375 research outputs found
URANIUM(IV) NITRATE AS A REDUCING AGENT FOR PLUTONIUM(IV) IN THE PUREX PROCESS
Practical procedures were demonstrated for partitioning plutonium from uranium in the Purex process with hydrazine-stabilized uranium(IV) nitrate as the reducing reagent for plutonium. Typical partitioning results were 0.04--0.2% loss of plutonium to the uranium product and less than 1 wt% uranium in the plutonium product. With uranium(IV) nitrate as a replacement for ferrous sulfamate, most solids in wastes from the Parex process can be eliminated. No undue hazard is expected from the use of hydrazine as the stabilizing agent. (auth
Brane World Cosmologies and Statistical Properties of Gravitational Lenses
Brane world cosmologies seem to provide an alternative explanation for the
present accelerated stage of the Universe with no need to invoke either a
cosmological constant or an exotic \emph{quintessence} component. In this paper
we investigate statistical properties of gravitational lenses for some
particular scenarios based on this large scale modification of gravity. We show
that a large class of such models are compatible with the current lensing data
for values of the matter density parameter
(). If one fixes to be , as suggested by
most of the dynamical estimates of the quantity of matter in the Universe, the
predicted number of lensed quasars requires a slightly open universe with a
crossover distance between the 4 and 5-dimensional gravities of the order of
.Comment: 6 pages, 3 figures, revte
G\"odel Incompleteness and the Black Hole Information Paradox
Semiclassical reasoning suggests that the process by which an object
collapses into a black hole and then evaporates by emitting Hawking radiation
may destroy information, a problem often referred to as the black hole
information paradox. Further, there seems to be no unique prediction of where
the information about the collapsing body is localized. We propose that the
latter aspect of the paradox may be a manifestation of an inconsistent
self-reference in the semiclassical theory of black hole evolution. This
suggests the inadequacy of the semiclassical approach or, at worst, that
standard quantum mechanics and general relavity are fundamentally incompatible.
One option for the resolution for the paradox in the localization is to
identify the G\"odel-like incompleteness that corresponds to an imposition of
consistency, and introduce possibly new physics that supplies this
incompleteness. Another option is to modify the theory in such a way as to
prohibit self-reference. We discuss various possible scenarios to implement
these options, including eternally collapsing objects, black hole remnants,
black hole final states, and simple variants of semiclassical quantum gravity.Comment: 14 pages, 2 figures; revised according to journal requirement
Self-Consistency and Calibration of Cluster Number Count Surveys for Dark Energy
Cluster number counts offer sensitive probes of the dark energy if and only
if the_evolution_ of the cluster mass versus observable relation(s) is well
calibrated. We investigate the potential for internal calibration by demanding
consistency in the counts as a function of the observable. In the context of a
constant dark energy equation of state, known initial fluctuation amplitude
expected from the CMB, universal underlying mass function, and an idealized
selection, we find that the ambiguity from the normalization of the
mass-observable relationships, or an extrapolation of external mass-observable
determinations from higher masses, can be largely eliminated with a
sufficiently deep survey, even allowing for an arbitrary evolution. More
generally, number counts as a function of both the redshift and the observable
enable strong consistency tests on assumptions made in modelling the
mass-observable relations and cosmology.Comment: 4 pages, 3 figures, submitted to PRD rapid communication
UV and EUV Instruments
We describe telescopes and instruments that were developed and used for
astronomical research in the ultraviolet (UV) and extreme ultraviolet (EUV)
regions of the electromagnetic spectrum. The wavelength ranges covered by these
bands are not uniquely defined. We use the following convention here: The EUV
and UV span the regions ~100-912 and 912-3000 Angstroem respectively. The
limitation between both ranges is a natural choice, because the hydrogen Lyman
absorption edge is located at 912 Angstroem. At smaller wavelengths,
astronomical sources are strongly absorbed by the interstellar medium. It also
marks a technical limit, because telescopes and instruments are of different
design. In the EUV range, the technology is strongly related to that utilized
in X-ray astronomy, while in the UV range the instruments in many cases have
their roots in optical astronomy. We will, therefore, describe the UV and EUV
instruments in appropriate conciseness and refer to the respective chapters of
this volume for more technical details.Comment: To appear in: Landolt-Boernstein, New Series VI/4A, Astronomy,
