Instrumental Resolution Effects Research Articles

About two-dimensional fits for the analysis of the scattering rates and renormalization functions from angle-resolved photoelectron spectroscopy data

A new method for the analysis of the scattering rates from angle-resolved photoelectron spectroscopy (ARPES) is presented and described in details. It takes into account experimental instrumental resolution and finite temperature effects. More accurate results are obtained in comparison with a standard, commonly used method. The application of the method is demonstrated for several examples commonly encountered in new quantum materials. Its usefulness is especially apparent in investigations of systems with strongly correlated electrons.

The fast radio burst dispersion measure distribution

ABSTRACT We compare the dispersion measure (DM) statistics of FRBs detected by the ASKAP and Parkes radio telescopes. We jointly model their DM distributions, exploiting the fact that the telescopes have different survey fluence limits but likely sample the same underlying population. After accounting for the effects of instrumental temporal and spectral resolution of each sample, we find that a fit between the modelled and observed DM distribution, using identical population parameters, provides a good fit to both distributions. Assuming a one-to-one mapping between DM and redshift for an homogeneous intergalactic medium (IGM), we determine the best-fitting parameters of the population spectral index, $\hat{\alpha }$, and the power-law index of the burst energy distribution, $\hat{\gamma }$, for different redshift evolutionary models. Whilst the overall best-fitting model yields $\hat{\alpha }=2.2_{-1.0}^{+0.7}$ and $\hat{\gamma }=2.0_{-0.1}^{+0.3}$, for a strong redshift evolutionary model, when we admit the further constraint of α = 1.5 we favour the best fit $\hat{\gamma }=1.5 \pm 0.2$ and the case of no redshift evolution. Moreover, we find no evidence that the FRB population evolves faster than linearly with respect to the star formation rate over the DM (redshift) range for the sampled population.

New capabilities for enhancement of RMCProfile: instrumental profiles with arbitrary peak shapes for structural refinements using the reverse Monte Carlo method

Reported here are the development and application of new capabilities in the RMCProfile software for structural refinements using the reverse Monte Carlo (RMC) method. An algorithm has been implemented to enable the use of arbitrary peak-shape functions in the modeling of Bragg diffraction patterns and instrumental resolution effects on total-scattering data. This capability eliminates the dependence of RMCProfile on preset functions, which are inadequate for data produced by some total-scattering instruments, e.g. NOMAD at the Spallation Neutron Source (SNS) at Oak Ridge, Tennessee, USA. The recently developed procedure for the instrument-resolution correction has been modified to improve its accuracy, which is critical for recovering nanoscale structure. The ability to measure fine details of local and nanoscale structures with high fidelity is required because such features are increasingly exploited in the design of materials with enhanced functional properties. The new methodology has been tested via RMC refinements of large-scale atomic configurations (distances up to 8 nm) for SrTiO3 using neutron total-scattering data collected on the Polaris and NOMAD time-of-flight powder diffractometers at the ISIS facility (Didcot, Oxfordshire, UK) and SNS, respectively. While the Polaris instrument is known to provide the high-quality data needed for RMC analysis, the similar and sound atomic configurations obtained from both instruments confirmed that the NOMAD data are also suitable for RMC refinements over a broad distance range.

On the Determination of Rotation Velocity and Dynamical Mass of Galaxies Based on Integrated H i Spectra

