8,840 research outputs found
On the relation between the Deuteron Form Factor at High Momentum Transfer and the High Energy Neutron-Proton Scattering Amplitude
A non-relativistic potential-model version of the factorization assumption,
used in perturbative QCD calculations of hadronic form factors, is used, along
with the Born approximation valid at high energies, to derive a remarkably
simple relationship between the impulse approximation contribution to the
deuteron form factor at high momentum transfer and the high energy
neutron-proton scattering amplitude. The relation states that the form factor
at a given value of is proportional to the scattering amplitude at a
specific energy and scattering angle. This suggests that an accurate
computation of the form factors at large requires a simultaneous
description of the phase-shifts at a related energy, a statement that seems
reasonable regardless of any derivation. Our form factor-scattering amplitude
relation is shown to be accurate for some examples. However, if the potential
consists of a strong short distance repulsive term and a strong longer ranged
attractive term, as typically occurs in many realistic potentials, the relation
is found to be accurate only for ridiculously large values of . More general
arguments, using only the Schroedinger equation, suggest a strong, but
complicated, relationship between the form factor and scattering amplitude.
Furthermore, the use of recently obtained soft potentials, along with an
appropriate current operator, may allow calculations of form factors that are
consistent with the necessary phase shifts.Comment: 14 pages, 4 figures, The discussion has been extended by including
numerical examples and general argument
Concepts and analysis for precision segmented reflector and feed support structures
Several issues surrounding the design of a large (20-meter diameter) Precision Segmented Reflector are investigated. The concerns include development of a reflector support truss geometry that will permit deployment into the required doubly-curved shape without significant member strains. For deployable and erectable reflector support trusses, the reduction of structural redundancy was analyzed to achieve reduced weight and complexity for the designs. The stiffness and accuracy of such reduced member trusses, however, were found to be affected to a degree that is unexpected. The Precision Segmented Reflector designs were developed with performance requirements that represent the Reflector application. A novel deployable sunshade concept was developed, and a detailed parametric study of various feed support structural concepts was performed. The results of the detailed study reveal what may be the most desirable feed support structure geometry for Precision Segmented Reflector/Large Deployable Reflector applications
The Quiescent X-ray Spectrum of Accreting Black Holes
The quiescent state is the dominant accretion mode for black holes on all
mass scales. Our knowledge of the X-ray spectrum is limited due to the
characteristic low luminosity in this state. Herein, we present an analysis of
the sample of dynamically-confirmed stellar-mass black holes observed in
quiescence in the \textit{Chandra/XMM-Newton/Suzaku} era resulting in a sample
of 8 black holes with 570 ks of observations. In contrast to the
majority of AGN where observations are limited by contamination from diffuse
gas, the stellar-mass systems allow for a clean study of the X-ray spectrum
resulting from the accretion flow alone. The data are characterized using
simple models. We find a model consisting of a power-law or thermal
bremsstrahlung to both provide excellent descriptions of the data, where we
measure and
respectively in the 0.3 -- 10 keV bandpass, at a median luminosity of . This result in discussed in the context of our
understanding of the accretion flow onto stellar and supermassive black holes
at low luminosities.Comment: 12 pages, 5 figures, 2 tables, MNRAS accepte
A Rapidly Spinning Black Hole Powers the Einstein Cross
Observations over the past 20 years have revealed a strong relationship
between the properties of the supermassive black hole (SMBH) lying at the
center of a galaxy and the host galaxy itself. The magnitude of the spin of the
black hole will play a key role in determining the nature of this relationship.
To date, direct estimates of black hole spin have been restricted to the local
Universe. Herein, we present the results of an analysis of 0.5 Ms of
archival Chandra observations of the gravitationally lensed quasar Q 2237+305
(aka the "Einstein-cross"), lying at a redshift of z = 1.695. The boost in flux
provided by the gravitational lens allows constraints to be placed on the spin
of a black hole at such high redshift for the first time. Utilizing state of
the art relativistic disk reflection models, the black hole is found to have a
spin of at the 90% confidence level. Placing a
lower limit on the spin, we find (4). The high value of
the spin for the black hole in Q 2237+305 lends
further support to the coherent accretion scenario for black hole growth. This
is the most distant black hole for which the spin has been directly constrained
to date.Comment: 5 pages, 3 figures, 1 table, formatted using emulateapj.cls. Accepted
for publication in ApJ
SONTRAC—a scintillating plastic fiber tracking detector for neutron and proton imaging spectroscopy
SONTRAC (SOlar Neutron TRACking imager and spectrometer) is a conceptual instrument intended to measure the energy and incident direction of 20–150 MeV neutrons produced in solar flares. The intense neutron background in a low-Earth orbit requires that imaging techniques be employed to maximize an instrument’s signal-to-noise ratio. The instrument is comprised of mutually perpendicular, alternating layers of parallel, scintillating, plastic fibers that are viewed by optoelectronic devices. Two stereoscopic views of recoil proton tracks are necessary to determine the incident neutron’s direction and energy. The instrument can also be used as a powerful energetic proton imager. Data from a fully functional 3-d prototype are presented. Early results indicate that the instrument’s neutron energy resolution is approximately 10% with the neutron incident direction determined to within a few degrees
Workshop on Institutional Aspects of Proliferation Resistance
Prepared for the U.S. Dept. of Energy under Contract no. EN-77-S-02-4571.A000.
Organized by the MIT Dept. of Nuclear Engineering, PSIA, and the U.S. Dept. of Energy
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