737 research outputs found
Radiation Damage of F8 Lead Glass with 20 MeV Electrons
Using a 20 MeV linear accelerator, we investigate the effects of
electromagnetic radiation on the optical transparency of F8 lead glass.
Specifically, we measure the change in attenuation length as a function of
radiation dose. Comparing our results to similar work that utilized a proton
beam, we conclude that F8 lead glass is more susceptible to proton damage than
electron damage.Comment: 5 pages, 6 figure
Preliminary modelling of crack nucleation and propagation in SiC/SiC accident-tolerant fuel during routine operational transients using peridynamics
Silicon carbide fibre in silicon carbide matrix composites (SiC/SiC) are a promising cladding for use in accident tolerant fuels (ATF) in current light water reactor (LWR) designs. However, as they are a radically different material from current metal clads, current thermomechanical simulation methods struggle to accurately predict their behaviour, especially regarding the potential development of cracks. Thus, a new peridynamic model for SiC/SiC cladding has been developed in the Abaqus finite element code. The material model was isotropic and considers matrix cracking and fibre pull-out. The thermal expansion, swelling and the degradation of the thermal conductivity are modelled under typical LWR irradiation conditions. The swelling on the outer surface is predicted to be greater than the inner surface due to the lower irradiation temperature, causing a tensile stress on the inside of the cladding; tension being more challenging for a ceramic than a metal. This stress increases during the decrease in power at the start of a typical pressurised water reactor refuelling outage and causes microcracking of the matrix on the cladding inner surface. In models without fibres, cracks would propagate through the cladding. If fibres are modelled, matrix cracking will extend to a depth of around 20% through the cladding from the inner surface, which is unlikely to be an acceptable design. If an inner monolith of SiC is additionally modelled, cracking propagates through the monolith and acts as a stress raiser for matrix cracking in the composite, and therefore does not constitute a design improvement. If an outer SiC monolith is modelled, fibre pull-out strain on the inner surface of the cladding was increased by just under 70%. No cracks are predicted in an outer monolith which may therefore remain gas-tight and thus a more suitable design. These predictions are consistent with experimental findings
Claw-free t-perfect graphs can be recognised in polynomial time
A graph is called t-perfect if its stable set polytope is defined by
non-negativity, edge and odd-cycle inequalities. We show that it can be decided
in polynomial time whether a given claw-free graph is t-perfect
Neutral and cationic half-sandwich arene ruthenium, Cp*Rh and Cp*Ir oximato and oxime complexes: Synthesis, structural, DFT and biological studies
The reaction of [(p-cymene)RuCl2]2 and [Cp*MCl2]2 (M = Rh/Ir) with chelating ligand 2-pyridylcyanoxime {pyC(CN)NOH} leads to the formation of neutral oximato complexes having the general formula [(arene)M{pyC(CN)NO}Cl] {arene = p-cymene, M = Ru, (1); Cp*, M = Rh (2);Cp*, M = Ir (3)}. Whereas the reaction of 2-pyridyl phenyloxime {pyC(Ph)NOH} and 2-thiazolyl methyloxime {tzC(Me)NOH} with precursor compounds afforded the cationic oxide complexes bearing formula [(arene)M{pyC(ph)NOH}Cl]+ and [(arene)M{tzC(Me)NOH}Cl]+{arene = p-cymene M = Ru, (4), (7); Cp*, M = Rh (5), (8); Cp*, M = Ir (6), (9)}. The cationic complexes were isolated as their hexafluorophosphate salts. All these complexes were fully characterized by analytical, spectroscopic and X-ray diffraction studies. The molecular structures of the complexes revealed typical piano stool geometry around the metal center within which the ligand acts as a NNĘš donor chelating ligand. The Chemo-sensitivity activities of the complexes evaluated against HT-29 (human colorectal cancer), and MIAPaCa-2 (human pancreatic cancer) cell line showed that the iridium-based complexes are much more potent than the ruthenium and rhodium analogues. Theoretical studies were carried out to have a deeper understanding about the charge distribution pattern and the various electronic transitions occurring in the complexes
Application of mineralogical, petrological and geochemical tools for evaluating the palaeohdrogeological evolution of the PADAMOT study sites
The role of Work Package (WP) 2 of the PADAMOT project â âPalaeohydrogeological Data
Measurementsâ - has been to study late-stage fracture mineral and water samples from
groundwater systems in Spain, Sweden, United Kingdom and the Czech Republic, with the aim
of understanding the recent palaeohydrogeological evolution of these groundwater systems. In
particular, the project sought to develop and evaluate methods for obtaining information about
past groundwater evolution during the Quaternary (about the last 2 million years) by examining
how the late-stage mineralization might record mineralogical, petrographical and geochemical
evidence of how the groundwater system may have responded to past geological and
climatological changes.