Abstract The integrated 21 cm H i emission profile of a galaxy encodes valuable information on the kinematics, spatial distribution, and dynamical state of its cold interstellar medium. The line width, in particular, reflects the rotation velocity of the galaxy, which, in combination with a size scale, can be used to constrain the dynamical mass of the system. We introduce a new method based on the concept of the curve of growth to derive a set of robust parameters to characterize the line width, asymmetry, and concentration of the integrated H i spectra. We use mock spectra to evaluate the performance of our method, to estimate realistic systematic uncertainties for the proposed parameters, and to correct the line widths for the effects of instrumental resolution and turbulence broadening. Using a large sample of nearby galaxies with available spatially resolved kinematics, we demonstrate that the newly defined line widths can predict the rotational velocities of galaxies to within an accuracy of ≲30 km s−1. We use the calibrated line widths, in conjunction with the empirical relation between the size and mass of H i disks, to formulate a prescription for estimating the dynamical mass within the H i-emitting region of gas-rich galaxies. Our formalism yields dynamical masses accurate to ∼0.3 dex based solely on quantities that can be derived efficiently and robustly from current and future extragalactic H i surveys. We further extend the dynamical mass calibration to the scale of the dark matter halo.

Polarized inelastic neutron scattering of nonreciprocal spin waves in MnSi

We report spin-polarized inelastic neutron scattering of the dynamical structure factor of the conical magnetic helix in the cubic chiral magnet MnSi. We find that the spectral weight of spin-flip scattering processes is concentrated on single branches for wavevector transfer parallel to the helix axis as inferred from well-defined peaks in the neutron spectra. In contrast, for wavevector transfers perpendicular to the helix the spectral weight is distributed among different branches of the magnon band structure as reflected in broader features of the spectra. Taking into account the effects of instrumental resolution, our experimental results are in excellent quantitative agreement with parameter-free theoretical predictions. Whereas the dispersion of the spin waves in MnSi appears to be approximately reciprocal at low energies and small applied fields, the associated spin-resolved spectral weight displays a pronounced non-reciprocity that implies a distinct non-reciprocal response in the limit of vanishing uniform magnetization at zero magnetic field.

Advancing reverse Monte Carlo structure refinements to the nanoscale

Over the past decade, the RMCProfile software package has evolved into a powerful computational framework for atomistic structural refinements using a reverse Monte Carlo (RMC) algorithm and multiple types of experimental data. However, realizing the full potential of this method, which can provide a consistent description of atomic arrangements over several length scales, requires a computational speed much higher than that permitted by the current software. This problem has been addressed via substantial optimization and development of RMCProfile, including the introduction of the new parallel-chains RMC algorithm. The computing speed of this software has been increased by nearly two orders of magnitude, as demonstrated using the refinements of a simulated structure with two distinct correlation lengths for the atomic displacements. The new developments provide a path for achieving even faster performance as more advanced computing hardware becomes available. This version of RMCProfile permits refinements of atomic configurations of the order of 500 000 atoms (compared to the current limit of 20 000), which sample interatomic distances up to 10 nm (versus 3 nm currently). Accurate, computationally efficient corrections of the calculated X-ray and neutron total scattering data have been developed to account for the effects of instrumental resolution. These corrections are applied in both reciprocal and real spaces, thereby enabling RMC fitting of an atomic pair distribution function, which is obtained as the Fourier transform of the total-scattering intensity, over the entire nanoscale distance range accessible experimentally.

The momentum distribution of liquid 3He

We present high-resolution neutron Compton scattering measurements of liquid 3He below its renormalized Fermi temperature. Theoretical predictions are in excellent agreement with the experimental data when instrumental resolution and final-state effects are accounted for. Our results resolve the long-standing inconsistency between theoretical and experimental estimates of the average atomic kinetic energy.

A generalized mean-squared displacement from inelastic fixed window scans of incoherent neutron scattering as a model-free indicator of anomalous diffusion confinement

Elastic fixed window scans of incoherent neutron scattering are an established and frequently employed method to study dynamical changes, usually over a broad temperature range or during a process such as a conformational change in the sample. In particular, the apparent mean-squared displacement can be extracted via a model-free analysis based on a solid physical interpretation as an effective amplitude of molecular motions. Here, we provide a new account of elastic and inelastic fixed window scans, defining a generalized mean-squared displacement for all fixed energy transfers. We show that this generalized mean-squared displacement in principle contains all information on the real mean-square displacement accessible in the instrumental time window. The derived formula provides a clear understanding of the effects of instrumental resolution on the apparent mean-squared displacement. Finally, we show that the generalized mean-square displacement can be used as a model-free indicator on confinement effects within the instrumental time window.

