1,933 research outputs found
Radiation Damage Studies of Silicon Photomultipliers
We report on the measurement of the radiation hardness of silicon
photomultipliers (SiPMs) manufactured by
Fondazione Bruno Kessler in Italy (1 mm and 6.2 mm), Center of
Perspective Technology and Apparatus in Russia (1 mm and 4.4 mm), and
Hamamatsu Corporation in Japan (1 mm). The SiPMs were irradiated using a
beam of 212 MeV protons at Massachusetts General Hospital, receiving fluences
of up to protons per cm with the SiPMs at operating
voltage. Leakage currents were read continuously during the irradiation. The
delivery of the protons was paused periodically to record scope traces in
response to calibrated light pulses to monitor the gains, photon detection
efficiencies, and dark counts of the SiPMs. The leakage current and dark noise
are found to increase with fluence. Te leakage current is found to be
proportional to the mean square deviation of the noise distribution, indicating
the dark counts are due to increased random individual pixel activation, while
SiPMs remain fully functional as photon detectors. The SiPMs are found to
anneal at room temperature with a reduction in the leakage current by a factor
of 2 in about 100 days.Comment: 35 pages, 25 figure
High-pressure structural investigation of several zircon-type orthovanadates
Room temperature angle-dispersive x-ray diffraction measurements on
zircon-type EuVO4, LuVO4, and ScVO4 were performed up to 27 GPa. In the three
compounds we found evidence of a pressure-induced structural phase
transformation from zircon to a scheelite-type structure. The onset of the
transition is near 8 GPa, but the transition is sluggish and the low- and
high-pressure phases coexist in a pressure range of about 10 GPa. In EuVO4 and
LuVO4 a second transition to a M-fergusonite-type phase was found near 21 GPa.
The equations of state for the zircon and scheelite phases are also determined.
Among the three studied compounds, we found that ScVO4 is less compressible
than EuVO4 and LuVO4, being the most incompressible orthovanadate studied to
date. The sequence of structural transitions and compressibilities are
discussed in comparison with other zircon-type oxides.Comment: 34 pages, 2 Tables, 11 Figure
Assessing the Performance of Recent Density Functionals for Bulk Solids
We assess the performance of recent density functionals for the
exchange-correlation energy of a nonmolecular solid, by applying accurate
calculations with the GAUSSIAN, BAND, and VASP codes to a test set of 24 solid
metals and non-metals. The functionals tested are the modified
Perdew-Burke-Ernzerhof generalized gradient approximation (PBEsol GGA), the
second-order GGA (SOGGA), and the Armiento-Mattsson 2005 (AM05) GGA. For
completeness, we also test more-standard functionals: the local density
approximation, the original PBE GGA, and the Tao-Perdew-Staroverov-Scuseria
(TPSS) meta-GGA. We find that the recent density functionals for solids reach a
high accuracy for bulk properties (lattice constant and bulk modulus). For the
cohesive energy, PBE is better than PBEsol overall, as expected, but PBEsol is
actually better for the alkali metals and alkali halides. For fair comparison
of calculated and experimental results, we consider the zero-point phonon and
finite-temperature effects ignored by many workers. We show how Gaussian basis
sets and inaccurate experimental reference data may affect the rating of the
quality of the functionals. The results show that PBEsol and AM05 perform
somewhat differently from each other for alkali metal, alkaline earth metal and
alkali halide crystals (where the maximum value of the reduced density gradient
is about 2), but perform very similarly for most of the other solids (where it
is often about 1). Our explanation for this is consistent with the importance
of exchange-correlation nonlocality in regions of core-valence overlap.Comment: 32 pages, single pdf fil
Spatio-Temporal Gap Analysis of OBIS-SEAMAP Project Data: Assessment and Way Forward
The OBIS-SEAMAP project has acquired and served high-quality marine mammal, seabird, and sea turtle data to the public since its inception in 2002. As data accumulated, spatial and temporal biases resulted and a comprehensive gap analysis was needed in order to assess coverage to direct data acquisition for the OBIS-SEAMAP project and for taxa researchers should true gaps in knowledge exist. All datasets published on OBIS-SEAMAP up to February 2009 were summarized spatially and temporally. Seabirds comprised the greatest number of records, compared to the other two taxa, and most records were from shipboard surveys, compared to the other three platforms. Many of the point observations and polyline tracklines were located in northern and central Atlantic and the northeastern and central-eastern Pacific. The Southern Hemisphere generally had the lowest representation of data, with the least number of records in the southern Atlantic and western Pacific regions. Temporally, records of observations for all taxa were the lowest in fall although the number of animals sighted was lowest in the winter. Oceanographic coverage of observations varied by platform for each taxa, which showed that using two or more platforms represented habitat ranges better than using only one alone. Accessible and published datasets not already incorporated do exist within spatial and temporal gaps identified. Other related open-source data portals also contain data that fill gaps, emphasizing the importance of dedicated data exchange. Temporal and spatial gaps were mostly a result of data acquisition effort, development of regional partnerships and collaborations, and ease of field data collection. Future directions should include fostering partnerships with researchers in the Southern Hemisphere while targeting datasets containing species with limited representation. These results can facilitate prioritizing datasets needed to be represented and for planning research for true gaps in space and time
