449 research outputs found
MODIFICATION OF CP-5 FUEL ASSEMBLIES FOR FUEL CAPSULE IRRADIATION EXPERIMENTS
BS>The standard vertical fuel tube assembly of the CP-5 research reactor was modified to permit diversion of heavywater coolant into the thimble for temperature-controlled irradiation experiments of capsules containing fuel specimens. The modification entailed revisions to the thimble, outer top shield plug and center shield plug. and helium sweep system. The modified thimble is provided with inlet actor flow system through the thimble cavity. The coolant in the thimble removes fission heat from the encapsulated samples introduced into the thimble cavity. Flow tests of the thimble were first performed on a flow stand, with final flow characteristics being determined in the reactor. A helium purge system was incorporated to remove dissociation gases trapped in the thimble cavity. Four configurations of capsules and capsule baskets were flow tested under conditions simulating operating flow conditions from 2- to 10-Mw reactor power. Irradiation capsules are now being satisfactorily operated in the modified thimbles at 2-Mw reactor power. (auth
Begomovirus disease complex: emerging threat to vegetable production systems of West and Central Africa
Vegetables play a major role in the livelihoods of the rural poor in Africa. Among major constraints to vegetable
production worldwide are diseases caused by a group of viruses belonging to the genus Begomovirus, family
Geminiviridae. Begomoviruses are plant-infecting viruses, which are transmitted by the whitefly vector Bemisia tabaci
and have been known to cause extreme yield reduction in a number of economically important vegetables around
the world. Several begomoviruses have been detected infecting vegetable crops in West and Central Africa (WCA).
Small single stranded circular molecules, alphasatellites and betasatellites, which are about half the size of their
helper begomovirus genome, have also been detected in plants infected by begomoviruses. In WCA, B. tabaci has
been associated with suspected begomovirus infections in many vegetable crops and weed species. Sequencing of
viral genomes from crops such as okra resulted in the identification of two previously known begomovirus species
(Cotton leaf curl Gezira virus and Okra yellow crinkle virus) as well as a new recombinant begomovirus species (Okra
leaf curl Cameroon virus), a betasatellite (Cotton leaf curl Gezira betasatellite) and new alphasatellites. Tomato and
pepper plants with leaf curling were shown to contain isolates of new begomoviruses, collectively referred to as
West African tomato-infecting begomoviruses (WATIBs), new alphasatellites and betasatellites. To study the potential
of weeds serving as begomovirus reservoirs, begomoviruses and satellites in the weed Ageratum conyzoides were
characterized. Sequence analyses showed that they were infected by isolates of a new begomovirus (Ageratum
leaf curl Cameroon virus) that belong to the WATIBs group, a new betasatellite (Ageratum leaf curl Cameroon
betasatellite), an alphasatellite and two types of defective recombinants between a begomovirus and an alphasatellite.
Putative recombinations were detected in begomovirus genomes for all four plant species studied, indicating that
recombination is an important mechanism for their evolution. A close relationship between the begomoviruses
infecting pepper and tomato and A. conyzoides and the detection of the same alphasatellite in them support the idea
that weeds are important reservoirs for begomoviruses and their satellites. With this high diversity, recombination
potential and transmission by B. tabaci, begomoviruses and ssDNA satellites pose a serious threat to crop production in
West and Central Africa
Single-Proton Removal Reaction Study of 16B
The low-lying level structure of the unbound system B has been
investigated via single-proton removal from a 35 MeV/nucleon C beam. The
coincident detection of the beam velocity B fragment and neutron allowed
the relative energy of the in-flight decay of B to be reconstructed. The
resulting spectrum exhibited a narrow peak some 85 keV above threshold. It is
argued that this feature corresponds to a very narrow (100 keV)
resonance, or an unresolved multiplet, with a dominant + configuration which decays by d-wave neutron
emission.Comment: 16 pages, 5 figures, 1 table, submitted to Phys. Lett.