WillItFit: a framework for fitting of constrained models to small-angle scattering data

A software framework for analysis of small-angle scattering data is presented. On the basis of molecular constraints and prior knowledge of the chemical composition of the sample, the software is capable of simultaneously fitting small-angle X-ray and neutron scattering data to analytical or semi-analytical models of biomacromolecules. The software features various fitting routines along with the possibility of incorporating instrumental resolution effects on the fit. Finally, trust region estimation, based on the profile likelihood strategy, is implemented. The algorithms and models are written in C, whereas the user interface is written in Python. Parallelization is implemented using the OpenMP extensions to C. The source code is available for free upon request orviathe associated code repository. The software runs on Linux, Windows and OSX and is available as an open-source initiative published under the General Publishing License.

Line-of-sight geometrical and instrumental resolution effects on intensity perturbations by sausage modes

Diagnostics of MHD waves in the solar atmosphere is a topic which often encounters problems of interpretation, due partly to the high complexity of the solar atmospheric medium. Forward modeling can significantly guide interpretation, bridging the gap between numerical simulations and observations, and increasing the reliability of mode identification for application of MHD seismology. In this work we aim at determining the characteristics of the fast MHD sausage mode in the corona on the modulation of observable quantities such as line intensity and spectral line broadening. Effects of line-of-sight angle, and spatial, temporal and spectral resolutions are considered. We take a cylindrical tube simulating a loop in a low-{\beta} coronal environment with an optically thin background, and let it oscillate with the fast sausage mode. A parametric study is performed. Among other results, we show that regardless of the ionisation state of the plasma, the variation of spectral line broadening can be significant, even for low intensity modulation. The nature of this broadening is not thermal but is mostly turbulent. This places spectrometers in clear advantage over imaging instruments for the detection of the sausage mode. The modulation of all quantities is considerably affected by the line-of-sight angle, and especially by the spatial and temporal resolution when these are on the order of the mode's wavelength and period. This places high constraints on instrumentation.

HerMES: A DEFICIT IN THE SURFACE BRIGHTNESS OF THE COSMIC INFRARED BACKGROUND DUE TO GALAXY CLUSTER GRAVITATIONAL LENSING

We have observed four massive galaxy clusters with the SPIRE instrument on the Herschel Space Observatory and measure a deficit of surface brightness within their central region after subtracting sources. We simulate the effects of instrumental sensitivity and resolution, the source population, and the lensing effect of the clusters to estimate the shape and amplitude of the deficit. The amplitude of the central deficit is a strong function of the surface density and flux distribution of the background sources. We find that for the current best fitting faint end number counts, and excellent lensing models, the most likely amplitude of the central deficit is the full intensity of the cosmic infrared background (CIB). Our measurement leads to a lower limit to the integrated total intensity of the CIB of I(250 microns) > 0.69_(-0.03)^(+0.03) (stat.)_(-0.06)^(+0.11) (sys.) MJy/sr, with more CIB possible from both low-redshift sources and from sources within the target clusters. It should be possible to observe this effect in existing high angular resolution data at other wavelengths where the CIB is bright, which would allow tests of models of the faint source component of the CIB.

Instrumental resolution effects in neutron scattering studies of protein dynamics

In this study, the dynamics of Green Fluorescent protein (GFP) are analyzed using three neutron scattering spectrometers. We focus on the effect of instrumental energy resolution in the analysis of the elastic incoherent structure factor (EISF) and mean square displacement (MSD). This topic still remains a source of controversy. Our data clearly demonstrate the presence of the resolution effect in the dynamic transition for hydrated protein and the onset of translational motions in hydration water consistent with previous results from quasielastic neutron scattering. The 190K onset of motions in hydration water observed at ∼1ns is also consistent with a Tg of hydration water below 190K.

