583 research outputs found
Transport of Sellafield-derived C-14 from the Irish Sea through the North Channel
Since the early 1950s, the Sellafield nuclear fuel reprocessing plant in Northwest England has released radio-carbon into the Irish Sea in a mainly inorganic form as part of its authorized liquid effluent discharge. In contrast to the trend in which the activities of most radionuclides in the Sellafield liquid effluent have decreased substantially, C-14 discharges have increased since 1994-95. This has largely been due to a policy change favoring marine discharges over atmospheric discharges. C-14 is radiologically important due to its long half life, mobility in the environment, and propensity for entering the food chain. Current models for radionuclide dispersal in the Irish Sea are based on a reversible equilibrium distribution coefficient (k(d)), an approach which has been shown to be inadequate for C-14. Development of predictive models for the fate of Sellafield-derived C-14 requires a thorough understanding of the biogeochemical fluxes between different carbon reservoirs and the processes controlling the net flux of C-14 out of the Irish Sea, through he North Channel. In this study., both an empirical and a halving time approach indicate that close to 100% of the C-14 that is discharged from Sellafield is dispersed beyond the Irish Sea on a time-scale of months in the form of DIC, with little transfer to the PIC, POC, and DOC fractions, indicating that the "dilute and disperse" mechanism is operating satisfactorily. This is consistent with previous research that indicated little transfer of C-14 to Irish Sea sediments, While significant C-14 enhancements have been observed in the biota of the Irish Sea, this observation is not necessarily in conflict with either of the above as the total biomass has to be taken into account in any calculations of C-14 retention within the Irish Sea
Sources of anthropogenic C-14 to the North Sea
The Sellafield nuclear fuel reprocessing plant on the northwest coast of England is the largest source of anthropogenic radiocarbon to the UK coastal environment. In a mid-1990s study of C-14 distribution around the UK coast, the pattern of dilution with increasing distance from Sellafield appeared to be perturbed by anomalously high C-14 activities in marine biota in the coastal environment of northeast England. This present study was undertaken during 1998 and 1999 to determine whether this C-14 enhancement was due to Sellafield or the nuclear power plants on the east coast. Seawater, seaweed (Fucus sp.), and mussel (Mytilus edulis) samples that were collected from the vicinity of the Torness and Hartlepool advanced gas-cooled reactor (AGR) nuclear power stations were all enhanced above the contemporary regional background activity derived from natural production and atmospheric nuclear weapons testing. We used previously published dilution factors and transfer times for Tc-99 between Sellafield and various points on the UK coast to determine likely Sellafield- derived C-14 contributions to the activities at the nuclear power plant sites. The results suggest that the activities observed at Torness, which are only marginally enhanced above the natural background activity, are possibly due to discharges from Sellafield; however, the significant C-14 enhancements at Hartlepool are not Sellafield-derived. Furthermore, since both reactors have the same fundamental design, the low activities at the Torness AGR imply that the activities at Hartlepool are not from the AGR, suggesting that there is an input of C-14 to the marine environment in the vicinity of Hartlepool which is probably non-nuclear-power related. However, there is no other authorized site in the area that could account for the observed C-14 enrichments; therefore, further research is required to ascertain the source of this C-14
Lower limb stiffness estimation during running: the effect of using kinematic constraints in muscle force optimization algorithms
The focus of this paper is on the effect of muscle force optimization algorithms on the human lower limb stiffness estimation. By using a forward dynamic neuromusculoskeletal model coupled with a muscle short-range stiffness model we computed the human joint stiffness of the lower limb during running. The joint stiffness values are calculated using two different muscle force optimization procedures, namely: Toque-based and Torque/Kinematic-based algorithm. A comparison between the processed EMG signal and the corresponding estimated muscle forces with the two optimization algorithms is provided. We found that the two stiffness estimates are strongly influenced by the adopted algorithm. We observed different magnitude and timing of both the estimated muscle forces and joint stiffness time profile with respect to each gait phase, as function of the optimization algorithm used
Wavy Strings: Black or Bright?
Recent developments in string theory have brought forth a considerable
interest in time-dependent hair on extended objects. This novel new hair is
typically characterized by a wave profile along the horizon and angular
momentum quantum numbers in the transverse space. In this work, we
present an extensive treatment of such oscillating black objects, focusing on
their geometric properties. We first give a theorem of purely geometric nature,
stating that such wavy hair cannot be detected by any scalar invariant built
out of the curvature and/or matter fields. However, we show that the tidal
forces detected by an infalling observer diverge at the `horizon' of a black
string superposed with a vibration in any mode with . The same
argument applied to longitudinal () waves detects only finite tidal
forces. We also provide an example with a manifestly smooth metric, proving
that at least a certain class of these longitudinal waves have regular
horizons.Comment: 45 pages, latex, no figure
Tachyon Condensation and Black Strings
We show that under certain conditions, closed string tachyon condensation
produces a topology changing transition from black strings to Kaluza-Klein
"bubbles of nothing." This can occur when the curvature at the horizon is much
smaller than the string scale, so the black string is far from the
correspondence point when it would make a transition to an excited fundamental
string. This provides a dramatic new endpoint to Hawking evaporation. A similar
transition occurs for black p-branes, and can be viewed as a nonextremal
version of a geometric transition. Applications to AdS black holes and the AdS
soliton are also discussed.Comment: 23 pages, 1 figure, v2: references adde
The Evidence for a Pentaquark Signal and Kinematic Reflections
Several recent experiments have reported evidence for a narrow baryon
resonance with positive strangeness () at a mass of 1.54 GeV/.
