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The Monte Carlo code MCSHAPE: Main features and recent developments
No full textMCSHAPE is a general purpose Monte Carlo code developed at the University of Bologna to simulate the diffusion of X- and gamma-ray photons with the special feature of describing the full evolution of the photon polarization state along the interactions with the target. The prevailing photon-matter interactions in the energy range 1–1000 keV, Compton and Rayleigh scattering and photoelectric effect, are considered. All the parameters that characterize the photon transport can be suitably defined: (i) the source intensity, (ii) its full polarization state as a function of energy, (iii) the number of collisions, and (iv) the energy interval and resolution of the simulation. It is possible to visualize the results for selected groups of interactions. MCSHAPE simulates the propagation in heterogeneous media of polarized photons (from synchrotron sources) or of partially polarized sources (from X-ray tubes). In this paper, the main features of MCSHAPE are illustrated with some examples and a comparison with experimental data.
The Monte Carlo code MCSHAPE: main features and recent developments. Available from: https://www.researchgate.net/publication/273261334_The_Monte_Carlo_code_MCSHAPE_main_features_and_recent_developments
Scattering computation on two targets using the vector code MCSHAPE
No full textUsing the Monte Carlo code MCSHAPE, some simulations have been made by varying the angle between the scattering plane with the incident beam (defined by the incident beam and the beam 1) and the scattering plane of the collision with the second target (defined by beam 1 and the outgoing beam). The code MCSHAPE, in fact, can simulate the behaviour of arbitrarily polarised photons and can follow the evolution of their polarisation state after the interaction with the atoms. The polarisation state of the photons is described using the Stokes parameters I, Q, U and V, having the dimension of an intensity and containing all the physical information about the polarisation state. Simulated experiments with a monochromatic unpolarised source of 59,54 keV (main gamma line of 241Am) and with an x-ray tube source have been considered. In the first case, the results of the simulations show that, after the 90° scattering in the first target, a part of the scattered beam (beam 1) is polarised (the degree of polarisation is a function of energy, and as it is shown, for some energies, 90% of the beam is polarised), but it is not fully polarised as for the single scattering (this is an effect of the multiple scattering in the target).The intensity collected by the detector, after the scattering with the second target, depends on the rotation between the first and the second pieces of the tube. The scattering is drastically reduced for a rotation angle around 90°, even if, due to multiple scattering, it is not zero
Simulation of the detector response function with the code MCSHAPE
No full textMCSHAPE is a Monte Carlo (MC) code for simulating the diffusion of X- and gamma-ray photons with arbitrary polarization state which takes into account photoelectric effect, and Compton and Rayleigh scattering. In this work, a general discussion on the detector response function in the framework of a MC code is presented, and is described how to include the influence of the detector into MCSHAPE. The energy resolution, being a characteristic of the type of detector, is added by using an independent postprocessing code called RESOLUTION instead of being comprised into the MC. Finally, some simulations with MCSHAPE v.261, taking into account the detector response function and the energy resolution, are compared with experimental data
Self-enhancement effects on XRF K-lines due to natural width.
No full textIt is well known that the energy levels in the atom are defined within an energy width. Any electronic transition involving two or more levels shows a natural width given by the sum of the widths of the participating levels. Moreover, any XRF line appears to have a Lorentzian-like distribution whose full width at half maximum is the natural width.
Since the Lorentzian distribution has long tails on both sides, the tails may cross above neighbor absorption edges giving rise to enhancement effects of a certain complexity.
In this article we discuss the self-enhancement effects on XRF K-lines in pure element samples having atomic numbers Z = 11-92. This study is performed with deterministic calculations
Contribution of inner shell Compton ionization to the X-ray fluorescence line intensity
No full textThe Compton effect is a potential ionization mechanism of atoms. It produces vacancies in inner shells that are filled with the same mechanism of atomic relaxation as the one following photo-absorption. This contribution to X-ray fluorescence emission is frequently neglected because the total Compton cross-section is apparently much lower than the photoelectric one at useful X-ray energies. However, a more careful analysis suggests that is necessary to consider single shell cross sections (instead of total cross sections) as a function of energy. In this article these Compton cross sections are computed for the shells K, L1-L3 and M1-M5 in the framework of the impulse approximation. By comparing the Compton and the photoelectric cross-section for each shell it is then possible to determine the extent of the Compton correction to the intensity of the corresponding characteristic lines. It is shown that for the K shell the correction becomes relevant for excitation energies which are too high to be influential in X-ray spectrometry. In contrast, for L and M shells the Compton contribution is relevant for medium-Z elements and medium energies. To illustrate the different grades of relevance of the correction, for each ionized shell, the energies for which the Compton contribution reaches the extent levels of 1, 5, 10, 20, 50 and 100% of the photoelectric one are determined for all the elements with Z=11–92. For practical applications it is provided a simple formula and fitting coefficients to compute average correction levels for the shells considered
Spectrum unfolding in X-ray spectrometry using the maximum entropy method
