601 research outputs found
Proposal to determine the Fermi-surface topology of a doped iron-based superconductor using bulk-sensitive Fourier-transform Compton scattering
No full textWe have carried out first-principles calculations of the Compton scattering spectra to demonstrate that the filling of the hole Fermi surface in LaO1?xFxFeAs produces a distinct signature in the Fourier-transformed Compton spectrum when the momentum transfer vector lies along the [100] direction. We thus show how the critical concentration xc, where hole Fermi-surface pieces are filled up and the superconductivity mediated by antiferromagnetic spin fluctuations is expected to be suppressed, can be obtained in a bulk-sensitive manner.Radiation, Radionuclides and ReactorsApplied Science
An inverse Compton scattering (ICS) model of pulsar emission - II. Frequency behavior of pulse profiles
No full textThe shapes of pulse profiles, especially their variations with respect to observing frequencies, are very important to understand emission mechanisms of pulsars. However, no previous attempt has been made to interpret their complicated phenomenology. In this paper, we present theoretical simulations for the integrated pulse profiles and their frequency evolution within the framework of the inverse Compton scattering (ICS) model proposed by Qiao (1988) and Qiao & Lin (1998). Using the phase positions of the pulse components predicted by the "beam-frequency figure" of the ICS model, we present Gaussian fits to the multi-frequency pulse profiles for some pulsars. It is shown that the model can reproduce various types of the frequency evolution behaviors of pulse profiles observed.Astronomy & AstrophysicsSCI(E)EI19ARTICLE3964-97137
A genetic variation map for chicken with 2.8 million single-nucleotide polymorphisms
No full textWe describe a genetic variation map for the chicken genome containing 2.8 million single-nucleotide polymorphisms (SNPs). This map is based on a comparison of the sequences of three domestic chicken breeds (a broiler, a layer and a Chinese silkie) with that of their wild ancestor, red jungle fowl. Subsequent experiments indicate that at least 90% of the variant sites are true SNPs, and at least 70% are common SNPs that segregate in many domestic breeds. Mean nucleotide diversity is about five SNPs per kilobase for almost every possible comparison between red jungle fowl and domestic lines, between two different domestic lines, and within domestic lines--in contrast to the notion that domestic animals are highly inbred relative to their wild ancestors. In fact, most of the SNPs originated before domestication, and there is little evidence of selective sweeps for adaptive alleles on length scales greater than 100 kilobases
Bulk Fermi surface and momentum density in heavily doped La2?xSrxCuO4 using high-resolution Compton scattering and positron annihilation spectroscopies
No full textWe have observed the bulk Fermi surface (FS) in an overdoped (x=0.3) single crystal of La2?xSrxCuO4 by using Compton scattering. A two-dimensional (2D) momentum density reconstruction from measured Compton profiles yields a clear FS signature in the third Brillouin zone along [100]. The quantitative agreement between density functional theory (DFT) calculations and momentum density experiment suggests that Fermi-liquid physics is restored in the overdoped regime. In particular the predicted FS topology is found to be in good accord with the corresponding experimental data. We find similar quantitative agreement between the measured 2D angular correlation of positron annihilation radiation (2D-ACAR) spectra and the DFT-based computations. However, 2D-ACAR does not give such a clear signature of the FS in the extended momentum space in either the theory or the experiment.RRR/Radiation, Radionuclides and ReactorsApplied Science
Role of Oxygen Electrons in the Metal-Insulator Transition in the Magnetoresistive Oxide La2-2xSr1+2xMn2O7 Probed by Compton Scattering
No full textWe have studied the [100]-[110] anisotropy of the Compton profile in the bilayer manganite. Quantitative agreement is found between theory and experiment with respect to the anisotropy in the two metallic phases (i.e., the low temperature ferromagnetic and the colossal magnetoresistant phase under a magnetic field of 7 T). Robust signatures of the metal-insulator transition are identified in the momentum density for the paramagnetic phase above the Curie temperature. We interpret our results as providing direct evidence for the transition from the metalliclike to the admixed ionic-covalent bonding accompanying the magnetic transition. The number of electrons involved in this phase transition is estimated. Our study demonstrates the sensitivity of the Compton scattering technique for identifying the number and type of electrons involved in the metal-insulator transition.Radiation, Radionuclides and ReactorsApplied Science
Measurement of radiative Bhabha and quasi-real Compton scattering
