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The true color composite is a widely used Earth observation product for displaying satellite imagery. As it is often used in communication with non-expert audiences in various media, it is desirable that such a product approximates the color perceived by the human eye. Additionally, as the Sentinel-2 mission with its high resolution multispectral imagery and short revisit times is delivering unprecedented amounts of data, any algorithm for computing the composite should be efficient. In this paper we define the natural color product, propose two efficient approaches for computing it, analyze the results, and implement the products on a satellite imagery service for interactive use. Our algorithms work on a per-pixel basis and hence parallelize naturally. The presented approaches are general and not limited to Sentinel-2 data.

Several total and partial photoionization cross section calculations, based on both theoretical and empirical approaches, are quantitatively evaluated with statistical analyses using a large collection of experimental data retrieved from the literature to identify the state of the art for modeling the photoelectric effect in Monte Carlo particle transport. Some of the examined cross section models are available in general purpose Monte Carlo systems, while others have been implemented and subjected to validation tests for the first time to estimate whether they could improve the accuracy of particle transport codes. The validation process identifies Scofield's 1973 non-relativistic calculations, tabulated in the Evaluated Photon Data Library(EPDL), as the one best reproducing experimental measurements of total cross sections. Specialized total cross section models, some of which derive from more recent calculations, do not provide significant improvements. Scofield's non-relativistic calculations are not surpassed regarding the compatibility with experiment of K and L shell photoionization cross sections either, although in a few test cases Ebel's parameterization produces more accurate results close to absorption edges. Modifications to Biggs and Lighthill's parameterization implemented in Geant4 significantly reduce the accuracy of total cross sections at low energies with respect to its original formulation. The scarcity of suitable experimental data hinders a similar extensive analysis for the simulation of the photoelectron angular distribution, which is limited to a qualitative appraisal., To be published in IEEE Trans. Nucl. Sci

We introduce a theoretical framework for the calculation of uncertainties affecting observables produced by Monte Carlo particle transport, which derive from uncertainties in physical parameters input into simulation. The theoretical developments are complemented by a heuristic application, which illustrates the method of calculation in a streamlined simulation environment., 12 pages, 9 figures

A system for in-vivo tracking of 192Ir source during high dose rate or pulsed dose rate brachytherapy treatment was built using 1 mm thick silicon pad detectors as image sensors and knife-edge lead pinholes as collimators. With source self-images obtained from a dual-pinhole system, location of the source could be reconstructed in three dimensions in real time. The system was tested with 192Ir clinical source (kerma rate in air at 1 m 2.38 Gy/h) in air and plexi-glass phantom. The locations of the source were tracked from a distance of 40 cm in a field of view of 20 × 20 × 20 cm3. Reconstruction precision, defined as the average distance between true and reconstructed source positions, with data collected in less than 1 s with 22 GBq 192Ir source was about 5 mm. The reconstruction precision was in our case mainly limited by imperfect alignment of detectors and pinholes. With perfect alignment the statistical error would allow precision of about 1 mm which could further be improved with larger detector placed at larger distance from the pinhole. However already the modest precision of few millimeters is sufficient for in-vivo detection of larger deviations from planned treatment caused by various misadministrations or malfunctioning of the brachytherapy treatment apparatus. Usage of silicon detectors offers a possibility for building a compact device which could be used as an independent online quality assurance system. In this paper details about sensors, readout system and reconstruction algorithm are described. Results from measurements with clinical source are presented.

This document briefly summarizes the status of an ongoing project concerning the assessment, validation, improvement and documentation of physics models for electron-photon transport relevant to multi-disciplinary experimental applications, including medical imaging simulation. Recent results on Geant4 electron backscattering validation are summarized. More extensive documentation can be found in dedicated publications in IEEE Transactions on Nuclear Science.