One-neutron removal reactions on neutron-rich psd-shell nuclei
A systematic study of high energy, one-neutron removal reactions on 23
neutron-rich, psd--shell nuclei (Z=5-9, A=12-25) has been carried out. The
longitudinal momentum distributions of the core fragments and corresponding
single-neutron removal cross sections are reported for reactions on a carbon
target. Extended Glauber model calculations, weighted by the spectroscopic
factors obtained from shell model calculations, are compared to the
experimental results. Conclusions are drawn regarding the use of such reactions
as a spectroscopic tool and spin-parity assignments are proposed for 15B, 17C,
19-21N, 21,23O, 23-25F. The nature of the weakly bound systems 14B and 15,17C
is discussed.Comment: 11 pages + 2 figure
Single-neutron transfer from 11Be gs via the (p,d) reaction with a radioactive beam
The 11Be(p,d)10Be reaction has been performed in inverse kinematics with a
radioactive 11Be beam of E/A = 35.3 MeV. Angular distributions for the 0+
ground state, the 2+, 3.37 MeV state and the multiplet of states around 6 MeV
in 10Be were measured at angles up to 16 deg CM by detecting the 10Be in a
dispersion-matched spectrometer and the coincident deuterons in a silicon
array. Distorted wave and coupled-channels calculations have been performed to
investigate the amount of 2+ core excitation in 11Be gs. The use of "realistic"
11Be wave functions is emphasised and bound state form factors have been
obtained by solving the particle-vibration coupling equations. This calculation
gives a dominant 2s component in the 11Be gs wave function with a 16% [2+ x 1d]
core excitation admixture. Cross sections calculated with these form factors
are in good agreement with the present data. The Separation Energy prescription
for the bound state wave function also gives satisfactory fits to the data, but
leads to a significantly larger [2 x 1d] component in 11Be gs.Comment: 39 pages, 12 figures. Accepted for publication in Nuclear Physics A.
Added minor corrections made in proof to pages 26 and 3
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
Shell evolution approaching the N=20 island of inversion : Structure of 26Na
The levels in 26Na with single particle character have been observed for the first time using the d(25Na, pγ) reaction at 5 MeV/nucleon. The measured excitation energies and the deduced spectroscopic factors are in good overall agreement with (0+1)hω shell model calculations performed in a complete spsdfp basis and incorporating a reduction in the N=20 gap. Notably, the 1p3/2 neutron configuration was found to play an enhanced role in the structure of the low-lying negative parity states in 26Na, compared to the isotone 28Al. Thus, the lowering of the 1p3/2 orbital relative to the 0f7/2 occurring in the neighbouring Z=10 and 12 nuclei - 25,27Ne and 27,29Mg - is seen also to occur at Z=11 and further strengthens the constraints on the modelling of the transition into the island of inversion
Atomic X-ray Spectroscopy of Accreting Black Holes
Current astrophysical research suggests that the most persistently luminous
objects in the Universe are powered by the flow of matter through accretion
disks onto black holes. Accretion disk systems are observed to emit copious
radiation across the electromagnetic spectrum, each energy band providing
access to rather distinct regimes of physical conditions and geometric scale.
X-ray emission probes the innermost regions of the accretion disk, where
relativistic effects prevail. While this has been known for decades, it also
has been acknowledged that inferring physical conditions in the relativistic
regime from the behavior of the X-ray continuum is problematic and not
satisfactorily constraining. With the discovery in the 1990s of iron X-ray
lines bearing signatures of relativistic distortion came the hope that such
emission would more firmly constrain models of disk accretion near black holes,
as well as provide observational criteria by which to test general relativity
in the strong field limit. Here we provide an introduction to this phenomenon.
While the presentation is intended to be primarily tutorial in nature, we aim
also to acquaint the reader with trends in current research. To achieve these
ends, we present the basic applications of general relativity that pertain to
X-ray spectroscopic observations of black hole accretion disk systems, focusing
on the Schwarzschild and Kerr solutions to the Einstein field equations. To
this we add treatments of the fundamental concepts associated with the
theoretical and modeling aspects of accretion disks, as well as relevant topics
from observational and theoretical X-ray spectroscopy.Comment: 63 pages, 21 figures, Einstein Centennial Review Article, Canadian
Journal of Physics, in pres
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