Resolving the Azimuthal Ambiguity in Vector Magnetogram Data with the Divergence-Free Condition: The Effects of Noise and Limited Spatial Resolution

We investigate how the azimuthal ambiguity in solar vector magnetogram data can be resolved by using the divergence-free property of magnetic fields. In a previous article, by Crouch, Barnes, and Leka (Solar Phys. 260, 271, 2009), error-free synthetic data were used to test several methods that each make a different assumption about how the divergence-free property can be used to resolve the ambiguity. In this paper this testing is continued with an examination of the effects of Poisson photon noise and limited instrumental spatial resolution. We find that all currently available methods based on the divergence-free property can produce undesirable results when photon noise or unresolved structure are present in the data. We perform a series of experiments aimed at improving the performance of the global minimisation method, which is the most promising of the methods. We present a two-step approach that produces reasonable results in tests using synthetic data. The first step of this approach involves the global minimisation of a combination of the absolute value of the approximation for the divergence and a smoothness constraint, which is designed to minimise the difference between the magnetic field in neighbouring pixels. In the second step, pixels with measurements known to be strongly affected by photon noise are revisited with a smoothing algorithm that also seeks to minimise the difference between the magnetic field in neighbouring pixels.

Analysis of randomly oriented structures by grazing-incidence small-angle neutron scattering

A formalism is presented which allows the quantitative evaluation of data from grazing-incidence small-angle neutron and X-ray scattering – GISANS and GISAXS – in the framework of the distorted wave Born approximation. While several aspects have been reported previously, this formalism combines solutions for scattering intensities in both reflection and transmission hemispheres, taking into account instrumental resolution effects. This formalism is applied to the case of GISANS from self-organized diblock copolymers, ordered in perpendicular lamellar structures on an Si wafer in randomly oriented short-range-ordered regions. The periodicity ofD= 85 (9) nm found for deuterated polystyrene–polybutadiene of molecular weight Mw= 165 kg mol−1and a molecular weight fraction of the deuterated polystyrene block of 52% is consistent with atomic force microscopy and specular neutron reflectivity results.

Protein Dynamics by Neutron Scattering: The Protein Dynamical Transition

Despite recent extensive efforts, the nature of the dynamics of biological macromolecules still remains unclear. in particular, contradicting models have been proposed for explaining the temperature behavior of the mean square displacement, MSD, and of the characteristic system time, τ. To solve this puzzle, different elastic incoherent neutron scattering experiments with different instrumental energy resolutions were performed on dry and hydrated lysozyme [S Magazu, et al. J. Phys. Chem. B 2011, 115:7736]. Characteristic system times have been correlated to the dynamical transition (DT) kinks in the MSDs (as is shown in the figure). The obtained results show that the DT is a finite instrumental energy resolution effect, more specifically, it appears when the characteristic system time intersects the resolution time of the spectrometer used; it does not imply any transition in the dynamical properties of the systems, and, it is not due to the fragile-to-strong dynamical crossover (FSC) in the characteristic system time temperature behavior. Furthermore, the obtained results confirm the change in τ(T) at T=220K, and show that it is not due to finite instrumental energy resolution effects and it is not connected to errors in the data analysis protocol.View Large Image | View Hi-Res Image | Download PowerPoint Slide