Several models for the Monte Carlo simulation of Compton scattering on electrons are quantitatively evaluated with respect to a large collection of experimental data retrieved from the literature. Some of these models are currently implemented in general purpose Monte Carlo systems; some have been implemented and evaluated for possible use in Monte Carlo particle transport for the first time in this study. Here we present first and preliminary results concerning total and differential Compton scattering cross sections., Comment: 5 pages, 3 figures, to be published in the Proceedings of IEEE Nuclear Science Symposium 2013

A systematic review of methods and data for the Monte Carlo simulation of photon interactions is in progress: it concerns a wide set of theoretical modeling approaches and data libraries available for this purpose. Models and data libraries are assessed quantitatively with respect to an extensive collection of experimental measurements documented in the literature to determine their accuracy; this evaluation exploits rigorous statistical analysis methods. The computational performance of the associated modeling algorithms is evaluated as well. An overview of the assessment of photon interaction models and results of the experimental validation are presented.

We present advances in the development of methods to predict the effect that uncertainties in physical data needed for generic Monte Carlo simulations induce on the observables resulting from the simulation. Under wide conditions the PDF for the input (uncertain) physical data determine univocally the PDF for the output of the simulation: this general result has clear and powerful applications both in the case of a single (or few) uncertain input data and in the case of many unknown variables, provided they are totally correlated or uncorrelated. Although this is a typical example of forward propagation problem, our results can be applicable also in the backward case, for instance when one is trying to assess conditions for a robust design.

Several physics methods for the simulation of the photoelectric effect are quantitatively evaluated with respect to a large collection of experimental data retrieved from the literature. They include theoretical and empirical calculations of total and partial cross sections, and calculations of the photoelectron angular distribution. Some of these models are currently implemented in general purpose Monte Carlo systems; some have been implemented and evaluated for possible use in Monte Carlo particle transport for the first time in this study., Comment: To be published in the Proceedings of IEEE Nuclear Science Symposium 2013

Various issues related to the complexity of apprais- ing the capabilities of physics models implemented in Monte Carlo simulation codes and the evolution of the functional quality the associated software are considered, such as the dependence on the experimental environment where the software operates and its sensitivity to detector characteristics. The concept of software validity as relative to the environment is illustrated by means of a real-life experimental test case. Methods and techniques to mitigate the risk of deteriorating the quality of the software are critically discussed: they concern various disciplines of the software development process. Quantitative validation of physics models is advocated as a method to appraise their capabilities objectively and to monitor the evolution of their associated software behavior., Comment: To be published in the Proceedings of IEEE Nuclear Science Symposium 2013

The simulation of PIXE (Particle Induced X-ray Emission) is discussed in the context of general-purpose Monte Carlo systems for particle transport. Dedicated PIXE codes are mainly concerned with the application of the technique to elemental analysis, but they lack the capability of dealing with complex experimental configurations. General-purpose Monte Carlo codes provide powerful tools to model the experimental environment in great detail, but so far they have provided limited functionality for PIXE simulation. This paper reviews recent developments that have endowed the Geant4 simulation toolkit with advanced capabilities for PIXE simulation, and related efforts for quantitative validation of cross sections and other physical parameters relevant to PIXE simulation.

Radioactive decays are of concern in a wide variety of applications using Monte-Carlo simulations. In order to properly estimate the quality of such simulations, knowledge of the accuracy of the decay simulation is required. We present a validation of the original Geant4 Radioactive Decay Module, which uses a per-decay sampling approach, and of an extended package for Geant4-based simulation of radioactive decays, which, in addition to being able to use a refactored per-decay sampling, is capable of using a statistical sampling approach. The validation is based on measurements of calibration isotope sources using a high purity Germanium (HPGe) detector; no calibration of the simulation is performed. For the considered validation experiment equivalent simulation accuracy can be achieved with per-decay and statistical sampling.

