NANO5 L. Quintieri (Art. 23)

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1 NANO5 L. Quintieri (Art. 23) 1 NANO5: Description of main objectives NANO5 is a Geant4-related R&D project. It was approved as part of INFN scientific program of Technology Research in September 2008, with start in January It gathers an international team of collaborating scientists at various institutes in Europe, Asia, Northern and Southern America. Geant4 1), 2) is an object oriented toolkit for the simulation of particle interactions with matter. It provides advanced functionalities for all the typical domains of detector simulation: geometry and material modeling, description of particle properties, physics processes, tracking, event and run management, user interface and visualization. Nevertheless, new experimental requirements have emerged in the recent years, which challenge the conventional scope of major Monte Carlo transport codes like Geant4. Research fields as nano-dosimetry, nanotechnology-based detectors, radiation effects on components in space and at high luminosity colliders, nuclear power, plasma physics etc. have shown the need of new methodological approaches to radiation transport simulation along with new physics functionalities in Geant4. The NANO5 project investigates conceptual and technological solutions to extend the current capabilities of the Geant4 toolkit to cope with the new experimental requirements and evaluates whether and how they can be supported by the Geant4 Kernel. In more detail, NANO5 investigates the possibility of introducing into Geant4 new physics functionality and implementing new methodological approach for radiation transport simulation in order to satisfy mainly the following requirements: To make possible to perform simulation at different scale in the same experimental environment (condensed-random-walk and discrete methods). In fact, in realistic cases, small-scale systems are often embedded in larger scale ones (i.e micro-component of an equipment inside an HEP experiment, cellular ans sub-cellular aggregate in biological system, etc). To exploit Monte Carlo and deterministic transport methods in the same simulation environment in case where performance issues are critical (nuclear reactor design). Developing of innovative design solutions in software architecture of Geant4 Kernel and consequent evaluation of their implications in terms of computational performances and quality assurance. 1.1 New Architectural Design Approach The main feature of the new software architectural design, at the present under study with NANO5, is the minimalist approach based on the policy class design, whose potentialities have been explored in detail by Andrei Alexandrescu 3). In brief, policy-based class design promotes to assemble a

2 class with complex behavior out of many little classes (called policies), each of which takes care of only one behavioral or structural aspect. A policy class is a template parameter used to transmit behavior. As the name suggests, a policy establishes an interface pertaining to a specific issue: it is possible to implement policies in various ways as long as the policy interface is respected. Because it is possible to mix and match policies, we can achieve a combinatorial set of behaviors by using a small core of elementary components. Alexandrescu description of policy classes suggests that their power is derived essentially from granularity and orthogonality. 2 Main activities performed at Frascati The work developed at Frascati has been essentially devoted to accomplish the following two tasks: 1. Application of the policy based design to implement the Photon Physics in Geant4 and study of the consequent effects it has on computational performance and quality assurance. 2. Set-up of an exhaustive data library as base of software developments to improve the PIXE (Particle Induced X-ray Emission) simulation in Geant4. The details of the given contributions are better explained in the following paragraphs. 2.1 New Photon Interaction Models implementation As start-up point, in order to test the implementation of the new architectural design, we focused on the electromagnetic physics package. In particular, a pilot project is currently in progress in the domain of photon interactions (Compton and Rayleigh scattering, photoelectric effect and photon conversion), with the aim of implementing the current Geant4 physics models in terms of the architectural design briefly described in 1.1. Performance measurements as well as first-hand evaluations of the capabilities and drawbacks of the policy-based design are, finally, studied. We have redesigned the generic photon process in such a way that it acts as a host class, deprived of intrinsic physics fucnctionality. Physics behavior is acquired through policy classes, respectively responsible for cross section and final state generation (passed as concrete classes). In this new approach, cross section and final state policies are orthogonal (that means independently implemented and computed). The main advantages of this approach are: Flexible configuration of processes at granular level. Transparency of physics Performance optimization for computationally intensive use cases Effortless Verification & Validation Figure 1 shows an example of the prototype design of photon interactions, applied to the Comptpn Scattering. Frascati has given a special contribution on the implementation (according the new architectural design) and consequent testing of the pair-production process in the Standard flavor 1. Thanks to the new design, we could appreciate an improved transparency of physics models, that is, in fact, exposed at a fine-grained level. 1 All photon interactions are implemented in Geant4 at least in 3 modeling variants, which are identified as Standard, Library-based and Penelope