No full textWe report on a study of radiative Bhabha and quasi-real Compton scattering at centre-of-mass energies between 50~{\GeV} and 170~{\GeV} and 20~{\GeV} and 140~{\GeV}, respectively, using the L3 detector at LEP. The analysis is based on data corresponding to an integrated luminosity of 232.2 \,\pb. A total of 2856 radiative Bhabha and 4641 Compton scattering events are collected. Total and differential cross sections for both reactions are presented and found to be in good agreement with QED expectations. Our measurement of Compton scattering at the highest energies obtained so far is used to derive exclusion limits on the coupling for the on-shell production of an excited electron \e^{\star} decaying into a \gamma\e pair in the mass range 20 \gev < m_{\e^{\star}} < 170 \gev
Maximum Likelihood Compton Polarimetry with the Compton Spectrometer and Imager
No full textInternational audienceAstrophysical polarization measurements in the soft gamma-ray band are becoming more feasible as detectors with high position and energy resolution are deployed. Previous work has shown that the minimum detectable polarization (MDP) of an ideal Compton polarimeter can be improved by ∼21% when an unbinned, maximum likelihood method (MLM) is used instead of the standard approach of fitting a sinusoid to a histogram of azimuthal scattering angles. Here we outline a procedure for implementing this maximum likelihood approach for real, nonideal polarimeters. As an example, we use the recent observation of GRB 160530A with the Compton Spectrometer and Imager. We find that the MDP for this observation is reduced by 20% when the MLM is used instead of the standard method
Compton scattering of quasi-real virtual photons at LEP
No full textCompton scattering of quasi-real virtual photons, gamma e(+/-) -\u3e gamma e(+/-), is studied with 0.6 fb(-1) of data collected by the L3 detector at the LEP e(+)e(-) collider at centre-of-mass energies root s = 189-209 GeV. About 4500 events produced by the interaction of virtual photons emitted by e(+/-) of one beam with e(-/+) of the opposite beam are collected for effective centre-of-mass energies of the photon-electron and photon-positron systems in the range from root s\u27 = 35 GeV up to root s\u27 = 175 GeV, the highest energy at which Compton scattering was ever probed. The cross sections of the gamma e(+/-) -\u3e gamma e(+/-) process as a function of root s\u27 and of the rest-frame scattering angle are measured, combined with previous U measurements down to root s\u27 similar or equal to 20 GeV, and found to agree with the QED expectations. (c) 2005 Published by Elsevier B.V
X-rays from Compton scattering around accreting black holes
No full textCompton scattering is one of the most important radiation processes from accreting compact objects. Hard X-rays can be produced by Compton upscattering of soft seed photons from the accretion disk. Here we use both theoretical and observational methods to investigate the hard X-ray emission around accreting Black Holes. We attempt to observationally constrain the geometry of the accretion flow using the observed spectra from the outburst of a transient black hole. The rapid rise of the hard X-ray flux is almost certainly associated with most of the disk mass moving inwards, so the optically thick disk should not extend down to the last stable orbit until the outburst peak. The low/hard state is seen at the start of the rise showing that this is probably associated with a truncated disk. Attempts to derive the inner disk radius from reflection model parameters are inconclusive due to the effects of ionization. We describe a new approach to numerically modelling Compton scattering around accreting sources by solving the distribution functions. This approach (based on work by Guilbert 1981) involves breaking the scattering into individual segments of space and time. We show how a coarse angle grid can be adapted to mimic an arbitrarily fine grid with very little increase in run-time. The resulting code automatically includes time dependent behaviour. We extend the code to calculate the time dependent, self consistent electron distribution resulting from the Compton cooling. This can be used even where the Compton cooling time is shorter than the light crossing time. We show that any system in which the seed photons are dominated by reprocessing should produce soft lags of the order of the light crossing time. Future observations, with more sophisticated satellites, may be able to identify this lag
Deeply Virtual Compton Scattering Cross Section at High Bjorken xB
No full textWe report high-precision measurements of the deeply virtual Compton scattering (DVCS) cross section at high values of the Bjorken variable xB. DVCS is sensitive to the generalized parton distributions of the nucleon, which provide a three-dimensional description of its internal constituents. Using the exact analytic expression of the DVCS cross section for all possible polarization states of the initial and final electron and nucleon, and final state photon, we present the first experimental extraction of all four helicity-conserving Compton form factors (CFFs) of the nucleon as a function of xB, while systematically including helicity flip amplitudes. In particular, the high accuracy of the present data demonstrates sensitivity to some very poorly known CFFs
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