The simulation of radioactive decays is a common task in Monte-Carlo systems such as Geant4. Usually, a system either uses an approach focusing on the simulations of every individual decay or an approach which simulates a large number of decays with a focus on correct overall statistics. The radioactive decay package presented in this work permits, for the first time, the use of both methods within the same simulation framework - Geant4. The accuracy of the statistical approach in our new package, RDM-extended, and that of the existing Geant4 per-decay implementation (original RDM), which has also been refactored, are verified against the ENSDF database. The new verified package is beneficial for a wide range of experimental scenarios, as it enables researchers to choose the most appropriate approach for their Geant4-based application.

We present results from a recently launched project to study computational issues related to the quantification of non statistical uncertainties in numerical (Monte Carlo) simulations: they derive from different areas of the process of simulation[1], like e.g. epistemic uncertainties[2], experimental errors in physical data, error propagation from the employed numerical algorithms, etc., This paper addresses the development of methods to predict the effects of a set of correlated, partially correlated or uncorrelated physical uncertainties on the observables produced in a Monte Carlo simulation. It also provides some insight on the computational effort needed and on the possible software solutions to be implemented in the kernel of Monte Carlo codes to facilitate the quantification of uncertainty in experimental use cases.

Ongoing investigations for the improvement of Geant4 accuracy and computational performance resulting by refactoring and reengineering parts of the code are discussed. Issues in refactoring that are specific to the domain of physics simulation are identified and their impact is elucidated. Preliminary quantitative results are reported., To be published in the Proc. CHEP (Computing in High Energy Physics) 2012

The Statistical Toolkit is an open source system specialized in the statistical comparison of distributions. It addresses requirements common to different experimental domains, such as simulation validation (e.g. comparison of experimental and simulated distributions), regression testing in the course of the software development process, and detector performance monitoring. Various sets of statistical tests have been added to the existing collection to deal with the one sample problem (i.e. the comparison of a data distribution to a function, including tests for normality, categorical analysis and the estimate of randomness). Improved algorithms and software design contribute to the robustness of the results. A simple user layer dealing with primitive data types facilitates the use of the toolkit both in standalone analyses and in large scale experiments., To be published in the Proc. of CHEP (Computing in High Energy Physics) 2012

A comprehensive analysis of the effects of Geant4 algorithms for condensed transport in detectors is in progress. The first phase of the project focuses on electron multiple scattering, and studies two related observables: the longitudinal pattern of energy deposition in various materials, and the fraction of backscattered particles. The quality of the simulation is evaluated through comparison with high precision experimental measurements; several versions of Geant4 are analyzed to provide an extensive overview of the evolution of Geant4 multiple scattering algorithms and of their contribution to simulation accuracy., Comment: To be published in the Proc. of CHEP (Computing in High Energy Physics) 2012

Several models for the simulation of photon elastic scattering are quantitatively evaluated with respect to a large collection of experimental data retrieved from the literature. They include models based on the form factor approximation, on S-matrix calculations and on analytical parameterizations; they exploit publicly available data libraries and tabulations of theoretical calculations. Some of these models are currently implemented in general purpose Monte Carlo systems; some have been implemented and evaluated for the first time in this paper for possible use in Monte Carlo particle transport. The analysis mainly concerns the energy range between 5 keV and a few MeV. The validation process identifies the newly implemented model based on second order S-matrix calculations as the one best reproducing experimental measurements. The validation results show that, along with Rayleigh scattering, additional processes, not yet implemented in Geant4 nor in other major Monte Carlo systems, should be taken into account to realistically describe photon elastic scattering with matter above 1 MeV. Evaluations of the computational performance of the various simulation algorithms are reported along with the analysis of their physics capabilities.

