5,124 research outputs found
Detailed Studies of Pixelated CZT Detectors Grown with the Modified Horizontal Bridgman Method
The detector material Cadmium Zinc Telluride (CZT), known for its high
resolution over a broad energy range, is produced mainly by two methods: the
Modified High-Pressure Bridgman (MHB) and the High-Pressure Bridgman (HPB)
process. This study is based on MHB CZT substrates from the company Orbotech
Medical Solutions Ltd. with a detector size of 2.0x2.0x0.5 cm^3, 8x8 pixels and
a pitch of 2.46 mm. Former studies have emphasized only on the cathode material
showing that high-work-function improve the energy resolution at lower
energies. Therfore, we studied the influence of the anode material while
keeping the cathode material constant. We used four different materials:
Indium, Titanium, Chromium and Gold with work-functions between 4.1 eV and 5.1
eV. The low work-function materials Indium and Titanium achieved the best
performance with energy resolutions: 2.0 keV (at 59 keV) and 1.9 keV (at 122
keV) for Titanium; 2.1 keV (at 59 keV) and 2.9 keV (at 122 keV) for Indium.
These detectors are very competitive compared with the more expensive ones
based on HPB material if one takes the large pixel pitch of 2.46 mm into
account. We present a detailed comparison of our detector response with 3-D
simulations, from which we determined the mobility-lifetime-products for
electrons and holes. Finally, we evaluated the temperature dependency of the
detector performance and mobility-lifetime-products, which is important for
many applications. With decreasing temperature down to -30C the breakdown
voltage increases and the electron mobility-lifetime-product decreases by about
30% over a range from 20C to -30C. This causes the energy resolution to
deteriorate, but the concomitantly increasing breakdown voltage makes it
possible to increase the applied bias voltage and restore the full performance.Comment: Accepted for publication in Astroparticle Physics, 25 pages, 13
figure
Formation of Hot Planets by a combination of planet scattering, tidal circularization, and Kozai mechanism
We have investigated the formation of close-in extrasolar giant planets
through a coupling effect of mutual scattering, Kozai mechanism, and tidal
circularization, by orbital integrations. We have carried out orbital
integrations of three planets with Jupiter-mass, directly including the effect
of tidal circularization. We have found that in about 30% runs close-in planets
are formed, which is much higher than suggested by previous studies. We have
found that Kozai mechanism by outer planets is responsible for the formation of
close-in planets. During the three-planet orbital crossing, the Kozai
excitation is repeated and the eccentricity is often increased secularly to
values close enough to unity for tidal circularization to transform the inner
planet to a close-in planet. Since a moderate eccentricity can remain for the
close-in planet, this mechanism may account for the observed close-in planets
with moderate eccentricities and without nearby secondary planets. Since these
planets also remain a broad range of orbital inclinations (even retrograde
ones), the contribution of this process would be clarified by more observations
of Rossiter-McLaughlin effects for transiting planets.Comment: 15 pages, 16 figures, Accepted for publication in Ap
Turbulence-generated proton-scale structures in the terrestrial magnetosheath
Recent results of numerical magnetohydrodynamic simulations suggest that in
collisionless space plasmas turbulence can spontaneously generate thin current
sheets. These coherent structures can partially explain intermittency and the
non-homogenous distribution of localized plasma heating in turbulence. In this
Letter Cluster multi-point observations are used to investigate the
distribution of magnetic field discontinuities and the associated small-scale
current sheets in the terrestrial magnetosheath downstream of a quasi-parallel
bow shock. It is shown experimentally, for the first time, that the strongest
turbulence generated current sheets occupy the long tails of probability
distribution functions (PDFs) associated with extremal values of magnetic field
partial derivatives. During the analyzed one hour long time interval, about a
hundred strong discontinuities, possibly proton-scale current sheets were
observed.Comment: 10 pages, 5 figures in The Astrophysical Journal Letters, Volume 819,
Number 1, 201
Developing a Compton Polarimeter to Measure Polarization of Hard X-Rays in the 50-300 keV Energy Range
This paper discusses the latest progress in the development of GRAPE
(Gamma-Ray Polarimeter Experiment), a hard X-ray Compton Polarimeter. The
purpose of GRAPE is to measure the polarization of hard X-rays in the 50-300
keV energy range. We are particularly interested in X-rays that are emitted
from solar flares and gamma-ray bursts (GRBs). Accurately measuring the
polarization of the emitted radiation from these sources will lead, to a better
understating of both the emission mechanisms and source geometries. The GRAPE
design consists of an array of plastic scintillators surrounding a central
high-Z crystal scintillator. We can monitor individual Compton scatters that
occur in the plastics and determine whether the photon is photo absorbed by the
high-Z crystal or not. A Compton scattered photon that is immediately photo
absorbed by the high-Z crystal constitutes a valid event. These valid events
provide us with the interaction locations of each incident photon and
ultimately produces a modulation pattern for the Compton scattering of the
polarized radiation. Comparing with Monte Carlo simulations of a 100% polarized
beam, the level of polarization of the measured beam can then be determined.