Study of a mini-array for the Linsley effect in cosmic-ray air showers
The arrival-time distribution of shower particles far from the core has been measured as a function of shower axis distance at the Akeno Observatory by a cluster of 64 scintillators (total area 16 m2) located near the centre of the 1 km2 array. More than 3500 1 km2 array events and about 300 20 km2 array events have been analysed. The authors compare parameters tau (exponential fit to arrival times), sigma (conventional dispersion) and tr (rise time) for the specification of the spread in arrival times. From a measurement of time spread, the shower axis distance can be determined with an uncertainty of 25 to 30% with this detector system. The resulting uncertainty in shower size, including a 60% uncertainty in particle density measurement, is 120 to 140%. The resultant smearing of changes in slope of the frequency spectrum of incident showers is calculated. The smearing masks all but the gross changes in slope. The method may be useful for anisotropy studies, when a system for finding direction is included.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/49016/2/jgv15i1p113.pd
Transformation Pathways of Silica under High Pressure
Concurrent molecular dynamics simulations and ab initio calculations show
that densification of silica under pressure follows a ubiquitous two-stage
mechanism. First, anions form a close-packed sub-lattice, governed by the
strong repulsion between them. Next, cations redistribute onto the interstices.
In cristobalite silica, the first stage is manifest by the formation of a
metastable phase, which was observed experimentally a decade ago, but never
indexed due to ambiguous diffraction patterns. Our simulations conclusively
reveal its structure and its role in the densification of silica.Comment: 14 pages, 4 figure
Chiral Polymerization in Open Systems From Chiral-Selective Reaction Rates
We investigate the possibility that prebiotic homochirality can be achieved
exclusively through chiral-selective reaction rate parameters without any other
explicit mechanism for chiral bias. Specifically, we examine an open network of
polymerization reactions, where the reaction rates can have chiral-selective
values. The reactions are neither autocatalytic nor do they contain explicit
enantiomeric cross-inhibition terms. We are thus investigating how rare a set
of chiral-selective reaction rates needs to be in order to generate a
reasonable amount of chiral bias. We quantify our results adopting a
statistical approach: varying both the mean value and the rms dispersion of the
relevant reaction rates, we show that moderate to high levels of chiral excess
can be achieved with fairly small chiral bias, below 10%. Considering the
various unknowns related to prebiotic chemical networks in early Earth and the
dependence of reaction rates to environmental properties such as temperature
and pressure variations, we argue that homochirality could have been achieved
from moderate amounts of chiral selectivity in the reaction rates.Comment: 15 pages, 6 figures, accepted for publication in Origins of Life and
Evolution of Biosphere
Multiple time scales in cataclysmic binaries. The low-field magnetic dwarf nova DO Draconis
We study the variability of the cataclysmic variable DO Dra, on time-scales
of between minutes and decades.
The characteristic decay time dt/dm=0.902(3) days/mag was estimated from our
3 nights of CCD R observations. The quiescent data show a photometric wave with
a cycle about 303(15)d. We analyzed the profile of the composite (or mean)
outburst. We discovered however, that a variety of different outburst heights
and durations had occurred, contrary to theoretical predictions. With
increasing maximum brightness, we find that the decay time also increases; this
is in contrast to the model predictions, which indicate that outbursts should
have a constant shape. This is interpreted as representing the presence of
outburst-to-outburst variability of the magnetospheric radius. A presence of a
number of missed weak narrow outbursts is predicted from this statistical
relationship. A new type of variability is detected, during 3 subsequent nights
in 2007: periodic (during one nightly run) oscillations with rapidly-decreasing
frequency from 86 to 47 cycles/day and a semi-amplitude increasing from 0.06 to
0. 10, during a monotonic brightness increase from 14. 27 to 14. 13. This
phenomenon was observed only during an unusually prolonged event of about 1 mag
brightening in 2007 (lasting till autumn), during which no (expected) outburst
was detected. We refer to this behaviour as to the transient periodic
oscillations (TPO). To study this new and interesting phenomenon, new regular
photometric and spectral (in a target of opportunity mode) observations are
required.Comment: 12pages, 8figures, accepted in Astronomy and Astrophysic
Gut microbiotaâdependent trimethylamine Nâoxide and cardiovascular outcomes in patients with prior myocardial infarction: A nested case control study from the PEGASUSâTIMI 54 trial
Background
Trimethylamine Nâoxide (TMAO) may have prothrombotic properties. We examined the association of TMAO quartiles with major adverse cardiovascular events (MACE) and the effect of TMAO on the efficacy of ticagrelor.