Three software systems, ERCS08, ISICS 2011 and \v{S}mit's code, that implement theoretical calculations of inner shell ionization cross sections by proton impact, are validated with respect to experimental data. The accuracy of the cross sections they generate is quantitatively estimated and inter-compared through statistical methods. Updates and extensions of a cross section data library relevant to PIXE simulation with Geant4 are discussed., Comment: To be published in IEEE Trans. Nucl. Sci., vol. 58, no.6, Dec. 2011

ISICS, originally a C language program for calculating K-, L- and M-shell ionization and X-ray production cross sections from ECPSSR and PWBA theory, has been reengineered into a C++ language class, named ISICSoo. The new software design enables the use of ISICS functionality in other software systems. The code, originally developed for Microsoft Windows operating systems, has been ported to Linux and Mac OS platforms to facilitate its use in a wider scientific environment. The reengineered software also includes some fixes to the original implementation, which ensure more robust computational results and a review of some physics parameters used in the computation. The paper describes the software design and the modifications to the implementation with respect to the previous version; it also documents the test process and provides some indications about the software performance., Preprint submitted to Computer Physics Communications

A survey of atomic binding energies used by general purpose Monte Carlo systems is reported. Various compilations of these parameters have been evaluated; their accuracy is estimated with respect to experimental data. Their effects on physical quantities relevant to Monte Carlo particle transport are highlighted: X-ray fluorescence emission, electron and proton ionization cross sections, and Doppler broadening in Compton scattering. The effects due to different binding energies are quantified with respect to experimental data. Among the examined compilations, EADL is found in general a less suitable option to optimize simulation accuracy; other compilations exhibit distinctive capabilities in specific applications, although in general their effects on simulation accuracy are rather similar. The results of the analysis provide quantitative ground for the selection of binding energies to optimize the accuracy of Monte Carlo simulation in experimental use cases. Recommendations on software design dealing with these parameters and on the improvement of data libraries for Monte Carlo simulation are discussed.

A system for in-vivo tracking of 192Ir source during high dose rate or pulsed dose rate brachytherapy treatment was built using 1 mm thick silicon pad detectors as image sensors and knife-edge lead pinholes as collimators. With source self-images obtained from a dual-pinhole system location of the source could be reconstructed in three dimensions in real time. The system was tested with 1 Ci 192Ir clinical source in air and plexi-glass phantom. The locations of the source were tracked from a distance of 40 cm in a field of view of 20 × 20 × 20 cm3. Reconstruction precision, defined as the average distance between true and reconstructed source positions, with data collected in less than 1 second with 1 Ci 192Ir source was about 5 mm. The reconstruction precision was in our case mainly limited by imperfect alignment of detectors and pinholes. With perfect alignment the statistical error would allow precision of about 1 mm which could further be improved with larger detector placed at larger distance from the pinhole. However also the modest precision of few millimeters is sufficient for in-vivo detection of larger deviations from planned treatment caused by various misadministrations or malfunctioning of the brachytherapy treatment apparatus. Usage of silicon detectors offers a possibility for building a compact device which could be used as an independent online quality assurance system. In this paper details about sensors, readout system and reconstruction algorithm are described. Results from measurements with clinical source are presented.

A system for in vivo tracking of 192Ir source during high dose rate (HDR) brachytherapy treatments has been built using silicon pad detectors as image sensors and knifeedge lead pinholes as collimators. HDR brachytherapy utilizes moving a small (2-4 mm long) radioactive (usually 192Ir) source within body cavity or tumor to deliver fractionated therapy dose to designated volume. The source is pulled and pushed within catheters (needles or applicators) which are operatively inserted into the target volume before the treatment. Positions and dwell times of the source in catheters are calculated by treatment planning programs. Various kind of errors and inaccuracies, such as displacements of catheters between treatment steps, can and do occur and often remain undetected since procedure only rely on a computer controlled mechanical delivery system (afterloader). An independent system for verifying of HDR treatment would therefore be desirable. With self-images obtained from a dual-pinhole system the source location can be reconstructed in three dimensions. Due to high speed of silicon detector readout system the source position can be determined in real time allowing on-line detection of deviations from planned treatment. The performance of the system was tested with 1 Ci 192Ir clinical source in air and plexi-glass phantom. The movements of the source could be tracked in a field of view of approximately 20 × 20 × 20 cm3 with absolute precision of about 10 millimeters and relative position of around 1.5 millimeter.