498 research outputs found
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
Correction for losses in optical birefringent networks, a concept
Technique determines the effects of losses upon the performance of a birefringent network and shows how the desired amplitude transmittance of the network may be corrected /or predistorted/, prior to synthesizing the birefringent network, to prevent the effects of crystal losses
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Germanium-Based Detectors for Gamma-Ray Imaging andSpectroscopy
Germanium-based detectors are the standard technology usedfor gamma-ray spectroscopy when high efficiency and excellent energyresolution are desired. By dividing the electrical contacts on thesedetectors into segments, the locations of the gamma-ray interactionevents within the detectors can be determined as well as the depositedenergies. This enables simultaneous gamma-ray imaging and spectroscopyand leads to applications in the areas of astronomy, nuclear physics,environmental remediation, nuclear nonproliferation, and homelandsecurity. Producing the fine-pitched electrode segmentation oftenrequired for imaging has been problematic in the past. To address thisissue, we have developed an amorphous-semiconductor contact technology.Using this technology, fully passivated detectors with closely spacedcontacts can be produced using a simple fabrication process. The currentstate of the amorphous-semiconductor contact technology and thechallenges that remain will be given in this paper
Bark Beetle and Wood Borer Infestation in the Greater Yellowstone Area During Four Postfire Years
Extensive surveys of bark beetles and wood bores in the Greater Yellowstone area were conducted in 1991 through 1993. The study objectives were to determine the effect of delayed tree mortality following the 1988 fires on mosaics of fire-killed and green tree stands, the relationship between fire injury and subsequent infestation, and the effect of insect buildup in fire injured trees on infestation rates for uninjured trees. Surveys were conducted adjacent to roads, and plots wee selected randomly. In 1991, 321 plots were measured, 198 plots in 1992, and 127 plots in 1993. Insects killed 12.6 percent of the Douglas-fir, 17.9 percent of the lodgepole pine, 6.6 percent of the Engelmann spruce, 7.5 percent of the subalpine fire, and 2.8 percent of the whitebark pine. Delayed mortality attributed to fire injury accounted for more mortality than insects. Both types of mortality greatly altered the original fire-killed/green tree mosaics that were apparent immediately after the 1988 fires. Insect infestation was strongly and positively correlated with the percent of the basal circumference of the tree that was fire killed in the species, except in Engelmann spruce where infestation peaked in the middle fire-injury class. Infestation in Douglas-fir, lodgepole pine, and Engelmann spruce increased through 1992 then declined in 1993. Although it cannot be said with certainty that insects built up in fire-injured trees and then caused increased infestation of uninjured trees, the high level of infestation suggests this is the case
Charge Solitons in 1-D Arrays of Serially Coupled Josephson Junctions
We study a 1-D array of Josephson coupled superconducting grains with kinetic
inductance which dominates over the Josephson inductance. In this limit the
dynamics of excess Cooper pairs in the array is described in terms of charge
solitons, created by polarization of the grains. We analyze the dynamics of
these topological excitations, which are dual to the fluxons in a long
Josephson junction, using the continuum sine-Gordon model. We find that their
classical relativistic motion leads to saturation branches in the I-V
characteristic of the array. We then discuss the semi-classical quantization of
the charge soliton, and show that it is consistent with the large kinetic
inductance of the array. We study the dynamics of a quantum charge soliton in a
ring-shaped array biased by an external flux through its center. If the
dephasing length of the quantum charge soliton is larger than the circumference
of the array, quantum phenomena like persistent current and coherent current
oscillations are expected. As the characteristic width of the charge soliton is
of the order of 100 microns, it is a macroscopic quantum object. We discuss the
dephasing mechanisms which can suppress the quantum behaviour of the charge
soliton.Comment: 26 pages, LaTex, 7 Postscript figure
Outbreak of Marburg hemorrhagic fever among miners in Kamwenge and Ibanda Districts, Uganda, 2007
Marburg hemorrhagic fever was detected among 4 miners in Ibanda District, Uganda, from June through September, 2007. Infection was likely acquired through exposure to bats or bat secretions in a mine in Kamwenge District, Uganda, and possibly human-to-human transmission between some patients. We describe the epidemiologic investigation and the health education response
Fluctuation theorem for currents and Schnakenberg network theory
A fluctuation theorem is proved for the macroscopic currents of a system in a
nonequilibrium steady state, by using Schnakenberg network theory. The theorem
can be applied, in particular, in reaction systems where the affinities or
thermodynamic forces are defined globally in terms of the cycles of the graph
associated with the stochastic process describing the time evolution.Comment: new version : 16 pages, 1 figure, to be published in Journal of
Statistical Physic
On Uniqueness of the Jump Process in Quantum Measurement Theory
We prove that, contrary to the standard quantum theory of continuous
observation, in the formalism of Event Enhanced Quantum Theory the stochastic
process generating individual sample histories of pairs (observed quantum
system, observing classical apparatus) is unique. This result gives a rigorous
basis to the previous heuristic argument of Blanchard and Jadczyk. Possible
implications of this result are discussed.Comment: 31 pages, LaTeX, article; e-mail contact [email protected]
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Demonstration of the effectiveness of basal area cutting to reduce tree killing by the mountain pine beetle in ponderosa pine, Crow and Northern Cheyenne Indian Reservations, Montana, 1984 : establishment report
During 1984, a demonstration project was established on the Crow and Northern Cheyenne Indian Reservations to determine the effectiveness of basal area cutting in second-growth ponderosa pine stands to reduce losses to the mountain pine beetle. This project will help develop management strategies for susceptible stands in eastern Montana. Four treatments, replicated twice on each Reservation, will be implemented: reduction of existing basal area through partial-cutting to 50, 65, and 80 square feet/acre and no-cutting (control). After treatment, each block will be monitored yearly for the first 5 years, then at 5-year intervals for 20 years
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