80 research outputs found
Ultrafast x-ray diffraction of laser-irradiated crystals
An apparatus has been developed for measuring time-dependent x-ray diffraction. X-ray pulses from an Advanced Light Source bend magnet are diffracted by a sagittally-focusing Si (111) crystal and then by a sample crystal, presently InSb (111). Laser pulses with 100 fs duration and a repetition rate of 1 KHz irradiate the sample inducing a phase transition. Two types of detectors are being employed: an x-ray streak camera and an avalanche photodiode. The streak camera is driven by a photoconductive switch and has a 2 ps temporal resolution determined by trigger jitter. The avalanche photodiode has high quantum efficiency and sufficient time resolution to detect single x-ray pulses in ALS two bunch or âcamshaftâ operation. A beamline is under construction dedicated for time resolved and micro-diffraction experiments. In the new beamline a toroidal mirror collects 3 mrad horizontally and makes a 1:1 image of the bend magnet source in the x-ray hutch. A laser induced phase transition has been observed in InSb occurring within 70 ps. © 1997 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87821/2/204_1.pd
Interplay of thermal and non-thermal effects in x-ray-induced ultrafast melting
X-ray laser-induced structural changes in silicon undergoing femtosecond
melting have been investigated by using an x-ray pump-x-ray probe technique.
The experimental results for different initial sample temperatures reveal that
the onset time and the speed of the atomic disordering are independent of the
initial temperature, suggesting that equilibrium atomic motion in the initial
state does not play a pivotal role in the x-ray-induced ultrafast melting. By
comparing the observed time-dependence of the atomic disordering and the
dedicated theoretical simulations, we interpret that the energy transfer from
the excited electrons to ions via electron-ion coupling (thermal effect) as
well as a strong modification of the interatomic potential due to electron
excitations (non-thermal effect) trigger the ultrafast atomic disordering. Our
finding of the interplay of thermal and non-thermal effects in the
x-ray-induced melting demonstrates that accurate modeling of intense x-ray
interactions with matter is essential to ensure a correct interpretation of
experiments using intense x-ray laser pulses
Transverse Polarisation of Quarks in Hadrons
We review the present state of knowledge regarding the transverse
polarisation (or transversity) distributions of quarks. After some generalities
on transverse polarisation, we formally define the transversity distributions
within the framework of a classification of all leading-twist distribution
functions. We describe the QCD evolution of transversity at leading and
next-to-leading order. A comprehensive treatment of non-perturbative
calculations of transversity distributions (within the framework of quark
models, lattice QCD and QCD sum rules) is presented. The phenomenology of
transversity (in particular, in Drell-Yan processes and semi-inclusive
leptoproduction) is discussed in some detail. Finally, the prospects for future
measurements are outlined.Comment: small changes, references added, as finally published in Physics
Report
In situ measurements of atmospheric O2 and CO2 reveal an unexpected O2 signal over the tropical Atlantic Ocean
We present the first meridional transects of atmospheric O2 and CO2 over the Atlantic Ocean. We combine these measurements into the tracer atmospheric potential oxygen (APO), which is a measure of the oceanic contribution to atmospheric O2 variations. Our new in situ measurement system, deployed on board a commercial container ship during 2015, performs as well as or better than existing similar measurement systems. The data show small short-term variability (hours to days), a step-change corresponding to the position of the Intertropical Convergence Zone (ITCZ), and seasonal cycles that vary with latitude. In contrast to data from the Pacific Ocean and to previous modeling studies, our Atlantic Ocean APO data show no significant bulge in the tropics. This difference cannot be accounted for by interannual variability in the position of the ITCZ or the Atlantic Meridional Mode Index and appears to be a persistent feature of the Atlantic Ocean system. Modeled APO using the TM3 atmospheric transport model does exhibit a significant bulge over the Atlantic and overestimates the interhemispheric gradient in APO over the Atlantic Ocean. These results indicate that either there are inaccuracies in the oceanic flux data products in the equatorial Atlantic Ocean region, or that there are atmospheric transport inaccuracies in the model, or a combination of both. Our shipboard O2 and CO2 measurements are ongoing and will reveal the long-term nature of equatorial APO outgassing over the Atlantic as more data become available
Earlier snowmelt may lead to late season declines in plant productivity and carbon sequestration in Arctic tundra ecosystems
Arctic warming is affecting snow cover and soil hydrology, with consequences for carbon sequestration in tundra ecosystems. The scarcity of observations in the Arctic has limited our understanding of the impact of covarying environmental drivers on the carbon balance of tundra ecosystems. In this study, we address some of these uncertainties through a novel record of 119 site-years of summer data from eddy covariance towers representing dominant tundra vegetation types located on continuous permafrost in the Arctic. Here we found that earlier snowmelt was associated with more tundra net CO2 sequestration and higher gross primary productivity (GPP) only in June and July, but with lower net carbon sequestration and lower GPP in August. Although higher evapotranspiration (ET) can result in soil drying with the progression of the summer, we did not find significantly lower soil moisture with earlier snowmelt, nor evidence that water stress affected GPP in the late growing season. Our results suggest that the expected increased CO2 sequestration arising from Arctic warming and the associated increase in growing season length may not materialize if tundra ecosystems are not able to continue sequestering CO2 later in the season.Peer reviewe
The Time-resolved Atomic, Molecular and Optical Science Instrument at the Linac Coherent Light Source
The newly constructed Time-resolved atomic, Molecular and Optical science
instrument (TMO), is configured to take full advantage of both linear
accelerators at SLAC National Accelerator Laboratory, the copper accelerator
operating at a repetition rate of 120 Hz providing high per pulse energy, as
well as the superconducting accelerator operating at a repetition rate of about
1 MHz providing high average intensity. Both accelerators build a soft X-ray
free electron laser with the new variable gab undulator section. With this
flexible light sources, TMO supports many experimental techniques not
previously available at LCLS and will have two X-ray beam focus spots in line.