Fracture-flow groundwater systems at six European sites were studied:
⢠Melechov Hill, in the Bohemian Massif of the Czech Republic: a shallow (0-100 m)
dilute groundwater flow system within the near-surface weathering zone in fractured
granitic rocks;
⢠Cloud Hill, in the English Midlands: a (~100 m) shallow dilute groundwater flow system
in fractured and dolomitized Carboniferous limestone;
⢠Los Ratones, in southwest Spain: an intermediate depth (0-500 m) dilute groundwater
flow system in fractured granitic rocks;
⢠Laxemar, in southeast Sweden: a deep (0-1000 m) groundwater flow system in fractured
granitic rocks. This is a complex groundwater system with potential recharge and
flushing by glacial, marine, lacustrine and freshwater during the Quaternary;
⢠Sellafield, northwest England: a deep (0-2000 m) groundwater flow system in fractured
Ordovician low-grade metamorphosed volcaniclastic rocks and discontinuous
Carboniferous Limestone, overlain by a Permo-Triassic sedimentary sequence with
fracture and matrix porosity. This is a complex coastal groundwater system with deep
hypersaline sedimentary basinal brines, and deep saline groundwaters in crystalline
basement rocks, overlain by a shallow freshwater aquifer system. The site was glaciated
several times during the Quaternary and may have been affected by recharge from glacial
meltwater;
⢠Dounreay, northeast Scotland: a deep (0-1400 m) groundwater flow system in fractured
Precambrian crystalline basement overlain by fractured Devonian sedimentary rocks.
This is within the coastal discharge area of a complex groundwater system, comprising
deep saline groundwater hosted in crystalline basement, overlain by a fracture-controlled
freshwater sedimentary aquifer system. Like Sellafield, this area experienced glaciation
and may potentially record the impact of glacial meltwater recharge.
In addition, a study has been made of two Quaternary sedimentary sequences in Andalusia in
southeastern Spain to provide a basis of estimating the palaeoclimatic history of the region that
could be used in any reconstruction of the palaeoclimatic history at the Los Ratones site:
⢠The Cúllar-Baza lacustrine sequence records information about precipitation and
palaeotemperature regimes, derived largely from the analysis of the stable isotope (δ18O
and δ13C) signatures from biogenic calcite (ostracod shells).
⢠The Padul Peat Bog sequence provided information on past vegetation cover and
palaeogroundwater inputs based on the study of fossil pollen and biomarkers as proxies
for past climate change.
Following on from the earlier EC 4th Framework EQUIP project, the focus of the PADAMOT
studies has been on calcite mineralization. Calcite has been identified as a late stage mineral, closely associated with hydraulically-conductive fractures in the present-day groundwater
systems at the ĂspĂś-Laxemar, Sellafield, Dounreay and Cloud Hill sites. At Los Ratones and
Melechov sites late-stage mineralization is either absent or extremely scarce, and both the
quantity and fine crystal size of any late-stage fracture mineralization relevant to Quaternary
palaeohydrogeological investigations is difficult to work with. The results from the material
investigated during the PADAMOT studies indicate that the fracture fillings at these sites are
related to hydrothermal activity, and so do not have direct relevance as Quaternary indicators.
Neoformed calcite has not been found at these two sites at the present depth of the investigations.
