443 research outputs found
Fragility and compressibility at the glass transition
Isothermal compressibilities and Brillouin sound velocities from the
literature allow to separate the compressibility at the glass transition into a
high-frequency vibrational and a low-frequency relaxational part. Their ratio
shows the linear fragility relation discovered by x-ray Brillouin scattering
[1], though the data bend away from the line at higher fragilities. Using the
concept of constrained degrees of freedom, one can show that the vibrational
part follows the fragility-independent Lindemann criterion; the fragility
dependence seems to stem from the relaxational part. The physical meaning of
this finding is discussed. [1] T. Scopigno, G. Ruocco, F. Sette and G. Monaco,
Science 302, 849 (2003)Comment: 4 pages, 2 figures, 2 tables, 33 references. Slightly changed after
refereein
The Raman coupling function in amorphous silica and the nature of the long wavelength excitations in disordered systems
New Raman and incoherent neutron scattering data at various temperatures and
molecular dynamic simulations in amorphous silica, are compared to obtain the
Raman coupling coefficient and, in particular, its low frequency
limit. This study indicates that in the limit
extrapolates to a non vanishing value, giving important indications on the
characteristics of the vibrational modes in disordered materials; in particular
our results indicate that even in the limit of very long wavelength the local
disorder implies non-regular local atomic displacements.Comment: Revtex, 4 ps figure
Co-firing of biomass and other wastes in fluidised bed systems
A project on co-firing in large-scale power plants burning coal is currently funded by the European Commission. It is called COPOWER. The project involves 10 organisations from 6 countries. The project involves combustion studies over the full spectrum of equipment size, ranging from small laboratory-scale reactors and pilot plants, to investigate fundamentals and operating parameters, to proving trials on a commercial power plant in Duisburg. The power plant
uses a circulating fluidized bed boiler. The results to be obtained are to be compared as function of scale-up. There are two different coals, 3 types of biomass and 2 kinds of waste materials are to be used for blending with coal for co-firing tests. The baseline values are obtained during a campaign of one month at the power station and the results are used for comparison with those to be obtained in other units of various sizes. Future tests will be implemented with the objective to achieve improvement on baseline values. The fuels to be used are already characterized. There are ongoing studies to determine reactivities of fuels and chars produced from the fuels. Reactivities are determined not only for individual fuels but also for blends to be used. Presently pilot-scale combustion tests are also undertaken to study the effect of blending coal with different types of biomass and waste materials. The potential for synergy to improve combustion is investigated. Early results will be reported in the Conference. Simultaneously, studies to verify the availability of biomass and waste materials in Portugal, Turkey and Italy have been undertaken. Techno-economic barriers for the future use of biomass and other waste materials are identified. The potential of using these materials in coal fired power stations has been assessed. The conclusions will also be reported
Tunka-Rex: the Cost-Effective Radio Extension of the Tunka Air-Shower Observatory
Tunka-Rex is the radio extension of the Tunka cosmic-ray observatory in
Siberia close to Lake Baikal. Since October 2012 Tunka-Rex measures the radio
signal of air-showers in coincidence with the non-imaging air-Cherenkov array
Tunka-133. Furthermore, this year additional antennas will go into operation
triggered by the new scintillator array Tunka-Grande measuring the secondary
electrons and muons of air showers. Tunka-Rex is a demonstrator for how
economic an antenna array can be without losing significant performance: we
have decided for simple and robust SALLA antennas, and we share the existing
DAQ running in slave mode with the PMT detectors and the scintillators,
respectively. This means that Tunka-Rex is triggered externally, and does not
need its own infrastructure and DAQ for hybrid measurements. By this, the
performance and the added value of the supplementary radio measurements can be
studied, in particular, the precision for the reconstructed energy and the
shower maximum in the energy range of approximately eV. Here
we show first results on the energy reconstruction indicating that radio
measurements can compete with air-Cherenkov measurements in precision.
Moreover, we discuss future plans for Tunka-Rex.Comment: Proceeding of UHECR 2014, Springdale, Utah, USA, accepted by JPS
Conference Proceeding
Quantitative reconstruction of precipitation changes on the NE Tibetan Plateau since the Last Glacial Maximum – extending the concept of pollen source area to pollen-based climate reconstructions from large lakes
Pollen records from large lakes have been used for quantitative
palaeoclimate reconstruction, but the influences that lake size (as a result
of species-specific variations in pollen dispersal patterns that smaller
pollen grains are more easily transported to lake centre) and taphonomy have
on these climatic signals have not previously been systematically
investigated. We introduce the concept of pollen source area to pollen-based
climate calibration using the north-eastern Tibetan
Plateau as our study area. We present a pollen data set collected from large
lakes in the arid to semi-arid region of central Asia. The influences that
lake size and the inferred pollen source areas have on pollen compositions
have been investigated through comparisons with pollen assemblages in
neighbouring lakes of various sizes. Modern pollen samples collected from
different parts of Lake Donggi Cona (in the north-eastern part of the
Tibetan Plateau) reveal variations in pollen assemblages within this large
lake, which are interpreted in terms of the species-specific dispersal and
depositional patterns for different types of pollen, and in terms of fluvial
input components. We have estimated the pollen source area for each lake
individually and used this information to infer modern climate data with
which to then develop a modern calibration data set, using both the
multivariate regression tree (MRT) and weighted-averaging partial least
squares (WA-PLS) approaches. Fossil pollen data from Lake Donggi Cona have
been used to reconstruct the climate history of the north-eastern part of
the Tibetan Plateau since the Last Glacial Maximum (LGM). The mean annual
precipitation was quantitatively reconstructed using WA-PLS: extremely dry
conditions are found to have dominated the LGM, with annual precipitation of
around 100 mm, which is only 32% of present-day precipitation. A
gradually increasing trend in moisture conditions during the Late Glacial is
terminated by an abrupt reversion to a dry phase that lasts for about 1000 yr
and coincides with "Heinrich event 1" in the North Atlantic
region. Subsequent periods corresponding to the Bølling/Allerød
interstadial, with annual precipitation (<i>P</i><sub>ann</sub>) of about 350 mm, and the
Younger Dryas event (about 270 mm <i>P</i><sub>ann</sub>) are followed by moist
conditions in the early Holocene, with annual precipitation of up to 400 mm.
