1,720,979 research outputs found
Photon Scattering by Compounds: an assessment of the validity of the independent Atomic Model Approximation
No full textThe validity of the independent atomic model (IAM) for x-ray scattering, which assumes that each atom in the sample scatters independently of the others, was examined for materials and irradiation geometries where interatomic and intermolecular cooperative effects are expected to provide a non-negligible contribution. Samples of water, fat, polymers and powder were investigated by means of a simple powder diffractometer in the interval of momentum transfer 0.017 ≤ χ ≤ 0.67 {χ = E[sin(θ/2)]/12.4, E being the incident photon energy and θ the scattering angle}. Accurate evaluation of self-absorption, air scattering and other contamination was carried out by means of a Monte Carlo code. Simulation of the incident photon spectra permitted an assessment of non-monochromaticity effect. This allowed the accurate evaluation of the scattering intensities in terms of fundamental impact parameters. All the materials investigated showed marked discrepancies from the corresponding IAM theoretical results, even for intervals of momentum transfer in which this model is usually claimed to be valid. However, this work shows that the errors introduced by corrections are relatively low, making it possible to validate the use of simple diffractometry to evaluate the scattering factors in real samples without having to resort to sophisticated irradiation facilities
Measurement of L X ray emission fom elements in 64_Z_73 interval stimulated by 59.54 keV photons
No full textAccurate measurements (mostly within 5%) of L-alpha L-beta and L-gamma x-ray emission cross sections were carried out on thick foils of elements lying in 64 <= Z <= 73 interval, stimulated by 59.54 keV photons. They can be of help as an overall reference to compare the theoretical and semi-empirical approaches by which the atomic parameters included in the expressions of L emission cross sections are evaluated. An appropriate extension of the expression linking the cross section to the counting rate measured in reflection geometry on thick targets makes it possible to overcome the difficulties arising from the width in energy of the L-beta and L-gamma groups and from the jump present in the attenuation coefficients of the target elements within the energy interval of their L-gamma emission lines
In vivo fat and soft tissue evaluation by photon scattering measurements
No full textThe authors propose a technique for tissue characterization based on the measurement of Compton scatering distribution obtained in backscattering geometry. Such a technique shows useful when the coherent peak is not easily detectabel and measurable as in the in vivo applications where the measurement time needs to be as short as possible. Furthermore this study shows that the Compton profile techniques can be used for tissue characterization in some situations as those, for instance, of in vivo x-ray fluorescence analysis of heavy metals
Molecular differential cross sections for x-ray coherent scattering in fat and polymethyl methacrylate
No full textMolecular differential cross sections for the coherent scattering of x- rays in polymethyl methacrylate (PMMA) and fat were determined from measured diffraction patterns in the interval χ = 0 to 6.4 nm-1 (χ = [sin(θ/2)]/λ;λ being the incident wavelength having the units of nm and θ the scattering angle). All measurements were performed at a controlled temperature of 23 °C. The final results for PMMA show overall agreement when compared with the data existing in the literature. However, some discrepancies with the results reported by Kosanetzky et al in 1987 are found at the first three maxima. The data for filtered fat material are reported here for the first time. Finally, data sets of molecular form factors for fat and PMMA wore compiled from the smoothed corrected experimental results by assuming these materials to be mono-molecular
Experimental K shell fluorescence yield of monocristalline Germanium
No full textThe result of an experimental evaluation of the K-shell fluorescence yield, omega K, of monocrystalline Ge is reported. The procedure makes use of the information contained in the escape peaks which are present in the characteristic X-ray spectra given by an HP Ge detector. Details of the experimental methodology, peak area evaluations and uncertainty treatments are described. A value omega K=0.549+or-0.011 is obtained and compared with recent values obtained from experiment, theory, interpolation or normalisation. The authors consider the effects of the outer-shell configuration and the use of theoretical photoelectric cross section values
Detailed Multiple Scattering Profile Evaluation in Collimated Photon Scattering Techniques
No full textThe spectra of gamma radiation, multiple-scattered by large volume samples, have been studied in different irradiation conditions and positions of the sensitive volume (SV) defined by the intersection of the incoming beam and the solid angle seen by a detector collimation. Monte Carlo simulations of up to third order scattering are performed on phantoms of small and large dimensions compared to the mean free path of the incoming photons. Qualitative and quantitative results are discussed in terms of both the equipment collimation and the peak integration limits of the recorded photon energy spectra. Experiments with 59.54 keV incident photons and water phantoms are reported in order to validate the simulation results. The experimental and simulated results clearly show that the non-linear dependence of the multiple-scattered fraction on the SV position inside the sample volume cannot be ignored in the assessment of Compton tomographic techniques
EDXRS Study of a New Collimatorfor Enhanced Scattering Techniques
No full texthis paper explores the possibilities offered by basic photon scattering phenomenology in the study of anew collimator devised to enhance the collection of photons undergoing a single scattering in a volumeof interest. In order to maximize the spatial resolution, a 90scattering geometry was considered. Thecollimator was composed of two parallel plates perpendicular to both the detector surface and the plane offirst scattering defined by the trajectories of the incident and scattered photons. In this way, photons whoseazimuthal angle ranges over a wide interval but with the same scattering angle could reach the detector.Experimental comparison between this collimation and the usual narrow one was performed at a photonenergy of 59.54 keV under the same sample and irradiation geometry. The first results showed that anenhancement of about 10-fold could be reached with regard to the main Compton and coherent peaks.On the other hand, the multiple scattering fraction only rose to a maximum of 20% in the worst case, ascompared with that observed in classical highly collimated geometry. The usual analytical capacity offeredby the scattering techniques was demonstrated to be still valid in this situation
New EDXRS exploitation of Compton scattering properties
No full textThe exploitation of the information that can be obtained from the energy analysis of Compton‐scattered photons is discussed. It is shown that the use of the high‐energy wing part of this energy distribution adds a further tool for EDXRS analysis other than the evaluation of the total fluence area of the Compton (or incoherent) distribution and the total fluence area of the Rayleigh (or coherent) peak. Special attention is paid to those situations for which easy and unambiguous coherent peak evaluation is prevented. Copyright © 1995 John Wiley & Sons, Ltd
Feasibility of in vivo tissue charachterisation by Compton scattering profile measurements
No full textThe authors investigate a technique for the characterisation of biological tissues, based on energy analysis of Compton scattered photons. The incident beam consists of 59.54 keV photons and a backscatter geometry is used. Compared to the technique based on the ratio of photons coherently scattered due to the Rayleigh effect and those incoherently scattered due to the Compton effect, the present technique provides similar results for mineral content evaluation, with an error lower than 1%. However it allows the determination of fat content in a soft tissue matrix - even in the cases in which the Rayleigh peaks is not measurable - although the error can rise up to 3%
Photon cross-section data from X-Ray diffraction pattern measurements: Correction procedure evaluations
No full textThe feasibility of evaluating absolute differential cross-sections of coherently scattered X-rays by exploiting diffraction pattern measurements has been tested in the case of plastic and biological samples. The investigation takes into account three powder diffractometers with different irradiation facilities. The main relevant correction factors have been evaluated by means of Monte Carlo and computational algorithm approaches. A careful evaluation of the effect of non-monochromatic incident photon beams allows this procedure to be valid without resorting to sophisticated irradiation facilities. Moreover, in comparing the experimental data with the theory, it is shown that some assumptions usually made when calculating theoretical scattering cross-sections are not valid when a scattering angle ranging from forward to backscattering values is to be considered. These results, together with other usual instrumental corrections, permit us to evaluate the differential scattering cross-sections within acceptable uncertainties
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