X-ray diffuse scattering from HMTA: analysis via a Monte Carlo model

2008 ◽  
Vol 64 (4) ◽  
pp. 456-465 ◽  
Author(s):  
D. J. Goossens ◽  
A. P. Heerdegen ◽  
T. R. Welberry
Keyword(s):  
Monte Carlo ◽  
Azelaic Acid ◽  
Diffuse Scattering ◽  
Cross Correlation ◽  
Monte Carlo Model ◽  
Limited Range ◽  
Thermally Induced ◽  
X Ray ◽  
Zero Correlation ◽  
Average Position

Hexamethylenetetramine (HMT, C6H12N4, also referred to as urotropin) and azelaic acid [A, HOOC—(CH2)7—COOH] form a co-crystal or adduct (HMTA, also referred to as urotropin azelate) which exhibits several structural phases as a function of temperature. At room temperature, the structure is orthorhombic, but shows substantial disorder. Here, this disorder is explored by analyzing the diffuse scattering from single crystals of HMTA via Monte Carlo simulation. The disorder is in part occupational, with two orientations of azelaic acid occurring, and in part thermally induced, which is to say dynamic. The occupational disorder can be thought of as a combination of limited-range in-plane (bc plane) negative correlations combined with effectively zero correlation between planes (along a), rather like stacking faults. Size effect, the cross-correlation between molecular orientation and displacement from average position, is required to reproduce the observed diffuse scattering.

scholarly journals Glimpses of the past activity of Sgr A★ inferred from X-ray echoes in Sgr C

2018 ◽  
Vol 610 ◽  
pp. A34 ◽  
Author(s):  
D. Chuard ◽  
R. Terrier ◽  
A. Goldwurm ◽  
M. Clavel ◽  
S. Soldi ◽  
...  
Keyword(s):  
Monte Carlo ◽  
Black Hole ◽  
Monte Carlo Model ◽  
Line Of Sight ◽  
X Rays ◽  
X Ray ◽  
Supermassive Black Hole ◽  
The Past ◽  
Sgr A ◽  
Past Activity

Context. For a decade now, evidence has accumulated that giant molecular clouds located within the central molecular zone of our Galaxy reflect X-rays coming from past outbursts of the Galactic supermassive black hole. However, the number of illuminating events as well as their ages and durations are still unresolved questions. Aims. We aim to reconstruct parts of the history of the supermassive black hole Sgr A★ by studying this reflection phenomenon in the molecular complex Sgr C and by determining the line-of-sight positions of its main bright substructures. Methods. Using observations made with the X-ray observatories XMM-Newton and Chandra and between 2000 and 2014, we investigated the variability of the reflected emission, which consists of a Fe Kα line at 6.4 keV and a Compton continuum. We carried out an imaging and a spectral analysis. We also used a Monte Carlo model of the reflected spectra to constrain the line-of-sight positions of the brightest clumps, and hence to assign an approximate date to the associated illuminating events. Results. We show that the Fe Kα emission from Sgr C exhibits significant variability in both space and time, which confirms its reflection origin. The most likely illuminating source is Sgr A★. On the one hand, we report two distinct variability timescales, as one clump undergoes a sudden rise and fall in about 2005, while two others vary smoothly throughout the whole 2000–2014 period. On the other hand, by fitting the Monte Carlo model to the data, we are able to place tight constraints on the 3D positions of the clumps. These two independent approaches provide a consistent picture of the past activity of Sgr A★, since the two slowly varying clumps are located on the same wavefront, while the third (rapidly varying) clump corresponds to a different wavefront, that is, to a different illuminating event. Conclusions. This work shows that Sgr A★ experienced at least two powerful outbursts in the past 300 yrs, and for the first time, we provide an estimation of their age. Extending this approach to other molecular complexes, such as Sgr A, will allow this two-event scenario to be tested further.

scholarly journals The interpretation and analysis of diffuse scattering using Monte Carlo simulation methods

