16,143 research outputs found
Manganese-56 coincidence-counting facility precisely measures neutron-source strength
Precise measurement of neutron-source strength is provided by a manganese 56 coincidence-counting facility using the manganese-bath technique. This facility combines nuclear instrumentation with coincidence-counting techniques to handle a wide variety of radioisotope-counting requirements
Approximate Range Emptiness in Constant Time and Optimal Space
This paper studies the \emph{-approximate range emptiness} problem, where the task is to represent a set of points from and answer emptiness queries of the form " ?" with a probability of \emph{false positives} allowed. This generalizes the functionality of \emph{Bloom filters} from single point queries to any interval length . Setting the false positive rate to and performing queries, Bloom filters yield a solution to this problem with space bits, false positive probability bounded by for intervals of length up to , using query time . Our first contribution is to show that the space/error trade-off cannot be improved asymptotically: Any data structure for answering approximate range emptiness queries on intervals of length up to with false positive probability , must use space bits. On the positive side we show that the query time can be improved greatly, to constant time, while matching our space lower bound up to a lower order additive term. This result is achieved through a succinct data structure for (non-approximate 1d) range emptiness/reporting queries, which may be of independent interest
Monitoring of the prompt radio emission from the unusual supernova 2004dj in NGC2403
Supernova 2004dj in the nearby spiral galaxy NGC2403 was detected optically
in July 2004. Peaking at a magnitude of 11.2, this is the brightest supernova
detected for several years. Here we present Multi-Element Radio Linked
Interferometer Network (MERLIN) observations of this source, made over a four
month period, which give a position of R.A. = 07h37m17.044s, Dec
=+65deg35'57.84" (J2000.0). We also present a well-sampled 5 GHz light curve
covering the period from 5 August to 2 December 2004. With the exception of the
unusual and very close SN 1987A, these observations represent the first
detailed radio light curve for the prompt emission from a Type II-P supernova.Comment: (1) Jodrell Bank Observatory (2) University of Valencia (3)
University of Sheffield 6 pages, 1 figure. To appear in ApJ letter
Stable and Unstable Circular Strings in Inflationary Universes
It was shown by Garriga and Vilenkin that the circular shape of nucleated
cosmic strings, of zero loop-energy in de Sitter space, is stable in the sense
that the ratio of the mean fluctuation amplitude to the loop radius is
constant. This result can be generalized to all expanding strings (of non-zero
loop-energy) in de Sitter space. In other curved spacetimes the situation,
however, may be different.
In this paper we develop a general formalism treating fluctuations around
circular strings embedded in arbitrary spatially flat FRW spacetimes. As
examples we consider Minkowski space, de Sitter space and power law expanding
universes. In the special case of power law inflation we find that in certain
cases the fluctuations grow much slower that the radius of the underlying
unperturbed circular string. The inflation of the universe thus tends to wash
out the fluctuations and to stabilize these strings.Comment: 15 pages Latex, NORDITA 94/14-
q-Deformed de Sitter/Conformal Field Theory Correspondence
Unitary principal series representations of the conformal group appear in the
dS/CFT correspondence. These are infinite dimensional irreducible
representations, without highest weights. In earlier work of Guijosa and the
author it was shown for the case of two-dimensional de Sitter, there was a
natural q-deformation of the conformal group, with q a root of unity, where the
unitary principal series representations become finite-dimensional cyclic
unitary representations. Formulating a version of the dS/CFT correspondence
using these representations can lead to a description with a finite-dimensional
Hilbert space and unitary evolution. In the present work, we generalize to the
case of quantum-deformed three-dimensional de Sitter spacetime and compute the
entanglement entropy of a quantum field across the cosmological horizon.Comment: 18 pages, 2 figures, revtex, (v2 reference added
Helium, Oxygen, Proton, and Electron (HOPE) Mass Spectrometer for the Radiation Belt Storm Probes Mission
