3,842 research outputs found
Investigation of Hamamatsu H8500 phototubes as single photon detectors
We have investigated the response of a significant sample of Hamamatsu H8500
MultiAnode PhotoMultiplier Tubes (MAPMTs) as single photon detectors, in view
of their use in a ring imaging Cherenkov counter for the CLAS12 spectrometer at
the Thomas Jefferson National Accelerator Facility. For this, a laser working
at 407.2nm wavelength was employed. The sample is divided equally into standard
window type, with a spectral response in the visible light region, and
UV-enhanced window type MAPMTs. The studies confirm the suitability of these
MAPMTs for single photon detection in such a Cherenkov imaging application
Structural Changes of Yellow Cameleon Domains Observed by Quantitative FRET Analysis and Polarized Fluorescence Correlation Spectroscopy
Förster resonance energy transfer (FRET) is a widely used method for monitoring interactions between or within biological macromolecules conjugated with suitable donor-acceptor pairs. Donor fluorescence lifetimes in absence and presence of acceptor molecules are often measured for the observation of FRET. However, these lifetimes may originate from interacting and noninteracting molecules, which hampers quantitative interpretation of FRET data. We describe a methodology for the detection of FRET that monitors the rise time of acceptor fluorescence on donor excitation thereby detecting only those molecules undergoing FRET. The large advantage of this method, as compared to donor fluorescence quenching method used more commonly, is that the transfer rate of FRET can be determined accurately even in cases where the FRET efficiencies approach 100% yielding highly quenched donor fluorescence. Subsequently, the relative orientation between donor and acceptor chromophores is obtained from time-dependent fluorescence anisotropy measurements carried out under identical conditions of donor excitation and acceptor detection. The FRET based calcium sensor Yellow Cameleon 3.60 (YC3.60) was used because it changes its conformation on calcium binding, thereby increasing the FRET efficiency. After mapping distances and orientation angles between the FRET moieties in YC3.60, cartoon models of this FRET sensor with and without calcium could be created. Independent support for these representations came from experiments where the hydrodynamic properties of YC3.60 under ensemble and single-molecule conditions on selective excitation of the acceptor were determined. From rotational diffusion times as found by fluorescence correlation spectroscopy and consistently by fluorescence anisotropy decay analysis it could be concluded that the open structure (without calcium) is flexible as opposed to the rather rigid closed conformation. The combination of two independent methods gives consistent results and presents a rapid and specific methodology to analyze structural and dynamical changes in a protein on ligand bindin
The nature of the continuum limit in the 2D gauge model
The RP(2) gauge model is studied in 2D. We use Monte-Carlo renormalization
techniques for blocking the mean spin-spin interaction, , and the mean gauge
field plaquette, . The presence of the O(3) renormalized trajectory is
verified and is consistent with the known three-loop beta-function. The
first-order `vorticity' transition observed by Solomon et al. is confirmed, and
the location of the terminating critical point is established. New scaling
flows in (,) are observed associated with a large exponent kappa in the
range 4~5. The scaling flows give rise to a strong cross-over effect between
regions of high and low vorticity and are likely to induce an apparent signal
for scaling in the cross-over region which we propose explains the scaling
observed for RP(2), RP(3) and SO(4)-matrix models. The signal for this `pseudo'
scaling will occur for the RP(2) spin model in the cross-over region which is
the region in which computer simulations are done. We find that the RP(2) spin
model is in the same universality class as the O(3) spin model but that it is
likely to require a very large correlation length before the true scaling of
this class sets in. We conjecture that the scaling flows are due either to the
influence of a nearby new renormalized trajectory or to the ghost of the
Kosterlitz-Thouless trajectory in the associated XY model.Comment: 29 pages, LATEX2e, 10 figures, uses styles[epsfig,latexsym
Test of the CLAS12 RICH large scale prototype in the direct proximity focusing configuration
A large area ring-imaging Cherenkov detector has been designed to provide
clean hadron identification capability in the momentum range from 3 GeV/c up to
8 GeV/c for the CLAS12 experiments at the upgraded 12 GeV continuous electron
beam accelerator facility of Jefferson Laboratory. The adopted solution
foresees a novel hybrid optics design based on aerogel radiator, composite
mirrors and high-packed and high-segmented photon detectors. Cherenkov light
will either be imaged directly (forward tracks) or after two mirror reflections
(large angle tracks). We report here the results of the tests of a large scale
prototype of the RICH detector performed with the hadron beam of the CERN T9
experimental hall for the direct detection configuration. The tests
demonstrated that the proposed design provides the required pion-to-kaon
rejection factor of 1:500 in the whole momentum range.Comment: 15 pages, 23 figures, to appear on EPJ
Scaling Study of Pure Gauge Lattice QCD by Monte Carlo Renormalization Group Method
The scaling behavior of pure gauge SU(3) in the region is
examined by a Monte Carlo Renormalization Group analysis. The coupling shifts
induced by factor 2 blocking are measured both on 32 and 16 lattices
with high statistics. A systematic deviation from naive 2-loop scaling is