The complete array is mounted on a flat-panel multi-anode photomultiplier tube
(MAPMT) that can measure the deposited energies resulting from the photon
interactions. The design of the detector allows for a large field-of-view (>pi
steradian), at the same time offering the ability to be close-packed with
multiple modules in order to reduce deadspace. We plan to present in this paper
the latest laboratory results obtained from GRAPE using partially polarized
radiation sources.Comment: 10 pages; conference paper presented at the SPIE conference "UV,
X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XIV." To be
published in SPIE Conference Proceedings, vol. 589
Optimization of Single-Sided Charge-Sharing Strip Detectors
Simulation of the charge sharing properties of single-sided CZT strip detectors with small anode pads are presented. The effect of initial event size, carrier repulsion, diffusion, drift, trapping and detrapping are considered. These simulations indicate that such a detector with a 150 µm pitch will provide good charge sharing between neighboring pads. This is supported by a comparison of simulations and measurements for a similar detector with a coarser pitch of 225 µm that could not provide sufficient sharing. The performance of such a detector used as a gamma-ray imager is discussed
Assembly of the Auditory Circuitry by a Hox Genetic Network in the Mouse Brainstem
Rhombomeres (r) contribute to brainstem auditory nuclei during development. Hox genes are determinants of rhombomere-derived fate and neuronal connectivity. Little is known about the contribution of individual rhombomeres and their associated Hox codes to auditory sensorimotor circuitry. Here, we show that r4 contributes to functionally linked sensory and motor components, including the ventral nucleus of lateral lemniscus, posterior ventral cochlear nuclei (VCN), and motor olivocochlear neurons. Assembly of the r4-derived auditory components is involved in sound perception and depends on regulatory interactions between Hoxb1 and Hoxb2. Indeed, in Hoxb1 and Hoxb2 mutant mice the transmission of low-level auditory stimuli is lost, resulting in hearing impairments. On the other hand, Hoxa2 regulates the Rig1 axon guidance receptor and controls contralateral projections from the anterior VCN to the medial nucleus of the trapezoid body, a circuit involved in sound localization. Thus, individual rhombomeres and their associated Hox codes control the assembly of distinct functionally segregated sub-circuits in the developing auditory brainstem
HATS-1b: The First Transiting Planet Discovered by the HATSouth Survey
We report the discovery of HATS-1b, a transiting extrasolar planet orbiting
the moderately bright V=12.05 G dwarf star GSC 6652-00186, and the first planet
discovered by HATSouth, a global network of autonomous wide-field telescopes.
HATS-1b has a period P~3.4465 d, mass Mp~1.86MJ, and radius Rp~1.30RJ. The host
star has a mass of 0.99Msun, and radius of 1.04Rsun. The discovery light curve
of HATS-1b has near continuous coverage over several multi-day periods,
demonstrating the power of using a global network of telescopes to discover
transiting planets.Comment: Submitted to AJ 10 pages, 5 figures, 6 table
Global existence problem in -Gowdy symmetric IIB superstring cosmology
We show global existence theorems for Gowdy symmetric spacetimes with type
IIB stringy matter. The areal and constant mean curvature time coordinates are
used. Before coming to that, it is shown that a wave map describes the
evolution of this system
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