263,097 research outputs found
Query-Efficient Locally Decodable Codes of Subexponential Length
We develop the algebraic theory behind the constructions of Yekhanin (2008)
and Efremenko (2009), in an attempt to understand the ``algebraic niceness''
phenomenon in . We show that every integer ,
where , and are prime, possesses the same good algebraic property as
that allows savings in query complexity. We identify 50 numbers of this
form by computer search, which together with 511, are then applied to gain
improvements on query complexity via Itoh and Suzuki's composition method. More
precisely, we construct a -query LDC for every positive
integer and a -query
LDC for every integer , both of length , improving the
queries used by Efremenko (2009) and queries used by Itoh and
Suzuki (2010).
We also obtain new efficient private information retrieval (PIR) schemes from
the new query-efficient LDCs.Comment: to appear in Computational Complexit
A Combined Compton and Coded-aperture Telescope for Medium-energy Gamma-ray Astrophysics
A future mission in medium-energy gamma-ray astrophysics would allow for many
scientific advancements, e.g. a possible explanation for the excess positron
emission from the Galactic Center, a better understanding of nucleosynthesis
and explosion mechanisms in Type Ia supernovae, and a look at the physical
forces at play in compact objects such as black holes and neutron stars.
Additionally, further observation in this energy regime would significantly
extend the search parameter space for low-mass dark matter. In order to achieve
these objectives, an instrument with good energy resolution, good angular
resolution, and high sensitivity is required. In this paper we present the
design and simulation of a Compton telescope consisting of cubic-centimeter
Cadmium Zinc Telluride (CdZnTe) detectors as absorbers behind a silicon tracker
with the addition of a passive coded mask. The goal of the design was to create
a very sensitive instrument that is capable of high angular resolution. The
simulated telescope showed achievable energy resolutions of 1.68 FWHM at
511 keV and 1.11 at 1809 keV, on-axis angular resolutions in Compton mode
of 2.63 FWHM at 511 keV and 1.30 FWHM at 1809 keV, and is
capable of resolving sources to at least 0.2 at lower energies with
the use of the coded mask. An initial assessment of the instrument in Compton
imaging mode yields an effective area of 183 cm at 511 keV and an
anticipated all-sky sensitivity of 3.6 x 10 photons cm s
for a broadened 511 keV source over a 2-year observation time. Additionally,
combining a coded mask with a Compton imager to improve point source
localization for positron detection has been demonstrated
The Resolved Asteroid Program - Size, shape, and pole of (52) Europa
With the adaptive optics (AO) system on the 10 m Keck-II telescope, we
acquired a high quality set of 84 images at 14 epochs of asteroid (52) Europa
on 2005 January 20. The epochs covered its rotation period and, by following
its changing shape and orientation on the plane of sky, we obtained its
triaxial ellipsoid dimensions and spin pole location. An independent
determination from images at three epochs obtained in 2007 is in good agreement
with these results. By combining these two data sets, along with a single epoch
data set obtained in 2003, we have derived a global fit for (52) Europa of
diameters (379x330x249) +/- (16x8x10) km, yielding a volume-equivalent
spherical-diameter of 315 +/- 7 km, and a rotational pole within 7 deg of [RA;
Dec] = [257,+12] in an Equatorial J2000 reference frame (ECJ2000: 255,+35).
Using the average of all mass determinations available forEuropa, we derive a
density of 1.5 +/- 0.4, typical of C-type asteroids. Comparing our images with
the shape model of Michalowski et al. (A&A 416, 2004), derived from optical
lightcurves, illustrates excellent agreement, although several edge features
visible in the images are not rendered by the model. We therefore derived a
complete 3-D description of Europa's shape using the KOALA algorithm by
combining our imaging epochs with 4 stellar occultations and 49 lightcurves. We
use this 3-D shape model to assess these departures from ellipsoidal shape.
Flat facets (possible giant craters) appear to be less distinct on (52) Europa
than on other C-types that have been imaged in detail. We show that fewer giant
craters, or smaller craters, is consistent with its expected impact history.