Bio-protective effects of homologous disaccharides on biological macromolecules

In this contribution the effects of the homologous disaccharides trehalose and sucrose on both water and hydrated lysozyme dynamics are considered by determining the mean square displacement (MSD) from elastic incoherent neutron scattering (EINS) experiments. The self-distribution function (SDF) procedure is applied to the data collected, by use of IN13 and IN10 spectrometers (Institute Laue Langevin, France), on trehalose and sucrose aqueous mixtures (at a concentration corresponding to 19 water molecules per disaccharide molecule), and on dry and hydrated (H(2)O and D(2)O) lysozyme also in the presence of the disaccharides. As a result, above the glass transition temperature of water, the MSD of the water-trehalose system is lower than that of the water-sucrose system. This result suggests that the hydrogen-bond network of the water-trehalose system is stronger than that of the water-sucrose system. Furthermore, by taking into account instrumental resolution effects it was found that the system relaxation time of the water-trehalose system is longer than that of the water-sucrose system, and the system relaxation time of the protein in a hydrated environment in the presence of disaccharides increases sensitively. These results explain the higher bioprotectant effectiveness of trehalose. Finally, the partial MSDs of sucrose/water and trehalose/water have been evaluated. It clearly emerges from the analysis that these are almost equivalent in the low-Q domain (0-1.7 Å(-1)) but differ substantially in the high-Q range (1.7-4 Å(-1)). These findings reveal that the lower structural sensitivity of trehalose to thermal changes is connected with the local spatial scale.

Puzzle of Protein Dynamical Transition

Despite recent extensive efforts, the nature of the dynamics of biological macromolecules still remains unclear. In particular, contradicting models have been proposed for explaining the temperature behavior of the mean square displacement, MSD, and of the system relaxation time, τ. To solve this puzzle, different neutron scattering experiments with different instrumental energy resolutions were performed on dry and hydrated lysozyme. The obtained results show that the so called dynamical transition: (i) is a finite instrumental energy resolution effect, and more specifically, it appears when the characteristic system relaxation time intersects the resolution time, (ii) it does not imply any transition in the dynamical properties of the systems, (iii) it is not due to the fragile-to-strong dynamical crossover (FSC) in the temperature behavior of the system relaxation time, differently to what S. H. Chen et al. proposed [Proc. Natl. Acad. Sci. U.S.A.2006, 103, 9012]. Furthermore, the obtained results confirm the change in the τ-temperature dependence at T = 220 K of S. H. Chen et al., and show that it is not due to finite instrumental energy resolution effects and it is not connected to numerical errors in the data analysis protocol, differently to what W. Doster et al. proposed [Phys. Rev. Lett.2010, 104, 098101].

Electron momentum distribution and Compton profiles of europium

The electron momentum distribution and Compton profiles of the rare earth element Eu was measured using HPGe photon detector. The target atoms were excited by means of 59.54 keV gamma rays from Am-241 radioactive source of strength 300 mCi. Elemental foil of uniform aerial density, and purity better than 99.9% was used as target. The data were recorded and analyzed using a 4 K multichannel analyzer. These data duly corrected for instrumental resolution effects, sample absorption, energy dependence of the differential Compton cross-section and double scattering, and compared with theoretical renormalized free atom (RFA) values.

Theoretical consideration of the optimal performance of atomic resolution holography

The recording of holograms with atomic resolution is usually affected by distortions, e.g., due to the finite size of the source and detector, twin images and various imperfections of the instrument some of which have already been analyzed in detail. Amongst others in the present paper we discuss the effect of statistical noise. It is shown that the level of statistical noise of the reconstructed image is not influenced by the number of data points in the hologram and their individual counting statistics but is determined by the total number of counts collected in the experiment. The effects of instrumental resolution, filtering methods and symmetry operations on the signal-to-noise ratio of the reconstructed image are investigated as well. These achievements form the basis of the optimization method for the signal-to-noise ratio of the reconstructed holographic image.

Phase-object approximation in small-angle neutron scattering experiments on silicon gratings

The phase-object approximation for neutron scattering based on a one-dimensional dynamic forward scattering theory is discussed and used to calculate the differential cross section of an object. It is shown that this approximation is valid in ultra-small-angle neutron scattering (USANS) and spin-echo small-angle neutron scattering (SESANS) experiments on silicon gratings. In the weak scattering limit, the phase-object approximation reduces to the kinematic or first Born approximation. The spatial coherence function is used to describe instrumental resolution effects. Measurements on three different instruments are in good agreement with calculation results. In the experiment with a time-of-flight SESANS instrument, the effect ofPendellösungwith object size is observed.