998 research outputs found
Photo-heating and the fate of hard photons during the reionisation of HeII by quasars
We use a combination of analytic and numerical arguments to consider the
impact of quasar photo-heating during HeII reionisation on the thermal
evolution of the intergalactic medium (IGM). We demonstrate that rapid (\Delta
z 10^4 K) photo-heating is difficult to achieve
across the entire IGM unless quasar spectra are significantly harder than
implied by current observational constraints. Although filtering of intrinsic
quasar radiation through dense regions in the IGM does increase the mean excess
energy per HeII photo-ionisation, it also weakens the radiation intensity and
lowers the photo-ionisation rate, preventing rapid heating over time intervals
shorter than the local photo-ionisation timescale. Moreover, the hard photons
responsible for the strongest heating are more likely to deposit their energy
inside dense clumps. The abundance of such clumps is, however, uncertain and
model-dependent, leading to a fairly large uncertainty in the photo-heating
rates. Nevertheless, although some of the IGM may be exposed to a hardened and
weakened ionising background for long periods, most of the IGM must instead be
reionised by the more abundant, softer photons and with accordingly modest
heating rates (\Delta T < 10^4 K). The repeated ionisation of fossil quasar
HeIII regions does not increase the net heating because the recombination times
in these regions typically exceed the IGM cooling times and the average time
lag between successive rounds of quasar activity. Detailed line-of-sight
radiative transfer simulations confirm these expectations and predict a rich
thermal structure in the IGM during HeII reionisation. [Abridged]Comment: 20 pages, 6 figures, accepted by MNRA
The Inhomogeneous Ionizing Background Following Reionization
We study the spatial fluctuations in the hydrogen ionizing background in the
epoch following reionization (z ~ 5--6). The rapid decrease with redshift in
the photon mean free path (m.f.p.), combined with the clustering of
increasingly rare ionizing sources, can result in a very inhomogenous ionizing
background during this epoch. We systematically investigate the probability
density functions (PDFs) and power spectra of ionizing flux, by varying several
parameters such as the m.f.p., minimum halo mass capable of hosting stars, and
halo duty cycle. In order to be versatile, we make use of analytic,
semi-numeric and numeric approaches. Our models show that the ionizing
background indeed has sizable fluctuations during this epoch sourced by the
clustering of sources, with the PDFs being a factor of few wide at half of the
maximum likelihood. The distributions also show marked asymmetries, with a
high-value tail set by clustering on small scales, and a shorter low-value tail
which is set by the mean free path. The power spectrum of the ionizing
background is much more sensitive to source properties than the PDF and can be
well-understood analytically with a framework similar to the halo model
(usually used to describe dark matter clustering). Nevertheless, we find that
Lya forest spectra are extremely insensitive to the details of the UVB, despite
marked differences in the PDFs and power spectra of our various ionizing
backgrounds. Assuming a uniform ionizing background only underestimates the
value of the mean ionization rate inferred from the Lya forest by a few
percent. Instead, analysis of the Lya forest is dominated by the uncertainties
in the density field. Thus, our results justify the common assumption of a
uniform ionizing background in Lya forest analysis even during this epoch.Comment: 11 pages, 11 figures, submitted to the MNRA
Campylobacter jejuni Multilocus Sequence Types in Humans, Northwest England, 2003–2004
MLST can be used to describe and analyze the epidemiology of campylobacteriosis in distinct human populations
Semi-numeric simulations of helium reionization and the fluctuating radiation background
Recent He II Lyman-alpha forest observations from 2.0 2.7. These results point to a fluctuating He-ionizing background, which may be due to the end of helium reionization of this era. We present a fast, semi-numeric procedure to approximate detailed cosmological simulations. We compute the distribution of dark matter halos, ionization state of helium, and density field at z = 3 in broad agreement with recent simulations. Given our speed and flexibility, we investigate a range of ionizing source and active quasar prescriptions. Spanning a large area of parameter space, we find order-of-magnitude fluctuations in the He II ionization rate in the post-reionization regime. During reionization, the fluctuations are even stronger and develop a bimodal distribution, in contrast to semi-analytic models and the hydrogen equivalent. These distributions indicate a low-level ionizing background even at significant He II fractions