Overall, asteroid (52) Europa is still well modeled as a smooth triaxial
ellipsoid with dimensions constrained by observations obtained over several
apparitions.Comment: Accepted for publication in Icaru
Effectiveness of appropriately trained nurses in preoperative assessment: randomised controlled equivalence/non-inferiority trial
Objective
To determine whether preoperative assessments carried out by appropriately trained nurses are inferior in quality to those carried out by preregistration house officers.
Design
Randomised controlled equivalence/non-inferiority trial. Setting Four NHS hospitals in three trusts. Three of the four were teaching hospitals.
Participants
All patients attending for assessment before general anaesthesia for general, vascular, urological, or breast surgery between April 1998 and March 1999.
Intervention
Assessment by one of three appropriately trained nurses or by one of several preregistration house officers.
Main outcome measures
History taken, physical examination, and investigations ordered. Measures evaluated by a specialist registrar in anaesthetics and placed in four categories: correct, overassessment, underassessment not affecting management, and underassessment possibly affecting management (primary outcome).
Results
1907 patients were randomised, and 1874 completed the study; 926 were assessed by house officers and 948 by nurses. Overall 121/948 (13%) assessments carried out by nurses were judged to have possibly affected management compared with 138/926 (15%) of those performed by house officers. Nurses were judged to be non-inferior to house officers in assessment, although there was variation among them in terms of the quality of history taking. The house officers ordered considerably more unnecessary tests than the nurses (218/926 (24%) v 129/948 (14%).
Conclusions
There is no reason to inhibit the development of nurse led preoperative assessment provided that the nurses involved receive adequate training. However, house officers will continue to require experience in preoperative assessment
Diffuse Galactic antimatter from faint thermonuclear supernovae in old stellar populations
Our Galaxy hosts the annihilation of a few low-energy
positrons every second. Radioactive isotopes capable of supplying such
positrons are synthesised in stars, stellar remnants, and supernovae. For
decades, however, there has been no positive identification of a main stellar
positron source leading to suggestions that many positrons originate from
exotic sources like the Galaxy's central super-massive black hole or dark
matter annihilation. %, but such sources would not explain the
recently-detected positron signal from the extended Galactic disk. Here we show
that a single type of transient source, deriving from stellar populations of
age 3-6 Gyr and yielding ~0.03 of the positron emitter Ti, can
simultaneously explain the strength and morphology of the Galactic positron
annihilation signal and the solar system abundance of the Ti decay
product Ca. This transient is likely the merger of two low-mass white
dwarfs, observed in external galaxies as the sub-luminous, thermonuclear
supernova known as SN1991bg-like.Comment: 28 pages main text with 4 figures in preprint style; 26 pages of
Supplementary Informatio
Developing a second generation Laue lens prototype: high reflectivity crystals and accurate assembly
Laue lenses are an emerging technology that will enhance gamma-ray telescope
sensitivity by one to two orders of magnitude in selected energy bands of the
\sim 100 keV to \sim 1.5 MeV range. This optic would be particularly well
adapted to the observation of faint gamma ray lines, as required for the study
of Supernovae and Galactic positron annihilation. It could also prove very
useful for the study of hard X-ray tails from a variety of compact objects,
especially making a difference by providing sufficient sensitivity for
polarization to be measured by the focal plane detector. Our group has been
addressing the two key issues relevant to improve performance with respect to
the first generation of Laue lens prototypes: obtaining large numbers of
efficient crystals and developing a method to fix them with accurate
orientation and dense packing factor onto a substrate. We present preliminary
results of an on-going study aiming to enable a large number of crystals
suitable for diffraction at energies above 500 keV. In addition, we show the
first results of the Laue lens prototype assembled using our beamline at SSL/UC
Berkeley, which demonstrates our ability to orient and glue crystals with
accuracy of a few arcsec, as required for an efficient Laue lens telescope.Comment: Published in the proceedings of the SPIE conference held in San Diego
in August 201
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