Tullio Basaglia

Validation Test of Geant4 Simulation of Electron Backscattering

Backscattering is a sensitive probe of the accuracy of electron scattering algorithms implemented in Monte Carlo codes. The capability of the Geant4 toolkit to describe realistically the fraction of electrons backscattered from a target volume is extensively and quantitatively evaluated in comparison with experimental data retrieved from the literature. The validation test covers the energy range between approximately 100 eV and 20 MeV, and concerns a wide set of target elements. Multiple and single electron scattering models implemented in Geant4, as well as preassembled selections of physics models distributed within Geant4, are analyzed with statistical methods. The evaluations concern Geant4 versions from 9.1 to 10.1. Significant evolutions are observed over the range of Geant4 versions, not always in the direction of better compatibility with experiment. Goodness-of-fit tests complemented by categorical analysis tests identify a configuration based on Geant4 Urban multiple scattering model in Geant4 version 9.1 and a configuration based on single Coulomb scattering in Geant4 10.0 as the physics options best reproducing experimental data above a few tens of keV. At lower energies only single scattering demonstrates some capability to reproduce data down to a few keV. Recommended preassembled physics configurations appear incapable of describing electron backscattering compatible with experiment. With the support of statistical methods, a correlation is established between the validation of Geant4-based simulation of backscattering and of energy deposition.

Geant4 in scientific literature

The Geant4 reference paper published in Nuclear Instruments and Methods A in 2003 has become the most cited publication in the whole Nuclear Science and Technology category of Thomson-Reuters Journal Citation Reports. It is currently the second most cited article among the publications authored by two major research institutes, CERN and INFN. An overview of Geant4 presence (and absence) in scholarly literature is presented; the patterns of Geant4 citations are quantitatively examined and discussed.

Investigation of Geant4 Simulation of Electron Backscattering

A test of Geant4 simulation of electron backscattering recently published in this journal prompted further investigation into the causes of the observed behaviour. An interplay between features of geometry and physics algorithms implemented in Geant4 is found to significantly affect the accuracy of backscattering simulation in some physics configurations.

Writing software or writing scientific articles

An analysis of publications related to high energy physics computing in refereed journals is presented. The distribution of papers associated to various fields of computing relevant to high energy physics is critically analyzed. The relative publication rate of software papers is evaluated in comparison to other closely related physics disciplines, such as nuclear physics, radiation protection and medical physics, and to hardware publications. The results hint to the fact that, in spite of the significant effort invested in high energy physics computing and its fundamental role in the experiments, this research area is underrepresented in scientific literature; nevertheless the analysis of citations highlights the significant impact of software publications in experimental research.

Physics methods for the simulation of photoionisation

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.

Writing Software or Writing Scientific Articles

An analysis of publications related to high energy physics computing in refereed journals is presented. The distribution of papers associated to various fields of computing relevant to high energy physics is critically analyzed. The relative publication rate of software papers is evaluated in comparison to other closely related physics disciplines, such as nuclear physics, radiation protection and medical physics, and to hardware publications. The results hint to the fact that, in spite of the significant effort invested in high energy physics computing and its fundamental role in the experiments, this research area is underrepresented in scientific literature; nevertheless the analysis of citations highlights the significant impact of software publications in experimental research.

Quantitative Test of the Evolution of Geant4 Electron Backscattering Simulation

Evolutions of Geant4 code have affected the simulation of electron backscattering with respect to previously published results. Their effects are quantified by analyzing the compatibility of the simulated electron backscattering fraction with a large collection of experimental data for a wide set of physics configuration options available in Geant4. Special emphasis is placed on two electron scattering implementations first released in Geant4 version 10.2: the Goudsmit-Saunderson multiple scattering model and a single Coulomb scattering model based on Mott cross section calculation. The new Goudsmit-Saunderson multiple scattering model appears to perform equally or less accurately than the model implemented in previous Geant4 versions, depending on the electron energy. The new Coulomb scattering model was flawed from a physics point of view, but computationally fast in Geant4 version 10.2; the physics correction released in Geant4 version 10.2p01 severely degrades its computational performance. Problems observed in electron backscattering simulation in previous publications have been addressed by evolutions in the Geant4 geometry domain.

Validation of Cross Sections for Monte Carlo Simulation of the Photoelectric Effect

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 Scofields 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. Scofields 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 Ebels parameterization produces more accurate results close to absorption edges. Modifications to Biggs and Lighthills 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.

First Assessment of ENDF/B-VIII and EPICS Atomic Data Libraries

This paper reports an extensive assessment of widely used evaluated atomic data libraries released in ENDF/B-VIII.0 and in EPICS2017 in early 2018. The new versions are intended to replace the data libraries currently used by major Monte Carlo particle transport codes to model electron and photon interactions with matter, which date back to the 1990s. The evaluation is performed from a user perspective and concerns various characteristics of the data, including their intrinsic consistency, the differences across their various formats and distribution sources, and the effects on computational performance associated with their use. The results of the tests demonstrate the impact of using the new data libraries in a Monte Carlo simulation environment and highlight some opportunities for improvement in future versions.

Experimental assessment of electron ionization cross sections

We report preliminary results of an extensive investigation of theoretical and semi-empirical calculations of electron impact ionization cross sections, detailed by individual shells: they encompass the well known tabulations of the EEDL data library (also distributed within ENDF/B-VII) used by Geant4, MCNP and other codes, recent calculations used in Penelope, as well as other models not yet used in general-purpose Monte Carlo transport codes. All models have been subject to a rigorous validation test against a wide collection of experimental measurements. Special attention has been devoted to possible sources of systematics affecting the validation process, both of physical and mathematical origin. As most of the data reported in the literature as experimental measurements of ionization cross sections actually derive from X-ray production measurements, the systematic effect of different compilations of fluorescence yields has been quantitatively assessed. The compatibility of calculated and experimental cross sections has been further examined with categorical analysis methods to determine whether the observed differences across the various models are statistically significant. The results of this validation process identify objectively and quantitatively the state of the art in modeling electron impact ionization; they are relevant for the improvement of ionization modeling in Monte Carlo codes.