Methods and Results
PEGASUSâTIMI 54 (Prevention of Cardiovascular Events in Patients With Prior Heart Attack Using Ticagrelor Compared to Placebo on a Background of Aspirin â Thrombolysis in Myocardial Infarction 54) randomized patients with prior myocardial infarction to ticagrelor or placebo (median followâup 33 months). Baseline plasma concentrations of TMAO were measured in a nested caseâcontrol study of 597 cases with cardiovascular death, myocardial infarction, or stroke (MACE) and 1206 controls matched for age, sex, and estimated glomerular filtration rate [eGFR]. Odds ratios (OR) were used for the association between TMAO quartiles and MACE, adjusting for baseline clinical characteristics (age, sex, eGFR, region, body mass index, hypertension, hypercholesterolemia, diabetes mellitus, smoking, peripheral artery disease, index event, aspirin dosage and treatment arm), and cardiovascular biomarkers (hsâTnT [highâsensitivity troponin T], hsâCRP [highâsensitivity Câreactive protein], NTâproBNP [NâterminalâproâBâtype natriuretic peptide]). Higher TMAO quartiles were associated with risk of MACE (OR for quartile 4 versus quartile 1, 1.43, 95% CI, 1.06â1.93, P trend=0.015). The association was driven by cardiovascular death (OR 2.25, 95% CI, 1.28â3.96, P trend=0.003) and stroke (OR 2.68, 95% CI, 1.39â5.17, P trend<0.001). After adjustment for clinical factors, the association persisted for cardiovascular death (ORadj 1.89, 95% CI, 1.03â3.45, P trend=0.027) and stroke (ORadj 2.01, 95% CI, 1.01â4.01, P trend=0.022), but was slightly attenuated after adjustment for cardiovascular biomarkers (cardiovascular death: ORadj 1.74, 95% CI, 0.88â3.45, P trend=0.079; and stroke: ORadj 1.82, 95% CI, 0.88â3.78, P trend=0.056). The reduction in MACE with ticagrelor was consistent across TMAO quartiles (P interaction=0.92).
Conclusions
Among patients with prior myocardial infarction, higher TMAO levels were associated with cardiovascular death and stroke but not with recurrent myocardial infarction. The efficacy of ticagrelor was consistent regardless of TMAO levels.
Registration
URL: https://www.cliniâcaltrâials.gov; Unique identifiers: PEGASUSâTIMI 54, NCT01225562
Electron Correlation and the c-axis Dispersion of Cu d_z^2: a New Band Structure for High Temperature Superconductors
Previously we showed the major effect of electron correlation in the cuprate
superconductors is to lower the energy of the Cu d_x^2-y^2/O p_sigma (x^2-y^2)
band with respect to the Cu d_z^2/O' p_z (z^2) band. In our 2D Hubbard model
for La_1.85Sr_0.15CuO_4 (LaSCO), the z^2 band is narrow and crosses the
standard x^2-y^2 band just below the Fermi level. In this work, we introduce
c-axis dispersion to the model and find the z^2 band to have considerable
anisotropic 3D character. An additional hole-like surface opens up in the z^2
band at (0,0,2pi/c) which expands with doping. At sufficient doping levels, a
symmetry allowed x^2-y^2/z^2 band crossing along the (0,0)-(pi,pi) direction of
the Brillouin zone appears at the Fermi level. At this point, Cooper pairs
between the two bands (e.g. (k uparrow x^2-y^2/k downarrow z^2)) can form,
providing the basis for the Interband Pairing Theory of superconductivity in
these materials.Comment: submitted to Phys. Rev. Lett. Related publications: Phys. Rev. B 58,
12303 (1998); Phys. Rev. B 58, 12323 (1998); cond-mat/9903088;
cond-mat/990310
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