Furthermore, the HCO3
- concentration in all the Los Ratones groundwaters is mainly controlled
by complex carbonate dissolution. The carbonate mineral saturation indices do not indicate
precipitation conditions, and this is consistent with the fact that neoformed calcite, ankerite or
dolomite have not been observed petrographically
Star and Planet Formation with ALMA: an Overview
Submillimeter observations with ALMA will be the essential next step in our
understanding of how stars and planets form. Key projects range from detailed
imaging of the collapse of pre-stellar cores and measuring the accretion rate
of matter onto deeply embedded protostars, to unravelling the chemistry and
dynamics of high-mass star-forming clusters and high-spatial resolution studies
of protoplanetary disks down to the 1 AU scale.Comment: Invited review, 8 pages, 5 figures; to appear in the proceedings of
"Science with ALMA: a New Era for Astrophysics". Astrophysics & Space
Science, in pres
Magnetization of a two-dimensional electron gas with a second filled subband
We have measured the magnetization of a dual-subband two-dimensional electron
gas, confined in a GaAs/AlGaAs heterojunction. In contrast to two-dimensional
electron gases with a single subband, we observe non-1/B-periodic, triangularly
shaped oscillations of the magnetization with an amplitude significantly less
than per electron. All three effects are explained by a
field dependent self-consistent model, demonstrating the shape of the
magnetization is dominated by oscillations in the confining potential.
Additionally, at 1 K, we observe small oscillations at magnetic fields where
Landau-levels of the two different subbands cross.Comment: 4 pages, 4 figure
Regional climate of the Larsen B embayment 1980â2014
Understanding the climate response of the Antarctic Peninsula ice sheet is vital for accurate predictions of sea-level rise. However, since climate models are typically too coarse to capture spatial variability in local scale meteorological processes, our ability to study specific sectors has been limited by the local fidelity of such models and the (often sparse) availability of observations. We show that a high-resolution (5.5 km Ă 5.5 km) version of a regional climate model (RACMO2.3) can reproduce observed interannual variability in the Larsen B embayment sufficiently to enable its use in investigating long-term changes in this sector. Using the model, together with automatic weather station data, we confirm previous findings that the year of the Larsen B ice shelf collapse (2001/02) was a strong melt year, but discover that total annual melt production was in fact ~30% lower than 2 years prior. While the year before collapse exhibited the lowest melting and highest snowfall during 1980â2014, the ice shelf was likely pre-conditioned for collapse by a series of strong melt years in the 1990s. Melt energy has since returned to pre-1990s levels, which likely explains the lack of further significant collapse in the region (e.g. of SCAR Inlet)
Genetic variation in PCAF, a key mediator in epigenetics, is associated with reduced vascular morbidity and mortality: evidence for a new concept from three independent prospective studies
Magnetic Reconnection in Extreme Astrophysical Environments
Magnetic reconnection is a basic plasma process of dramatic rearrangement of
magnetic topology, often leading to a violent release of magnetic energy. It is
important in magnetic fusion and in space and solar physics --- areas that have
so far provided the context for most of reconnection research. Importantly,
these environments consist just of electrons and ions and the dissipated energy
always stays with the plasma. In contrast, in this paper I introduce a new
direction of research, motivated by several important problems in high-energy
astrophysics --- reconnection in high energy density (HED) radiative plasmas,
where radiation pressure and radiative cooling become dominant factors in the
pressure and energy balance. I identify the key processes distinguishing HED
reconnection: special-relativistic effects; radiative effects (radiative
cooling, radiation pressure, and Compton resistivity); and, at the most extreme
end, QED effects, including pair creation. I then discuss the main
astrophysical applications --- situations with magnetar-strength fields
(exceeding the quantum critical field of about 4 x 10^13 G): giant SGR flares
and magnetically-powered central engines and jets of GRBs. Here, magnetic
energy density is so high that its dissipation heats the plasma to MeV
temperatures. Electron-positron pairs are then copiously produced, making the
reconnection layer highly collisional and dressing it in a thick pair coat that
traps radiation. The pressure is dominated by radiation and pairs. Yet,
radiation diffusion across the layer may be faster than the global Alfv\'en
transit time; then, radiative cooling governs the thermodynamics and
reconnection becomes a radiative transfer problem, greatly affected by the
ultra-strong magnetic field. This overall picture is very different from our
traditional picture of reconnection and thus represents a new frontier in
reconnection research.Comment: Accepted to Space Science Reviews (special issue on magnetic
reconnection). Article is based on an invited review talk at the
Yosemite-2010 Workshop on Magnetic Reconnection (Yosemite NP, CA, USA;
February 8-12, 2010). 30 pages, no figure
- âŚ