Baryons with cannot be conventional states and the reports have
thus generated much theoretical speculation about the nature of possible
baryons, including a 5-quark, or pentaquark, interpretation. We show that
narrow enhancements in the effective mass spectrum can be generated as
kinematic reflections resulting from the decay of mesons, such as the
, the and the .Comment: 4 pages, 4 figure
On the Spin Gap Phase of Strongly-Correlated Electrons
We discuss the possible existence of a spin-gap phase in the low-doping
regime of strongly-correlated two-dimensional electrons within the gauge field
description of the t-J model. The spin-gap phase was recently shown by Ubbens
and Lee to be destroyed by gauge field quantum fluctuations for a single-layer
2D system in the absence of disorder and for a full gap. We show that the same
conclusion applies both in the dirty limit and for the case of a gapless spinon
condensate.Comment: 7 pages, uuencoded Postscript, including 1 figur
Explaining LSND by a decaying sterile neutrino
We propose an explanation of the LSND evidence for electron antineutrino
appearance based on neutrino decay. We introduce a heavy neutrino, which is
produced in pion and muon decays because of a small mixing with muon neutrinos,
and then decays into a scalar particle and a light neutrino, predominantly of
the electron type. We require values of few eV, being the
neutrino--scalar coupling and the heavy neutrino mass, e.g. in the
range from 1 keV to 1 MeV and . Performing a fit to
the LSND data as well as all relevant null-result experiments, we show that all
data can be explained within this decay scenario. In the minimal version of the
decay model, we predict a signal in the upcoming MiniBooNE experiment
corresponding to a transition probability of the same order as seen in LSND. In
addition, we show that extending our model to two nearly degenerate heavy
neutrinos it is possible to introduce CP violation in the decay, which can lead
to a suppression of the signal in MiniBooNE running in the neutrino mode. We
briefly discuss signals in future neutrino oscillation experiments, we show
that our scenario is compatible with bounds from laboratory experiments, and we
comment on implications in astrophysics and cosmology.Comment: 23 pages, 5 figures, minor improvements, matches published versio
Pyrochlore Photons: The U(1) Spin Liquid in a S=1/2 Three-Dimensional Frustrated Magnet
We study the S=1/2 Heisenberg antiferromagnet on the pyrochlore lattice in
the limit of strong easy-axis exchange anisotropy. We find, using only standard
techniques of degenerate perturbation theory, that the model has a U(1) gauge
symmetry generated by certain local rotations about the z-axis in spin space.
Upon addition of an extra local interaction in this and a related model with
spins on a three-dimensional network of corner-sharing octahedra, we can write
down the exact ground state wavefunction with no further approximations. Using
the properties of the soluble point we show that these models enter the U(1)
spin liquid phase, a novel fractionalized spin liquid with an emergent U(1)
gauge structure. This phase supports gapped S^z = 1/2 spinons carrying the U(1)
``electric'' gauge charge, a gapped topological point defect or ``magnetic''
monopole, and a gapless ``photon,'' which in spin language is a gapless,
linearly dispersing S^z = 0 collective mode. There are power-law spin
correlations with a nontrivial angular dependence, as well as novel U(1)
topological order. This state is stable to ALL zero-temperature perturbations
and exists over a finite extent of the phase diagram. Using a convenient
lattice version of electric-magnetic duality, we develop the effective
description of the U(1) spin liquid and the adjacent soluble point in terms of
Gaussian quantum electrodynamics and calculate a few of the universal
properties. The resulting picture is confirmed by our numerical analysis of the
soluble point wavefunction. Finally, we briefly discuss the prospects for
understanding this physics in a wider range of models and for making contact
with experiments.Comment: 22 pages, 14 figures. Further minor changes. To appear in Phys. Rev.
Pocket Monte Carlo algorithm for classical doped dimer models
We study the correlations of classical hardcore dimer models doped with
monomers by Monte Carlo simulation. We introduce an efficient cluster
algorithm, which is applicable in any dimension, for different lattices and
arbitrary doping. We use this algorithm for the dimer model on the square
lattice, where a finite density of monomers destroys the critical confinement
of the two-monomer problem. The monomers form a two-component plasma located in
its high-temperature phase, with the Coulomb interaction screened at finite
densities. On the triangular lattice, a single pair of monomers is not
confined. The monomer correlations are extremely short-ranged and hardly change
with doping.Comment: 6 pages, REVTeX
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