs at a modulation frequency corresponding to the sum of the
molecular binding energy and the relative kinetic energy of the atom pair. An
atom pair with an off-resonant energy has a probability of association which
oscillates with a frequency and time-varying amplitude which are primarily
dependent on its detuning. Increasing the amplitude of the modulation tends to
result in less energetic atom pairs being resonantly coupled to the molecular
state, and also alters the dynamics of the transfer from continuum states with
off-resonant energies. This leads to maxima and minima in the total conversion
from the gas as a function of the modulation amplitude. Increasing the
temperature of the gas leads to an increase in the modulation frequency
providing the best fit to the thermal distribution, and weakens the resonant
frequency dependence of the conversion. Mean-field effects can alter the
optimal modulation frequency and lead to the excitation of higher modes. Our
simulations predict that resonant association can be effective for binding
energies of order MHz.Comment: 8 pages latex, figures revised, references updated and typos
correcte
Surfactant-induced migration of a spherical drop in Stokes flow
In Stokes flows, symmetry considerations dictate that a neutrally-buoyant
spherical particle will not migrate laterally with respect to the local flow
direction. We show that a loss of symmetry due to flow-induced surfactant
redistribution leads to cross-stream drift of a spherical drop in Poiseuille
flow. We derive analytical expressions for the migration velocity in the limit
of small non-uniformities in the surfactant distribution, corresponding to
weak-flow conditions or a high-viscosity drop. The analysis predicts that the
direction of migration is always towards the flow centerline.Comment: Significant extension with additional text, figures, equations, et
Radio-frequency dressing of multiple Feshbach resonances
We demonstrate and theoretically analyze the dressing of several proximate
Feshbach resonances in Rb-87 using radio-frequency (rf) radiation. We present
accurate measurements and characterizations of the resonances, and the dramatic
changes in scattering properties that can arise through the rf dressing. Our
scattering theory analysis yields quantitative agreement with the experimental
data. We also present a simple interpretation of our results in terms of
rf-coupled bound states interacting with the collision threshold.Comment: 4+ pages, 3 figures, 1 table; revised introduction & references to
reflect published versio
Simulations of a Scintillator Compton Gamma Imager for Safety and Security
We are designing an all-scintillator Compton gamma imager for use in security
investigations and remediation actions involving radioactive threat material.
To satisfy requirements for a rugged and portable instrument, we have chosen
solid scintillator for the active volumes of both the scatter and absorber
detectors. Using the BEAMnrc/EGSnrc Monte Carlo simulation package, we have
constructed models using four different materials for the scatter detector:
LaBr_3, NaI, CaF_2 and PVT. We have compared the detector performances using
angular resolution, efficiency, and image resolution. We find that while PVT
provides worse performance than that of the detectors based entirely on
inorganic scintillators, all of the materials investigated for the scatter
detector have the potential to provide performance adequate for our purposes.Comment: Revised text and figures, Presented at SORMA West 2008, Published in
IEEE Transactions on Nuclear Scienc
Spin Susceptibility and Gap Structure of the Fractional-Statistics Gas
This paper establishes and tests procedures which can determine the electron
energy gap of the high-temperature superconductors using the model
with spinon and holon quasiparticles obeying fractional statistics. A simpler
problem with similar physics, the spin susceptibility spectrum of the spin 1/2
fractional-statistics gas, is studied. Interactions with the density
oscillations of the system substantially decrease the spin gap to a value of
, much less than the mean-field value of
. The lower few Landau levels remain visible, though broadened
and shifted, in the spin susceptibility. As a check of the methods, the
single-particle Green's function of the non-interacting Bose gas viewed in the
fermionic representation, as computed by the same approximation scheme, agrees
well with the exact results. The same mechanism would reduce the gap of the
model without eliminating it.Comment: 35 pages, written in REVTeX, 16 figures available upon request from
[email protected]
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