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Remote Laser Diffraction Particle Size Distribution Analyzer
In support of a radioactive slurry sampling and physical characterization task, an “off-the-shelf” laser diffraction (classical light scattering) particle size analyzer was utilized for remote particle size distribution (PSD) analysis. Spent nuclear fuel was previously reprocessed at the Idaho Nuclear Technology and Engineering Center (INTEC—formerly recognized as the Idaho Chemical Processing Plant) which is on DOE’s INEEL site. The acidic, radioactive aqueous raffinate streams from these processes were transferred to 300,000 gallon stainless steel storage vessels located in the INTEC Tank Farm area. Due to the transfer piping configuration in these vessels, complete removal of the liquid can not be achieved. Consequently, a “heel” slurry remains at the bottom of an “emptied” vessel. Particle size distribution characterization of the settled solids in this remaining heel slurry, as well as suspended solids in the tank liquid, is the goal of this remote PSD analyzer task. A Horiba Instruments Inc. Model LA-300 PSD analyzer, which has a 0.1 to 600 micron measurement range, was modified for remote application in a “hot cell” (gamma radiation) environment. This technology provides rapid and simple PSD analysis, especially down in the fine and microscopic particle size regime. Particle size analysis of these radioactive slurries down in this smaller range was not previously achievable—making this technology far superior than the traditional methods used. Successful acquisition of this data, in conjunction with other characterization analyses, provides important information that can be used in the myriad of potential radioactive waste management alternatives
Localized Heating Near a Rigid Spherical Inclusion in a Viscoelastic Binder Material Under Compressional Plane Wave Excitation
High-frequency mechanical excitation has been shown to generate heat within composite energetic materials and even induce reactions in single energetic crystals embedded within an elastic binder. To further the understanding of how wave scattering effects attributable to the presence of an energetic crystal can result in concentrated heating near the inclusion, an analytical model is developed. The stress and displacement solutions associated with the scattering of compressional plane waves by a spherical obstacle (Pao and Mow, 1963, “Scattering of Plane Compressional Waves by a Spherical Obstacle,” J. Appl. Phys., 34(3), pp. 493–499) are modified to account for the viscoelastic effects of the lossy media surrounding the inclusion (Gaunaurd and Uberall, 1978, “Theory of Resonant Scattering From Spherical Cavities in Elastic and Viscoelastic Media,” J. Acoust. Soc. Am., 63(6), pp. 1699–1712). The results from this solution are then utilized to estimate the spatial heat generation due to the harmonic straining of the material, and the temperature field of the system is predicted for a given duration of time. It is shown that for certain excitation and sample configurations, the elicited thermal response near the inclusion may approach, or even exceed, the decomposition temperatures of various energetic materials. Although this prediction indicates that viscoelastic heating of the binder may initiate decomposition of the crystal even in the absence of defects such as initial voids or debonding between the crystal and binder, the thermal response resulting from this bulk heating phenomenon may be a precursor to dynamic events associated with such crystal-scale effects
The ionizing background at the end of overlap
One of the most sought-after signatures of reionization is a rapid increase
in the ionizing background (usually measured through the Lyman-alpha optical
depth toward distant quasars). Conventional wisdom associates this with the
"overlap" phase when ionized bubbles merge, allowing each source to affect a
much larger volume. We argue that this picture fails to describe the transition
to the post-overlap Universe, where Lyman-limit systems absorb ionizing photons
over moderate lengthscales (20-100 Mpc). Using an analytic model, we compute
the probability distribution of the amplitude of the ionizing background
throughout reionization, including both discrete ionized bubbles and
Lyman-limit systems (parameterized by an attenuation length). We show that
overlap does not by itself cause a rapid increase in the ionizing background or
a rapid decrease in the mean Lyman-alpha transmission toward distant quasars.
More detailed semi-numeric models support these conclusions. We argue that
rapid changes should instead be interpreted as evolution in the attenuation
length itself, which may or may not be directly related to overlap.Comment: submitted to MNRAS, 7 pages, 5 figure
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