Astrophysics, and Cosmology; Instruments and Methods, ed. J.E. Truemper,
Springer-Verlag, Berlin, 201
Different W cluster deposition regimes in pulsed laser ablation observed by in situ Scanning Tunneling Microscopy
We report on how different cluster deposition regimes can be obtained and
observed by in situ Scanning Tunneling Microscopy (STM) by exploiting
deposition parameters in a pulsed laser deposition (PLD) process. Tungsten
clusters were produced by nanosecond Pulsed Laser Ablation in Ar atmosphere at
different pressures and deposited on Au(111) and HOPG surfaces. Deposition
regimes including cluster deposition-diffusion-aggregation (DDA), cluster
melting and coalescence and cluster implantation were observed, depending on
background gas pressure and target-to-substrate distance which influence the
kinetic energy of the ablated species. These parameters can thus be easily
employed for surface modification by cluster bombardment, deposition of
supported clusters and growth of films with different morphologies. The
variation in cluster mobility on different substrates and its influence on
aggregation and growth mechanisms has also been investigated.Comment: 12 pages (3 figures); Surface Science (accepted
Evidence for Shock Acceleration and Intergalactic Magnetic Fields in a Large-Scale Filament of Galaxies ZwCl 2341.1+0000
We report the discovery of large-scale diffuse radio emission from what
appears to be a large-scale filamentary network of galaxies in the region of
cluster ZwCl 2341.1+0000, and stretching over an area of at least Mpc in diameter. Multicolour CCD observations yield photometric
redshifts indicating that a significant fraction of the optical galaxies in
this region is at a redshift of z=0.3. This is supported by spectroscopic
measurements of 4 galaxies in the SDSS survey at a mean z=0.27. We present VLA
images at 20 cm (NVSS) and 90 cm wavelengths, showing the detailed radio
structure of the filaments. Comparison with the VLA high resolution FIRST radio
survey shows that the diffuse emission is not due to known individual point
sources. The diffuse radio-emission has a spectral index , and is most likely synchrotron emission from relativistic charged
particles in an inter-galactic magnetic field. Furthermore, this optical/radio
structure is detected in X-rays by the ROSAT all-sky survey. It has a 0.1--2.4
keV luminosity of about erg s and shows an extended highly
non-relaxed morphology. These observations suggest that ZwCl 2341.1+0000 is
possibly a proto-cluster of galaxies in which we are witnessing the process of
structure formation. We show (both analytically and by numerical simulations)
that the energetics of accretion shocks generated in forming large-scale
structures are sufficient to produce enough high energy cosmic-ray (CR)
electrons required to explain the observed radio emission, provided a magnetic
field of strength of about 1 micro Gauss is present there.Thus it is the first
evidence of cosmic-ray particle acceleration and magnetic fields occuring on a
super-cluster scale. (Abridged)Comment: Replaced with the published version. The published paper can be
accessed from http://www.elsevier.com/gej-ng/10/33/29/71/56/53/article.htm
Cosmology with clusters of galaxies
In this Chapter I review the role that galaxy clusters play as tools to
constrain cosmological parameters. I will concentrate mostly on the application
of the mass function of galaxy clusters, while other methods, such as that
based on the baryon fraction, are covered by other Chapters of the book. Since
most of the cosmological applications of galaxy clusters rely on precise
measurements of their masses, a substantial part of my Lectures concentrates on
the different methods that have been applied so far to weight galaxy clusters.
I provide in Section 2 a short introduction to the basics of cosmic structure
formation. In Section 3 I describe the Press--Schechter (PS) formalism to
derive the cosmological mass function, then discussing extensions of the PS
approach and the most recent calibrations from N--body simulations. In Section
4 I review the methods to build samples of galaxy clusters at different
wavelengths. Section 5 is devoted to the discussion of different methods to
derive cluster masses. In Section 6 I describe the cosmological constraints,
which have been obtained so far by tracing the cluster mass function with a
variety of methods. Finally, I describe in Section 7 the future perspectives
for cosmology with galaxy clusters and the challenges for clusters to keep
playing an important role in the era of precision cosmology.Comment: 49 pages, 19 figures, Lectures for 2005 Guillermo Haro Summer School
on Clusters, to appear in "Lecture notes in Physics" (Springer
Composition and evaluation of the antimicrobial activity of the essential oil of Senecio selloi Spreng DC.
Feeding frequency and growth performance of juvenile pink shrimp (Farfantepenaeus paulensis)
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