607 research outputs found
MEASUREMENT OF AVERAGE NEUTRON ENERGIES FOR (,n) NEUTRON SOURCES
A method is presented for measuring the average energy of the neutrons from a source. The attenuation of the neutrons by polyethylene is measured by the use of a long counter in good geometry. The attenuation length is a sensitive function of the neutron energy. The average neutron energies from several ( alpha ,n) sources were measwed and agree well with values obtained by other techniques. (auth
Observational Similarities and Potential Connections Between Luminous Ultrasoft NLS1s and BALQSOs
Luminous ultrasoft NLS1s and low-ionization BALQSOs share many properties,
and they both represent important extremes of the active galaxy phenomenon. We
briefly discuss their observational similarities as well as potential physical
connections between them, concentrating on the X-ray point of view. We present
several ways by which potential connections might be further tested.Comment: Invited talk presented at the Joint MPE,AIP,ESO workshop on NLS1s,
Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at
http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
Constraining the variation of the coupling constants with big bang nucleosynthesis
We consider the possibility of the coupling constants of the gauge interactions at the time of big bang nucleosynthesis
having taken different values from what we measure at present, and investigate
the allowed difference requiring the shift in the coupling constants not
violate the successful calculation of the primordial abundances of the light
elements. We vary gauge couplings and Yukawa couplings (fermion masses) using a
model in which their relative variations are governed by a single scalar field,
dilaton, as found in string theory. The results include a limit on the fine
structure constant
, which is
two orders stricter than the limit obtained by considering the variation of
alone.Comment: 7 page
Scaling Behavior of Anomalous Hall Effect and Longitudinal Nonlinear Response in High-Tc Superconductors
Based on existing theoretical model and by considering our longitudinal
nonlinear response function, we derive a nonliear equation in which the mixed
state Hall resistivity can be expressed as an analytical function of magnetic
field, temperature and applied current. This equation enables one to compare
quantitatively the experimental data with theoretical model. We also find some
new scaling relations of the temperature and field dependency of Hall
resistivity. The comparison between our theoretical curves and experimental
data shows a fair agreement.Comment: 4 pages, 3 figure
Hall-conductivity sign change and fluctuations in amorphous NbGe films
The sign change in the Hall conductivity has been studied in thin amorphous
NbGe0.3) films. By changing the film thickness it is
shown that the field at which the sign reversal occurs shifts to lower values
(from above to below the mean-field transition field ) with increasing
film thickness. This effect can be understood in terms of a competition between
a positive normal and a negative fluctuation contribution to the Hall
conductivity.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
The closest elastic tensor of arbitrary symmetry to an elasticity tensor of lower symmetry
The closest tensors of higher symmetry classes are derived in explicit form
for a given elasticity tensor of arbitrary symmetry. The mathematical problem
is to minimize the elastic length or distance between the given tensor and the
closest elasticity tensor of the specified symmetry. Solutions are presented
for three distance functions, with particular attention to the Riemannian and
log-Euclidean distances. These yield solutions that are invariant under
inversion, i.e., the same whether elastic stiffness or compliance are
considered. The Frobenius distance function, which corresponds to common
notions of Euclidean length, is not invariant although it is simple to apply
using projection operators. A complete description of the Euclidean projection
method is presented. The three metrics are considered at a level of detail far
greater than heretofore, as we develop the general framework to best fit a
given set of moduli onto higher elastic symmetries. The procedures for finding
the closest elasticity tensor are illustrated by application to a set of 21
moduli with no underlying symmetry.Comment: 48 pages, 1 figur
Anthropogenic Space Weather
Anthropogenic effects on the space environment started in the late 19th
century and reached their peak in the 1960s when high-altitude nuclear
explosions were carried out by the USA and the Soviet Union. These explosions
created artificial radiation belts near Earth that resulted in major damages to
several satellites. Another, unexpected impact of the high-altitude nuclear
tests was the electromagnetic pulse (EMP) that can have devastating effects
over a large geographic area (as large as the continental United States). Other
anthropogenic impacts on the space environment include chemical release ex-
periments, high-frequency wave heating of the ionosphere and the interaction of