No full textThe solution of the unfolding problem is an ever-present issue in X-ray spectrometry. The maximum entropy technique solves this problem by taking advantage of some known a priori physical information and by ensuring an outcome with only positive values. This method is implemented in MAXED (MAXimum Entropy Deconvolution), a software code contained in the package UMG (Unfolding with MAXED and GRAVEL) developed at PTB and distributed by NEA Data Bank. This package contains also the code GRAVEL (used to estimate the precision of the solution). This article introduces the new code UMESTRAT (Unfolding Maximum Entropy STRATegy) which applies a semi-automatic strategy to solve the unfolding problem by using a suitable combination of MAXED and GRAVEL for applications in X-ray spectrometry. Some examples of the use of UMESTRAT are shown, demonstrating its capability to remove detector artifacts from the measured spectrum consistently with the model used for the detector response function (DRF)
Improvement of the detector resolution in X-ray spectrometry by using the maximum entropy method
No full textIn every X-ray spectroscopy measurement the influence of the detection system causes loss of information. Different mechanisms contribute to form the so-called detector response function (DRF): the detector efficiency, the escape of photons as a consequence of photoelectric or scattering interactions, the spectrum smearing due to the energy resolution, and, in solid states detectors (SSD), the charge collection artifacts. To recover the original spectrum, it is necessary to remove the detector influence by solving the so-called inverse problem. The maximum entropy unfolding technique solves this problem by imposing a set of constraints, taking advantage of the known a priori information and preserving the positive-defined character of the X-ray spectrum. This method has been included in the tool UMESTRAT (Unfolding Maximum Entropy STRATegy), which adopts a semi-automatic strategy to solve the unfolding problem based on a suitable combination of the codes MAXED and GRAVEL, developed at PTB. In the past UMESTRAT proved the capability to resolve characteristic peaks which were revealed as overlapped by a Si SSD, giving good qualitative results. In order to obtain quantitative results, UMESTRAT has been modified to include the additional constraint of the total number of photons of the spectrum, which can be easily determined by inverting the diagonal efficiency matrix. The features of the improved code are illustrated with some examples of unfolding from three commonly used SSD like Si, Ge, and CdTe. The quantitative unfolding can be considered as a software improvement of the detector resolution
Detailed calculation of inner-shell impact ionization to use in photon transport codes
No full textSecondary electrons can modify the intensity of the XRF characteristic lines by means of a mechanism known as inner-shell impact ionization (ISII). The ad-hoc code KERNEL (which calls the PENELOPE package) has been used to characterize the electron correction in terms of angular, spatial and energy distributions. It is demonstrated that the angular distribution of the characteristic photons due to ISII can be safely considered as isotropic, and that the source of photons from electron interactions is well represented as a point source. The energy dependence of the correction is described using an analytical model in the energy range 1–150 keV, for all the emission lines (K, L and M) of the elements with atomic numbers Z=11–92. It is introduced a new photon kernel comprising the correction due to ISII, suitable to be adopted in photon transport codes (deterministic or Monte Carlo) with a minimal effort. The impact of the correction is discussed for the most intense K (Kα1,Kα2,Kβ1) and L (Lα1,Lα2) lines
Understanding the X-ray emission spectrum after excitation with a source of X-rays: From theory to experiment
Full text linkThe modified Boltzmann-Chandrasekhar equation of transport for photons is the proper framework for describing the photon radiation field with a complete description of the polarization state. The characterization of the radiation field requires a detailed knowledge of the interactions of photons with mater and comprises also the contribution of the secondary electrons to the photon field through mechanisms like inner impact ionization and bremsstrahlung. It will be shown a solution obtained without the need of solving the coupled transport electrons-photons. With all these interactions, the theoretical characterization of the X-ray spectrum of emission after excitation with a source of X-rays can be straightforwardly obtained from the albedo solution to the equation. In this work it will be privileged a Monte Carlo (MC) solution. However, this solution is still far from an experimental measurement modified by the radiation detection devices, comprised the pulse electronics. In this work we put together a MC simulation able to get a detailed transport solution and a complete characterization of the contributions of the detection chain. It is discussed the influence of the single contributions and how they combine to make that a simulated X-ray spectrum matches well a real measurement
Electron contribution to photon transport in coupled photon-electron problems: inner-shell impact ionization correction to XRF
No full textThe Monte Carlo code PENELOPE (coupled electron-photon Monte Carlo) has been used to compute the effect of the secondary electrons on the X-ray fluorescence characteristic lines. The mechanism that produces this contribution is the inner-shell impact ionization. The ad hoc code KERNEL (which calls the PENELOPE library) has been used to simulate a forced first collision at the origin of coordinates. The electron correction (produced by the secondary electrons and their multiple scattering) has been studied in terms of angle, space and energy. The energy dependence has been quantified in the interval 1-150keV, for all the emission lines (K, L and M) of the elements with atomic numbers Z=11-92. For each characteristic line, the energy dependence is described by simple parametric expressions corresponding to the five energy regions delimited by the K, L1, L2 and L3 absorption edges. It has been introduced a new photon kernel comprising the correction due to inner-shell impact ionization. The new kernel is suitable to be adopted in photon transport codes (either deterministic or Monte Carlo) with a minimal effort. Finally, the new kernel has been studied for different elements and lines to trace a general behavior
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