4,475 research outputs found
Optimization of a Langmuir-Taylor detector for lithium
This paper describes the construction and optimization of a Langmuir-Taylor
detector for lithium, using a rhenium ribbon. The absolute detection
probability of this very sensitive detector is measured and the dependence of
this probability with oxygen pressure and surface temperature is studied.
Sources of background signal and their minimization are also discussed in
details. And a comparison between our data concerning the response time of the
detector and literature values is given. A theoretical analysis has been made:
this analysis supports the validity of the Saha-Langmuir law to relate the
ionization probability to the work function. Finally, the rapid variations of
the work function with oxygen pressure and temperature are explained by a
chemical equilibrium model.Comment: 11 pages, 7 figures, to appear in Rev. Sci. Instru
Random-matrix approach to the statistical compound nuclear reaction at low energies using the Monte-Carlo technique
Using a random-matrix approach and Monte-Carlo simulations, we generate
scattering matrices and cross sections for compound-nucleus reactions. In the
absence of direct reactions we compare the average cross sections with the
analytic solution given by the Gaussian Orthogonal Ensemble (GOE) triple
integral, and with predictions of statistical approaches such as the ones due
to Moldauer, to Hofmann, Richert, Tepel, and Weidenm\"{u}ller, and to Kawai,
Kerman, and McVoy. We find perfect agreement with the GOE triple integral and
display the limits of validity of the latter approaches. We establish a
criterion for the width of the energy-averaging interval such that the relative
difference between the ensemble-averaged and the energy-averaged scattering
matrices lies below a given bound. Direct reactions are simulated in terms of
an energy-independent background matrix. In that case, cross sections averaged
over the ensemble of Monte-Carlo simulations fully agree with results from the
Engelbrecht-Weidenm\"{u}ller transformation. The limits of other approximate
approaches are displayed
Dispersive Gap Mode of Phonons in Anisotropic Superconductors
We estimate the effect of the superconducting gap anisotropy in the
dispersive gap mode of phonons, which is observed by the neutron scattering on
borocarbide superconductors. We numerically analyze the phonon spectrum
considering the electron-phonon coupling, and examine contributions coming from
the gap suppression and the sign change of the pairing function on the Fermi
surface. When the sign of the pairing function is changed by the nesting
translation, the gap mode does not appear. We also discuss the suppression of
the phonon softening of the Kohn anomaly due to the onset of superconductivity.
We demonstrate that observation of the gap dispersive mode is useful for
sorting out the underlying superconducting pairing function.Comment: 7 pages, 12 figures, to be published in J. Phys. Soc. Jp
Transverse effects in multifrequency Raman generation
The theory of ultrabroadband multifrequency Raman generation is extended, for the first time, to allow for beam-propagation effects in one and two transverse dimensions. We show that a complex transverse structure develops even when diffraction is neglected. In the general case, we examine how the ultrabroadband multifrequency Raman generation process is affected by the intensity, phase quality, and width of the input beams, and by the length of the Raman medium. The evolution of power spectra, intensity profiles, and global characteristics of the multifrequency beams are investigated and explained. In the two-dimensional transverse case, bandwidths comparable to the optical carrier frequency, spanning the whole visible spectrum and beyond, are still achievable
Initial POLAR MFE observation of substorm signatures in the polar magnetosphere
This paper studies substorm influences in the polar magnetosphere using data from the POLAR magnetic field experiment (MFE). The POLAR spacecraft remains in the high altitude polar magnetosphere for extended periods around apogee. There it can stay at nearly constant altitude through all phases of a substorm, which was not possible on previous missions. We report such an event on March 28, 1996. Ground magnetometers monitored substorm activity, while the POLAR spacecraft, directly over the pole at (â0.8, â0.6, 8.5) RE in GSM coordinates, observed a corresponding perturbation in the total magnetic field strength. The total magnetic field first increased, then recovered toward quiet levels, consistent with erosion of magnetic flux from the dayside magnetosphere, followed by transport of that flux to the magnetotail, and eventual onset of tail reconnection and the return of that magnetic flux to the dayside magnetosphere
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Dielectrophoretic Manipulation of Particles and Lymphocytes
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.A particle manipulation and sorting device using the dielectrophoretic (DEP) force is described in this study. The device consists of âladder-typeâ, âflip-typeâ and âoblique rail-typeâ electrode regions. The ladder-type and rail-type electrodes can generate a DEP force distribution that captures the particles, the DEP force of which is ânegativeâ (repulsion force), in the area located at the center of the electrodes. The particles can then be guided individually along the electrode. In addition to this, the ladder-type electrode can align the particles with equal spacing in the streamwise direction. Using the âflip-typeâ electrode, which pushes the particles away, in combination with these electrodes, the direction of the particle can be selected with high accuracy, reliability and response. In the first half of this paper, numerical simulation was carried out to calculate the particle motion and evaluate the performance of the ladder-type electrode. Several models were validated to investigate the influences of the non-uniformity of the electric field and the electric interaction of the surface charges and polarizations. Measurement using the high-speed camera was then carried out to investigate the motions of the particles and sorting reliability. The trajectories and the probability density functions of the particles at the inlet and outlet of the electrode region showed that by using these electrodes the particles can be aligned, sorted and guided accurately
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