10,822 research outputs found
Gas migration in pre-compacted bentonite under elevated pore-water pressure conditions
Pre-compacted bentonite has long been proposed as a primary component of an engineered barrier system for the safe geological disposal of radioactive waste. Selection of properties such as the clay composition, compaction-state and clay-to-sand ratio varies in different disposal concepts. However, a sound understanding of the gas transport properties of the barrier material is often considered a necessary part of safety case development for a geological disposal facility. In this study, results are presented from two gas injection experiments conducted in Mx80 bentonite, under elevated pore-water pressure conditions. Test observations indicate that the conditions necessary for gas to enter this material are remarkably consistent, irrespective of the applied water pressure. As expected, an association is noted between the total stress experienced by the clay and the gas pressure at the moment of entry. Gas migration is interpreted as occurring by the formation and propagation of dilatant pathways within the bentonite. Local pore-pressure and stress measurements indicate that significant reworking of the clay can occur, resulting in meta-stable episodes of ‘pressure-cycling’, as gas seeks a stable escape pathway. These findings demonstrate the potential for ‘phases’ of pathway development and propagation within the buffer, resulting in successive migration episodes over the repository lifetime. Experiments also show the potential for gas entry into the buffer to occur as a result of declining pore-water pressure conditions. As such, the influence of significant deviations from hydrostatic conditions (for example, resulting from glacial loading) should not be neglected when considering gas interaction with the buffer over long timescales
Electromagnetic multipole theory for optical nanomaterials
Optical properties of natural or designed materials are determined by the
electromagnetic multipole moments that light can excite in the constituent
particles. In this work we present an approach to calculate the multipole
excitations in arbitrary arrays of nanoscatterers in a dielectric host medium.
We introduce a simple and illustrative multipole decomposition of the electric
currents excited in the scatterers and link this decomposition to the classical
multipole expansion of the scattered field. In particular, we find that
completely different multipoles can produce identical scattered fields. The
presented multipole theory can be used as a basis for the design and
characterization of optical nanomaterials
Review:advances in the accuracy and traceability of metalloprotein measurements using isotope dilution inductively coupled plasma mass spectrometry
Total Molecular Gas Masses of Planck - Herschel Selected Strongly Lensed Hyper Luminous Infrared Galaxies
We report the detection of CO(1 - 0) line emission from seven Planck and
Herschel selected hyper luminous (LIR(8-1000um) > 10^13Lsun) infrared galaxies
with the Green Bank Telescope (GBT). CO(1 - 0) measurements are a vital tool to
trace the bulk molecular gas mass across all redshifts. Our results place tight
constraints on the total gas content of these most apparently luminous high-z
star-forming galaxies (apparent IR luminosities of LIR > 10^(13-14) Lsun),
while we confirm their predetermined redshifts measured using the Large
Millimeter Telescope, LMT (zCO = 1.33 - 3.26). The CO(1 - 0) lines show similar
profiles as compared to Jup = 2 -4 transitions previously observed with the
LMT. We report enhanced infrared to CO line luminosity ratios of
= 110 (pm 22) Lsun(K km s^-1 pc^-2)^-1 compared to normal
star-forming galaxies, yet similar to those of well-studied IR-luminous
galaxies at high-z. We find average brightness temperature ratios of =
0.93 (2 sources), = 0.34 (5 sources), and = 0.18 (1 source). The
r31 and r41 values are roughly half the average values for SMGs. We estimate
the total gas mass content as uMH2 = (0.9 - 27.2) x 10^11(alphaCO/0.8)Msun,
where u is the magnification factor and alphaCO is the CO line luminosity to
molecular hydrogen gas mass conversion factor. The rapid gas depletion times
are, on average, tau = 80 Myr, which reveal vigorous starburst activity, and
contrast the Gyr depletion timescales observed in local, normal star-forming
galaxies.Comment: published in MNRAS, 9pages, 5fig
The Scattering of Electromagnetic Waves from Two-Dimensional Randomly Rough Penetrable Surfaces
An accurate and efficient numerical simulation approach to electromagnetic
wave scattering from two-dimensional, randomly rough, penetrable surfaces is
presented. The use of the M\"uller equations and an impedance boundary
condition for a two-dimensional rough surface yields a pair of coupled
two-dimensional integral equations for the sources on the surface in terms of
which the scattered field is expressed through the Franz formulas. By this
approach, we calculate the full angular intensity distribution of the scattered
field that is due to a finite incident beam of -polarized light. We
specifically check the energy conservation (unitarity) of our simulations (for
the non-absorbing case). Only after a detailed numerical treatment of {\em
both} diagonal and close-to-diagonal matrix elements is the unitarity condition
found to be well-satisfied for the non-absorbing case (), a
result that testifies to the accuracy of our approach.Comment: Revtex, 4 pages, 2 figure
Bentonite permeability at elevated temperature
Repository designs frequently favour geological disposal of radioactive waste with a backfill material occupying void space around the waste. The backfill material must tolerate the high temperatures produced by decaying radioactive waste to prevent its failure or degradation, leading to increased hydraulic conductivity and reduced sealing performance. The results of four experiments investigating the effect of temperature on the permeability of a bentonite backfill are presented. Bentonite is a clay commonly proposed as the backfill in repository designs because of its high swelling capacity and very low permeability. The experiments were conducted in two sets of purpose-built, temperature controlled apparatus, designed to simulate isotropic pressure and constant volume conditions within the testing range of 4–6 MPa average effective stress. The response of bentonite during thermal loading at temperatures up to 200 °C was investigated, extending the previously considered temperature range. The results provide details of bentonite’s intrinsic permeability, total stress, swelling pressure and porewater pressure during thermal cycles. We find that bentonite’s hydraulic properties are sensitive to thermal loading and the type of imposed boundary condition. However, the permeability change is not large and can mostly be accounted for by water viscosity changes. Thus, under 150 °C, temperature has a minimal impact on bentonite’s hydraulic permeabilit