A drier trend after 9 cal. ka BP is followed by a second wet phase in the
middle Holocene, lasting until 4.5 cal. ka BP. Relatively steady conditions
with only slight fluctuations then dominate the late Holocene, resulting in
the present climatic conditions. The climate changes since the LGM have been
primarily driven by deglaciation and fluctuations in the intensity of the
Asian summer monsoon that resulted from changes in the Northern Hemisphere
summer solar insolation, as well as from changes in the North Atlantic
climate through variations in the circulation patterns and intensity of the
westerlies
Status and Plans for the Array Control and Data Acquisition System of the Cherenkov Telescope Array
The Cherenkov Telescope Array (CTA) is the next-generation atmospheric
Cherenkov gamma-ray observatory. CTA will consist of two installations, one in
the northern, and the other in the southern hemisphere, containing tens of
telescopes of different sizes. The CTA performance requirements and the
inherent complexity associated with the operation, control and monitoring of
such a large distributed multi-telescope array leads to new challenges in the
field of the gamma-ray astronomy. The ACTL (array control and data acquisition)
system will consist of the hardware and software that is necessary to control
and monitor the CTA arrays, as well as to time-stamp, read-out, filter and
store -at aggregated rates of few GB/s- the scientific data. The ACTL system
must be flexible enough to permit the simultaneous automatic operation of
multiple sub-arrays of telescopes with a minimum personnel effort on site. One
of the challenges of the system is to provide a reliable integration of the
control of a large and heterogeneous set of devices. Moreover, the system is
required to be ready to adapt the observation schedule, on timescales of a few
tens of seconds, to account for changing environmental conditions or to
prioritize incoming scientific alerts from time-critical transient phenomena
such as gamma ray bursts. This contribution provides a summary of the main
design choices and plans for building the ACTL system.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
Radio measurements of the energy and the depth of the shower maximum of cosmic-ray air showers by Tunka-Rex
We reconstructed the energy and the position of the shower maximum of air
showers with energies PeV applying a method using radio
measurements performed with Tunka-Rex. An event-to-event comparison to
air-Cherenkov measurements of the same air showers with the Tunka-133
photomultiplier array confirms that the radio reconstruction works reliably.
The Tunka-Rex reconstruction methods and absolute scales have been tuned on
CoREAS simulations and yield energy and values consistent
with the Tunka-133 measurements. The results of two independent measurement
seasons agree within statistical uncertainties, which gives additional
confidence in the radio reconstruction. The energy precision of Tunka-Rex is
comparable to the Tunka-133 precision of , and exhibits a
uncertainty on the absolute scale dominated by the amplitude calibration of the
antennas. For , this is the first direct experimental
correlation of radio measurements with a different, established method. At the
moment, the resolution of Tunka-Rex is approximately g/cm. This resolution can probably be improved by deploying additional
antennas and by further development of the reconstruction methods, since the
present analysis does not yet reveal any principle limitations.Comment: accepted for publication by JCA
Tunka-Rex: energy reconstruction with a single antenna station (ARENA 2016)
The Tunka-Radio extension (Tunka-Rex) is a radio detector for air showers in
Siberia. From 2012 to 2014, Tunka-Rex operated exclusively together with its
host experiment, the air-Cherenkov array Tunka-133, which provided trigger,
data acquisition, and an independent air-shower reconstruction. It was shown
that the air-shower energy can be reconstructed by Tunka-Rex with a precision
of 15\% for events with signal in at least 3 antennas, using the radio
amplitude at a distance of 120\,m from the shower axis as an energy estimator.
Using the reconstruction from the host experiment Tunka-133 for the air-shower
geometry (shower core and direction), the energy estimator can in principle
already be obtained with measurements from a single antenna, close to the
reference distance. We present a method for event selection and energy
reconstruction, requiring only one antenna, and achieving a precision of about
20\%. This method increases the effective detector area and lowers thresholds
for zenith angle and energy, resulting in three times more events than in the
standard reconstruction
Towards a cosmic-ray mass-composition study at Tunka Radio Extension (ARENA 2016)
The Tunka Radio Extension (Tunka-Rex) is a radio detector at the TAIGA
facility located in Siberia nearby the southern tip of Lake Baikal. Tunka-Rex
measures air-showers induced by high-energy cosmic rays, in particular, the
lateral distribution of the radio pulses. The depth of the air-shower maximum,
which statistically depends on the mass of the primary particle, is determined
from the slope of the lateral distribution function (LDF). Using a
model-independent approach, we have studied possible features of the
one-dimensional slope method and tried to find improvements for the
reconstruction of primary mass. To study the systematic uncertainties given by
different primary particles, we have performed simulations using the CONEX and
CoREAS software packages of the recently released CORSIKA v7.5 including the
modern high-energy hadronic models QGSJet-II.04 and EPOS-LHC. The simulations
have shown that the largest systematic uncertainty in the energy deposit is due
to the unknown primary particle. Finally, we studied the relation between the
polarization and the asymmetry of the LDF.Comment: ARENA proceedings, 4 pages, updated reference
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