2007 ◽  
Vol 64 (1) ◽  
pp. 23-32 ◽  
Author(s):  
T. R. Welberry ◽  
D. J. Goossens
Keyword(s):  
Monte Carlo Simulation ◽  
Monte Carlo ◽  
Diffuse Scattering ◽  
Simulation Methods ◽  
Neutron Sources ◽  
X Ray ◽  
Average Structure

Studies of diffuse scattering had a prominent place in the first issue ofActa Crystallographica60 years ago at a time when conventional crystallography (determination of the average structure from Bragg peaks) was in its infancy. Since that time, conventional crystallography has developed enormously while diffuse-scattering analysis has seemingly lagged well behind. The paper highlights some of the extra difficulties involved in the measurement, interpretation and analysis of diffuse scattering and plots the progress that has been made. With the advent of the latest X-ray and neutron sources, area detectors and the ever-increasing power of computers, most disorder problems are now tractable. Two recent contrasting examples are described which highlight what can be achieved by current methods.

Monte Carlo Simulation of the X-Ray Fluorescence Spectra from Multielement Homogeneous and Heterogeneous Samples

1986 ◽  
Vol 30 ◽  
pp. 121-132 ◽  
Author(s):  
A. M. Yacout ◽  
R. P. Gardner ◽  
K. Verghese
Keyword(s):  
Monte Carlo ◽  
Monte Carlo Model ◽  
Pulse Height ◽  
Computation Time ◽  
Cubic Splines ◽  
Photon Spectrum ◽  
Detector Response ◽  
X Rays ◽  
Simple Assumption ◽  
X Ray

AbstractA Monte Carlo model that predicts the entire photon, spectrum for energy-dispersive X-ray fluorescence (EDXRF) analyzers excited by radio-isotope sources from multielement homogeneous samples is developed and demonstrated. The components of the photon spectrum include: (1) the and Kα and Kβ characteristic primary, secondary and tertiary X rays from both the unscattered and scattered source photons, (2) the characteristic X rays excited by other characteristic X rays that have been scattered, and (3) the scattered source photons from single, double, and multiple scatters in the sample.The computer code NCSMCXF based on this model has been developed. It is capable of handling up to 20 elements per sample and provides a detailed account of the intensities of the X rays and backscattered source photons per unit source decay as well as a summary of the relative intensities from all elements present in the sample. Cubic splines are used within the code for photoelectric and total scattering cross sections and two-variable cubic splines for angular coherent and incoherent scattering distributions for efficiency in both computation time and storage. The code also provides the pulse-height spectrum of the sample by using the appropriate Si(Li) detector response function. The Monte Carlo predictions for benchmark experimental results on two alloy samples of known composition indicate that the model is very accurate. This approach is capable of replacing most of the experimental work presently required in EDXRF quantitative analysis.A previous Monte Carlo model that uses the simple assumption of spherical homogeneous particles to approximate sample heterogeneities has been modified to improve the computer execution time requirements for the heterogeneous sample case. A new technique for photon tracking in this medium is used and reduces the computation time requirement by half.

A Monte Carlo Model for Retrospective Analysis of Shield Design in a Diagnostic X-ray Room

1993 ◽  
Vol 65 (2) ◽  
pp. 164-171 ◽  
Author(s):  
Robert Metzger ◽  
Rebecca Richardson ◽  
Kenneth A. Riper
Keyword(s):  
Monte Carlo ◽  
Retrospective Analysis ◽  
Monte Carlo Model ◽  
X Ray ◽  
Shield Design

scholarly journals Monte Carlo model of the scanning beam digital x-ray (SBDX) source

2012 ◽  
Vol 57 (22) ◽  
pp. 7381-7394 ◽  
Author(s):  
M Bazalova ◽  
M D Weil ◽  
B Wilfley ◽  
E E Graves
Keyword(s):  
Monte Carlo ◽  
Monte Carlo Model ◽  
X Ray ◽  
Scanning Beam
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