The HOPE mass spectrometer of the Radiation Belt Storm Probes (RBSP) mission (renamed the Van Allen Probes) is designed to measure the in situ plasma ion and electron fluxes over 4Ï sr at each RBSP spacecraft within the terrestrial radiation belts. The scientific goal is to understand the underlying physical processes that govern the radiation belt structure and dynamics. Spectral measurements for both ions and electrons are acquired over 1 eV to 50 keV in 36 log-spaced steps at an energy resolution ÎE FWHM/Eâ15 %. The dominant ion species (H+, He+, and O+) of the magnetosphere are identified using foil-based time-of-flight (TOF) mass spectrometry with channel electron multiplier (CEM) detectors. Angular measurements are derived using five polar pixels coplanar with the spacecraft spin axis, and up to 16 azimuthal bins are acquired for each polar pixel over time as the spacecraft spins. Ion and electron measurements are acquired on alternate spacecraft spins. HOPE incorporates several new methods to minimize and monitor the background induced by penetrating particles in the harsh environment of the radiation belts. The absolute efficiencies of detection are continuously monitored, enabling precise, quantitative measurements of electron and ion fluxes and ion species abundances throughout the mission. We describe the engineering approaches for plasma measurements in the radiation belts and present summaries of HOPE measurement strategy and performance
Circular String-Instabilities in Curved Spacetime
We investigate the connection between curved spacetime and the emergence of
string-instabilities, following the approach developed by Loust\'{o} and
S\'{a}nchez for de Sitter and black hole spacetimes. We analyse the linearised
equations determining the comoving physical (transverse) perturbations on
circular strings embedded in Schwarzschild, Reissner-Nordstr\"{o}m and de
Sitter backgrounds. In all 3 cases we find that the "radial" perturbations grow
infinitely for (ring-collapse), while the "angular"
perturbations are bounded in this limit. For we find that
the perturbations in both physical directions (perpendicular to the string
world-sheet in 4 dimensions) blow up in the case of de Sitter space. This
confirms results recently obtained by Loust\'{o} and S\'{a}nchez who considered
perturbations around the string center of mass.Comment: 24 pages Latex + 2 figures (not included). Observatoire de Paris,
Meudon No. 9305
The Generalised Raychaudhuri Equations : Examples
Specific examples of the generalized Raychaudhuri Equations for the evolution
of deformations along families of dimensional surfaces embedded in a
background dimensional spacetime are discussed. These include string
worldsheets embedded in four dimensional spacetimes and two dimensional
timelike hypersurfaces in a three dimensional curved background. The issue of
focussing of families of surfaces is introduced and analysed in some detail.Comment: 8 pages (Revtex, Twocolumn format). Corrected(see section on string
worldsheets), reorganised and shortened slightl
A New Measurement of Cosmic Ray Composition at the Knee
The Dual Imaging Cerenkov Experiment (DICE) was designed and operated for
making elemental composition measurements of cosmic rays near the knee of the
spectrum at several PeV. Here we present the first results using this
experiment from the measurement of the average location of the depth of shower
maximum, , in the atmosphere as a function of particle energy. The value
of near the instrument threshold of ~0.1 PeV is consistent with
expectations from previous direct measurements. At higher energies there is
little change in composition up to ~5 PeV. Above this energy is deeper
than expected for a constant elemental composition implying the overall
elemental composition is becoming lighter above the knee region. These results
disagree with the idea that cosmic rays should become on average heavier above
the knee. Instead they suggest a transition to a qualitatively different
population of particles above 5 PeV.Comment: 7 pages, LaTeX, two eps figures, aas2pp4.sty and epsf.sty included,
accepted by Ap.J. Let
Focusing of timelike worldsheets in a theory of strings
An analysis of the generalised Raychaudhuri equations for string world sheets
is shown to lead to the notion of focusing of timelike worldsheets in the
classical Nambu-Goto theory of strings. The conditions under which such effects
can occur are obtained . Explicit solutions as well as the Cauchy initial value
problem are discussed. The results closely resemble their counterparts in the
theory of point particles which were obtained in the context of the analysis of
spacetime singularities in General Relativity many years ago.Comment: 14 pages, RevTex, no figures, extended, to appear in Phys Rev
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