The Formation of the First Stars II. Radiative Feedback Processes and Implications for the Initial Mass Function
We consider the radiative feedback processes that operate during the
formation of the first stars, including the photodissociation of H_2, Ly-alpha
radiation pressure, formation and expansion of an HII region, and disk
photoevaporation. These processes may inhibit continued accretion once the
stellar mass has reached a critical value, and we evaluate this mass separately
for each process. Photodissociation of H_2 in the local dark matter minihalo
occurs relatively early in the growth of the protostar, but we argue this does
not affect subsequent accretion since by this time the depth of the potential
is large enough for accretion to be mediated by atomic cooling. However,
neighboring starless minihalos can be affected. Ionization creates an HII
region in the infalling envelope above and below the accretion disk. Ly-alpha
radiation pressure acting at the boundary of the HII region is effective at
reversing infall from narrow polar directions when the star reaches ~20-30Msun,
but cannot prevent infall from other directions. Expansion of the HII region
beyond the gravitational escape radius for ionized gas occurs at masses
~50-100Msun, depending on the accretion rate and angular momentum of the
inflow. However, again, accretion from the equatorial regions can continue
since the neutral accretion disk has a finite thickness and shields a
substantial fraction of the accretion envelope from direct ionizing flux. At
higher stellar masses, ~140Msun in the fiducial case, the combination of
declining accretion rates and increasing photoevaporation-driven mass loss from
the disk act to effectively halt the increase in the protostellar mass. We
identify this process as the mechanism that terminates the growth of Population
III stars... (abridged)Comment: 31 pages, including 10 figures, accepted to Ap
Gas network development in a precompacted bentonite experiment: evidence of generation and evolution
In a deep geological disposal facility for radioactive waste, precompacted bentonite is proposed as a sealing material for the isolation of boreholes, disposal galleries and deposition holes. The advective movement of repository gas in bentonite has been linked to the development of new porosity and propagation of dilatant pathways. For the first time we present a detailed analysis of stress field data during the generation and evolution of a gas network. A new experimental dataset, from a highly instrumented test, clearly shows the strong coupling between stress, gas pressure and flow in bentonite. Multiple discrete propagation events are observed, demonstrating spatial variability and time-dependency as permeability within the clay develops. Analysis of the stress data before, during and after gas entry indicates a heterogeneous stress field initially develops, resulting from the development of these pathways. The flow network is dynamic and continues to spatially evolve after gas entry, such that permeability under these conditions must be time-dependent in nature. Perturbation of the stress field is significant before all major gas outflow events, presumably resulting from the requirement to propagate an effective gas network before outflow is possible. In contrast, no major flow perturbations are detected which did not correlate with fluctuations in the stress field. The controls on the distribution and geometry of the resulting flow network are unclear, as well as its long-term evolution and stability. These will be beneficial in the assessment of gas pressure evolution as part of safety case development
Four-Body Effects in Globular Cluster Black Hole Coalescence
In the high density cores of globular clusters, multibody interactions are
expected to be common, with the result that black holes in binaries are
hardened by interactions. It was shown by Sigurdsson & Hernquist (1993) and
others that 10 solar mass black holes interacting exclusively by three-body
encounters do not merge in the clusters themselves, because recoil kicks the
binaries out of the clusters before the binaries are tight enough to merge.
Here we consider a new mechanism, involving four-body encounters. Numerical
simulations by a number of authors suggest that roughly 20-50% of binary-binary
encounters will eject one star but leave behind a stable hierarchical triple.
If the orbital plane of the inner binary is strongly tilted with respect to the
orbital plane of the outer object, a secular Kozai resonance, first
investigated in the context of asteroids in the Solar System, can increase the
eccentricity of the inner body significantly. We show that in a substantial
fraction of cases the eccentricity is driven to a high enough value that the
inner binary will merge by gravitational radiation, without a strong
accompanying kick. Thus the merged object remains in the cluster; depending on
the binary fraction of black holes and the inclination distribution of
newly-formed hierarchical triples, this mechanism may allow massive black holes
to accumulate through successive mergers in the cores of globular clusters. It
may also increase the likelihood that stellar-mass black holes in globular
clusters will be detectable by their gravitational radiation.Comment: Submitted to ApJ Letters (includes emulateapj.sty
Achromatizing a liquid-crystal spectropolarimeter: Retardance vs Stokes-based calibration of HiVIS
Astronomical spectropolarimeters can be subject to many sources of systematic
error which limit the precision and accuracy of the instrument. We present a
calibration method for observing high-resolution polarized spectra using
chromatic liquid-crystal variable retarders (LCVRs). These LCVRs allow for
polarimetric modulation of the incident light without any moving optics at
frequencies >10Hz. We demonstrate a calibration method using pure Stokes input
states that enables an achromatization of the system. This Stokes-based
deprojection method reproduces input polarization even though highly chromatic
instrument effects exist. This process is first demonstrated in a laboratory
spectropolarimeter where we characterize the LCVRs and show example
deprojections. The process is then implemented the a newly upgraded HiVIS
spectropolarimeter on the 3.67m AEOS telescope. The HiVIS spectropolarimeter
has also been expanded to include broad-band full-Stokes spectropolarimetry
using achromatic wave-plates in addition to the tunable full-Stokes
polarimetric mode using LCVRs. These two new polarimetric modes in combination
with a new polarimetric calibration unit provide a much more sensitive
polarimetric package with greatly reduced systematic error.Comment: Accepted in PAS
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