VLF waves with the radiation belts. This paper reviews the fundamental physical
process behind these phenomena and discusses the observations of their impacts.Comment: 71 pages, 35 figure
The Development and Use of Sustainability Criteria in SuRF-UK’s Sustainable Remediation Framework
Sustainability considerations have become widely recognised in contaminated land management and are now accepted as an important component of remediation planning and implementation around the world. The Sustainable Remediation Forum for the UK (SuRF-UK) published guidance on sustainability criteria for consideration in drawing up (or framing) assessments, organised across 15 “headline” categories, five for the environment element of sustainability, five for the social, and five for the economic. This paper describes how the SuRF-UK indicator guidance was developed, and the rationale behind its structure and approach. It describes its use in remediation option appraisal in the UK, and reviews the international papers that have applied or reviewed it. It then reviews the lessons learned from its initial use and the opinions and findings of international commentators, and concludes with recommendations on how the indicator categories might be further refined in the future. The key findings of this review are that the SuRF-UK framework and indicator guidance is well adopted into practice in the UK. It is widely recognised as the most appropriate mechanism to support sustainability-based decision making in contaminated land decision making. It has influenced the development of other national and international guidance and standards on sustainable remediation. However, there is room for some fine tuning of approach based on the lessons learned during its application
X-ray Spectroscopy and Variability of AGN Detected in the 2 Ms Chandra Deep Field-North Survey
We investigate the nature of the faint X-ray source population through X-ray
spectroscopy and variability analyses of 136 AGN detected in the 2 Ms Chandra
Deep Field-North survey with > 200 background-subtracted 0.5-8.0 keV counts
[F(0.5-8.0 keV)=(1.4-200)e-15 erg cm^{-2} s^{-1}]. Our preliminary spectral
analyses yield median spectral parameters of Gamma=1.61 and intrinsic
N_H=6.2e21 cm^{-2} (z=1 assumed when no redshift available) when the AGN
spectra are fitted with a simple absorbed power-law model. However,
considerable spectral complexity is apparent (e.g., reflection, partial
covering) and must be taken into account to model the data accurately.
Moreover, the choice of spectral model (i.e., free vs. fixed photon index) has
a pronounced effect on the derived N_H distribution and, to a lesser extent,
the X-ray luminosity distribution. Ten of the 136 AGN (~7%) show significant Fe
Kalpha emission-line features with equivalent widths in the range 0.1-1.3 keV.
Two of these emission-line AGN could potentially be Compton thick (i.e., Gamma
< 1.0 and large Fe Kalpha equivalent width). Finally, we find that 81 (~60%) of
the 136 AGN show signs of variability, and that this fraction increases
significantly (~80-90%) when better photon statistics are available.Comment: Submitted to Advances in Space Research for New X-ray Results from
Clusters of Galaxies and Black Holes (Oct 2002; Houston, TX), eds. C. Done,
E.M. Puchnarewicz, M.J. Ward. Requires cospar.sty (6 pgs, 10 figs
Constraints on Dark Matter Annihilation in Clusters of Galaxies with the Fermi Large Area Telescope
Nearby clusters and groups of galaxies are potentially bright sources of
high-energy gamma-ray emission resulting from the pair-annihilation of dark
matter particles. However, no significant gamma-ray emission has been detected
so far from clusters in the first 11 months of observations with the Fermi
Large Area Telescope. We interpret this non-detection in terms of constraints
on dark matter particle properties. In particular for leptonic annihilation
final states and particle masses greater than ~200 GeV, gamma-ray emission from
inverse Compton scattering of CMB photons is expected to dominate the dark
matter annihilation signal from clusters, and our gamma-ray limits exclude
large regions of the parameter space that would give a good fit to the recent
anomalous Pamela and Fermi-LAT electron-positron measurements. We also present
constraints on the annihilation of more standard dark matter candidates, such
as the lightest neutralino of supersymmetric models. The constraints are
particularly strong when including the fact that clusters are known to contain
substructure at least on galaxy scales, increasing the expected gamma-ray flux
by a factor of ~5 over a smooth-halo assumption. We also explore the effect of
uncertainties in cluster dark matter density profiles, finding a systematic
uncertainty in the constraints of roughly a factor of two, but similar overall
conclusions. In this work, we focus on deriving limits on dark matter models; a
more general consideration of the Fermi-LAT data on clusters and clusters as
gamma-ray sources is forthcoming.Comment: accepted to JCAP, Corresponding authors: T.E. Jeltema and S. Profumo,
minor revisions to be consistent with accepted versio
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