Thereby, TMO supports Atomic, Molecular and Optical (AMO), strong-field and
nonlinear science and will host a designated new dynamic reaction microscope
with a sub-micron X-ray focus spot. The flexible instrument design is optimized
for studying ultrafast electronic and molecular phenomena and can take full
advantage of the sub-femtosecond soft X-ray pulse generation program
Evolution of fluid expulsion and concentrated hydrate zones across the southern Hikurangi subduction margin, New Zealand: An analysis from depth migrated seismic data
Identification of methane sources controlling hydrate distribution and concentrations in continental margins remains a major challenge in gas hydrate research. Lack of deep fluid samples and high quality regional scale seismic reflection data may lead to underestimation of the significance of fluid escape from subducting and compacting sediments in the global inventory of methane reaching the hydrate zone, the water column and the atmosphere. The distribution of concentrated hydrate zones in relation to focused fluid flow across the southern Hikurangi subduction margin was investigated using high quality, long offset (10 km streamer), pre-stack depth migrated multichannel seismic data. Analysis of low P wave velocity zones, bright-reverse polarity reflections and dim-amplitude anomalies reveals pathways for gas escape and zones of gas accumulation. The study shows the structural and stratigraphic settings of three main areas of concentrated hydrates: (1) the Opouawe Bank, dominated by focused periodic fluid input along thrust faults sustaining dynamic hydrate concentrations and gas chimneys development; (2) the frontal anticline, with a basal set of protothrusts controlling permeability for fluids from deeply buried and subducted sediments sustaining hydrate concentrations at the crest of the anticline; and (3) the Hikurangi Channel, with buried sand dominated channels hosting significant amounts of gas beneath the base of the hydrate zone. In sand dominated channels gas injection into the hydrate zone favors highly concentrated hydrate accumulations. The evolution of fluid expulsion controlling hydrate formation offshore southern Hikurangi is described in stages during which different methane sources (in situ, buried and thermogenic) have been dominant
On the relational dynamics of caring: a psychotherapeutic approach to emotional and power dimensions of womenâs care work
Care is double-edged and paradoxical, inspiring a vast range of strong feelings in both
care-givers and care-recipients. This paper draws on ideas about psychotherapeutic
relationships to offer a theorisation of the complex emotional and power dynamics and
imaginative geographies of care. Examining the humanistic approach developed by Carl
Rogers as well as the psychoanalytic tradition, I advance an interpretation of
psychotherapeutic practices that foregrounds the fundamental importance of the
emotional and power-inflected relationship between practitioners and those with whom
they work. I show how different traditions offer conceptualisations of the shape of
therapeutic relationships that are highly relevant to consideration of the emotional and
power dynamics of giving and receiving care. Against this background I discuss current
debates about care, emotions and power, drawing especially on feminist and disability
perspectives and arguing that psychotherapeutic approaches offer a powerful lens
through which to understand the emotional and power dynamics of caring relationships.
I conclude by emphasising how this theorisation helps to illuminate ubiquitous features
of womenâs care work
Multiple Scenario Generation of Subsurface Models:Consistent Integration of Information from Geophysical and Geological Data throuh Combination of Probabilistic Inverse Problem Theory and Geostatistics
Neutrinos with energies above 1017 eV are detectable with the Surface Detector Array of the Pierre Auger Observatory. The identification is efficiently performed for neutrinos of all flavors interacting in the atmosphere at large zenith angles, as well as for Earth-skimming \u3c4 neutrinos with nearly tangential trajectories relative to the Earth. No neutrino candidates were found in 3c 14.7 years of data taken up to 31 August 2018. This leads to restrictive upper bounds on their flux. The 90% C.L. single-flavor limit to the diffuse flux of ultra-high-energy neutrinos with an E\u3bd-2 spectrum in the energy range 1.0
7 1017 eV -2.5
7 1019 eV is E2 dN\u3bd/dE\u3bd < 4.4
7 10-9 GeV cm-2 s-1 sr-1, placing strong constraints on several models of neutrino production at EeV energies and on the properties of the sources of ultra-high-energy cosmic rays
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