8,153 research outputs found
Capability of Cherenkov Telescopes to Observe Ultra-fast Optical Flares
The large optical reflector (~ 100 m^2) of a H.E.S.S. Cherenkov telescope was
used to search for very fast optical transients of astrophysical origin. 43
hours of observations targeting stellar-mass black holes and neutron stars were
obtained using a dedicated photometer with microsecond time resolution. The
photometer consists of seven photomultiplier tube pixels: a central one to
monitor the target and a surrounding ring of six pixels to veto background
events. The light curves of all pixels were recorded continuously and were
searched offline with a matched-filtering technique for flares with a duration
of 2 us to 100 ms. As expected, many unresolved (500 us)
background events originating in the earth's atmosphere were detected. In the
time range 3 to 500 us the measurement is essentially background-free, with
only eight events detected in 43 h; five from lightning and three presumably
from a piece of space debris. The detection of flashes of brightness ~ 0.1 Jy
and only 20 us duration from the space debris shows the potential of this setup
to find rare optical flares on timescales of tens of microseconds. This
timescale corresponds to the light crossing time of stellar-mass black holes
and neutron stars.Comment: Accepted for publication in Astroparticle Physics, 8 pages, 9
figures, 1 tabl
Computer simulation of crystallization kinetics with non-Poisson distributed nuclei
The influence of non-uniform distribution of nuclei on crystallization
kinetics of amorphous materials is investigated. This case cannot be described
by the well-known Johnson-Mehl-Avrami (JMA) equation, which is only valid under
the assumption of a spatially homogeneous nucleation probability. The results
of computer simulations of crystallization kinetics with nuclei distributed
according to a cluster and a hardcore distribution are compared with JMA
kinetics. The effects of the different distributions on the so-called Avrami
exponent are shown. Furthermore, we calculate the small-angle scattering
curves of the simulated structures which can be used to distinguish
experimentally between the three nucleation models under consideration.Comment: 14 pages including 7 postscript figures, uses epsf.sty and
ioplppt.st
Easy implementable algorithm for the geometric measure of entanglement
We present an easy implementable algorithm for approximating the geometric
measure of entanglement from above. The algorithm can be applied to any
multipartite mixed state. It involves only the solution of an eigenproblem and
finding a singular value decomposition, no further numerical techniques are
needed. To provide examples, the algorithm was applied to the isotropic states
of 3 qubits and the 3-qubit XX model with external magnetic field.Comment: 9 pages, 3 figure
Electrodynamics of Media
Contains reports on two research projects.Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E)M.I.T. Sloan Fund for Basic Researc
Radiation damage in lithium-counterdoped n/p silicon solar cells
Lithium counterdoped n+/p silicon solar cells were irradiated with 1 MV electrons and their post irradiation performance and low temperature annealing properties were compared to that of the 0.35 ohm cm control cells. Cells fabricated from float zone and Czochralski grown silicon were investigated. It was found that the float zone cells exhibited superior radiation resistance compared to the control cells, while no improvement was noted for the Czochralski grown cells. Room temperature and 60 C annealing studies were conducted. The annealing was found to be a combination of first and second order kinetics for short times. It was suggested that the principal annealing mechanism was migration of lithium to a radiation induced defect with subsequent neutralization of the defect by combination with lithium. The effects of base lithium gradient were investigated. It was found that cells with negative base lithium gradients exhibited poor radiation resistance and performance compared to those with positive or no lithium gradients; the latter being preferred for overall performance and radiation resistance
A comparison of the floras of the Colchester (No. 2) Coal and Francis Creek Shale
Abundant data from spore studies of the Colchester (no. 2) Coal Member and from investigations of plant compressions in the Francis Creek Shale provide an opportunity to compare the flora of the coal with that of the overlying shale in the northeastern part of the Illinois Basin. As both floras were investigated by different methods and since different systems of form genera were used, it is first necessary to review the plant taxa found in the 2 facies and to arrange them according to major plant groups. Paleoenvironmental interpretations of Pennsylvanian floras are rare and widely scattered in the literature; therefore some of the research on fossil spores and plant assemblages from other strata is discussed in this report. Finally, the report presents an interpretation of paleoecological conditions that existed during deposition of peat and mud, which eventually formed the No. 2 Coal and Francis Creek Shale
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