clearly seen. The mean field and effective coupling constant schemes explain
part, but not all of the deviation. It can be accounted for by a suitable
change of coupling constant, including a correction term in the
2-loop lattice -function. Based on this improvement,
is estimated to be from the analysis of the string tension .Comment: 4 pages of A4 format including 7-postscript figure
Identifying factors limiting legume biomass production in a heterogeneous on-farm environment
Multipurpose legumes provide a wide range of benefits to smallholder production systems in the tropics. The degree of system improvement after legume introduction depends largely on legume biomass production, which in turn depends on the legumes' adaptation to environmental conditions. For Canavalia brasiliensis (canavalia), an herbaceous legume that has been recently introduced in the Nicaraguan hillsides, different approaches were tested to define the biophysical factors limiting biomass production on-farm, by combining information from topsoil chemical and physical properties, topography and soil profiles. Canavalia was planted in rotation with maize during two successive years on 72 plots distributed over six farms and at contrasting landscape positions. Above-ground biomass production was similar for both years and varied from 448 to 5357 kg/ha, with an average of 2117 kg/ha. Topsoil properties, mainly mineral nitrogen (N; ranging 25-142 mg/kg), total N (Ntot; 415-2967 mg/kg), soil organic carbon (SOC; 3-38 g/kg) and pH (5·3-7·1), significantly affected canavalia biomass production but explained only 0·45 of the variation. Topography alone explained 0·32 of the variation in canavalia biomass production. According to soil profiles descriptions, the best production was obtained on profiles with a root aggregation index close to randomness, i.e. with no major obstacles for root growth. When information from topsoil properties, topography and soil profiles was combined through a stepwise multiple regression, the model explained 0·61 of the variation in canavalia biomass (P<0·001) and included soil depth (0·5-1·70 m), slope position, amount of clay (19-696 kg/m2) and stones (7-727 kg/m2) in the whole profile, and SOC and N content in the topsoil. The linkages between topsoil properties, topography and soil profiles were further evaluated through a principal component analysis (PCA) to define the best landscape position for canavalia cultivation. The three data sets generated and used in the present study were found to be complementary. The profile description demonstrated that studies documenting heterogeneity in soil fertility should also consider deeper soil layers, especially for deep-rooted plants such as canavalia. The combination of chemical and physical soil properties with soil profile and topographic properties resulted in a holistic understanding of soil fertility heterogeneity and shows that a landscape perspective must be considered when assessing the expected benefits from multipurpose legumes in hillside environment
Measurement of the half-life of the T= mirror decay of Ne and its implication on physics beyond the standard model
The superallowed mixed mirror decay
of Ne to F is excellently suited for high precision studies of
the weak interaction. However, there is some disagreement on the value of the
half-life. In a new measurement we have determined this quantity to be
= s, which differs
from the previous world average by 3 standard deviations. The impact of this
measurement on limits for physics beyond the standard model such as the
presence of tensor currents is discussed.Comment: 5 pages, 3 figures, 1 tabl
Thrombin A-Chain: Activation Remnant or Allosteric Effector?
Although prothrombin is one of the most widely studied enzymes in biology, the role of the thrombin A-chain has been neglected in comparison to the other domains. This paper summarizes the current data on the prothrombin catalytic domain A-chain region and the subsequent thrombin A-chain. Attention is given to biochemical characterization of naturally occurring prothrombin A-chain mutations and alanine scanning mutants in this region. While originally considered to be simply an activation remnant with little physiologic function, the thrombin A-chain is now thought to play a role as an allosteric effector in enzymatic reactions and may also be a structural scaffold to stabilize the protease domain
Arago (1810): the first experimental result against the ether
95 years before Special Relativity was born, Arago attempted to detect the
absolute motion of the Earth by measuring the deflection of starlight passing
through a prism fixed to the Earth. The null result of this experiment gave
rise to the Fresnel's hypothesis of an ether partly dragged by a moving
substance. In the context of Einstein's Relativity, the sole frame which is
privileged in Arago's experiment is the proper frame of the prism, and the null
result only says that Snell's law is valid in that frame. We revisit the
history of this premature first evidence against the ether theory and calculate
the Fresnel's dragging coefficient by applying the Huygens' construction in the
frame of the prism. We expose the dissimilar treatment received by the ray and
the wave front as an unavoidable consequence of the classical notions of space
and time.Comment: 16 pages. To appear in European Journal of Physic
ОБМЕН СООБЩЕНИЯМИ В МИКС-СЕТЯХ
Рассмотрена идея Д.Чаума организации анонимной микс-сети с пересылкой сообщений
через каскад миксов (передаточных узлов) и последовательным шифрованием всех
промежуточных результатов. Описаны задачи, решаемые миксами в ходе обеспечения
анонимности связи, обговариваются проблемы ее безопасности
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