3 NANO5 AT LNF : Focussed on condensed-discrete transport agnetic Physic (policy-based design), Estimation of the consequent ce (still in progress), Software verification and validation (V&V) processes for use UML diagram of the new EM Physics design G4CrossSectionDataLib, G4GeneratorComptonDataLib G4ComptonDataLib TCrossSection TGenerator G4TRDPhotonProcess G4CrossSectionComptonPenelope,G4GeneratorComptonPenelope e.g. Compton scattering G4CrossSectionComptonStandard,G4GeneratorComptonDataLib G4CrossSectionComptonStandard,G4GeneratorComptonStandard G4ComptonStandardDataLib etc. G4ComptonPenelope G4ComptonStandard malist approach: a generic process acts as a host class, which is deprived of intrinsic red through policy classes, respectively responsible for cross section and final state cess is independent from the model that determines the cross-section and final state Figure 1: Main Feature of the Policy-Based prototype, illustrated for Compton Scattering Geant4 Standard EM Physics : Pair-Production Cross-section old implementation versus new implemetation compared with NIST data Berylium cpu time [s] avg % diff over the energy range between NIST data and new implementation is 22% (with 6% at 1.5 MeV) e+06 1e+07 1e+08 new implementation NIST old implementation ttering (40 kev photon ) on several atoms Performance gain: ~30% 10 Preliminary performance measurements in 1 a few simple cases of photon interactions indicate a gain of the order of 30% in 0.1 avg % diff over the energy range between NIST data and new implementation is 5 % computational speed with respect to equivalent physics 0.01 implementations in the current Geant4 design scheme; Geant4 Standard EM Physics : Pair-Production Cross-section old implementation versus new implemetation compared with NIST data Copper e+06 1e+07 1e+08 new implementation NIST old implementation Figure 2: Photon conversion cross-section validation with respect to NIST reference data for Be and Cu Validation of the implemented models The plots in Fig. 2 refer to the case of photon conversion cross-section comparison with respect to NIST reference data for two cases: Be and Cu. Discrepancies between the implementation in Geant4 9.1 standard electromagnetic package and the Official User Documentation of the crosssection model for photon conversion have been identified and reported to the maintainers of the original Geant4 implementations. The observed model behavior is shown in Fig. 3 (on the left). According to the Geant4 Physics Manual, above 100 GeV the cross-section for photon conversion, based on the Bethe-Heitler model, should be constant. The agility of NANO5 electromagnetic physics design allowed a quick re-implementation of the cross-section computation consistent with the specifications of Geant4 Physics Reference Manual. Other implementations of this cross section based on alternative models documented in literature are in progress; for instance, according to work reported in reference 4) the cross section above 1 TeV is expected to fall as a function

4 of energy. Implementations by Geant4 standard electromagnetic group have been announced in Geant4 development plans GeV GeV e+06 1e+07 G4BetheHeitler Current version of Geant e+06 1e+07 G4CrossSectionGammaConversionStandard Nano5 according Reference Manual Figure 3: Differences between the implementation in Geant4.9.1 standard electromagnetic package and the User Documentation of the cross section calculation for photon conversion Effects of the new architectural design on the computational performance and quality assurance The testing of basic physics components (like atomic cross sections or features of the final state models) is greatly facilitated wrt to the current Geant4 version: being associated with low level objects like policy classes, they can be verified and validated independently, while in the current design scheme a full-scale Geant4 based application is necessary to study even low-level physics entities. The gain in simplification of the physics testing code has been estimated as of approximately two orders of magnitude in terms of reduction of instruction lines to execute 2, while the computational resources needed for the test operation have been reduced dramatically. These achievements are relevant to the quality assurance of Geant4 physics. Preliminary results of the electromagnetic physics pilot project indicate a performance improvement associated with the policy-based design. In table 1 we report the performance comparison (in terms of cpu time) between the Geant4.9.1 and NANO5 design for the estimation of the final status in a simple test for Compton scattering of photon with energy 40 kev on several atoms. Preliminary performance measurements in a few simple cases of photon interactions indicate a gain of the order of 30% in computational speed with respect to equivalent physics implementations in the current Geant4 design scheme. Other tests are in progress to check if the computational performance are affected also in all the other Photon Physics Models that have been reimplemented, up to now. 2 The test for comparison of basic Geant4 electromagnetic physics features against NIST Physical Reference Data requires more than 4000 lines of code in a fully scale Geant4 based application. Equivalent tests for the physics parameters related to photons can be performed through simple tests, consisting of few tens of lines only and running very fast on a laptop computer.

5 Table 1: CPU time[s] for estimation of final status for Compton scattering of 40 kev Photons on several atoms (Penelope). Atom Policy-based Geant4 9.1 Gain design C % Si % Cu % W % 40 kev, 10 6 events, Intel Core2 Duo Processor E6420, 2.13 GHz, 4GB RAM 2.2 Developments on PIXE Simulation with Geant4 At the present time Geant4 does not provide adequate capabilities for the simulation of PIXE in realistic case, as documented in 5). For this reason, the capabilities of the Geant4 toolkit have been extended, in the frame of NANO5, by enabling the generation of PIXE associated with K, L and M shells for protons and α particles, providing a variety of cross section models. In this context, an extensive ionisation cross section data library has been created as support of the development process. This extended data library represents essentially the main contribution given by Frascati for PIXE improvement in Geant4. The adopted data-driven strategy and the software design improve the computational performance over previous Geant4 models. The validity of the implemented models has been quantitatively estimated with respect to experimental data, as shown in 5) The PIXE data Library The PIXE simulation in Geant4 identifies three main fields with associated responsibilities: the hadron ionization process, the creation of a vacancy in the shell occupancy resulting from ionisation, the deexcitation of the ionised atom with the associated generation of X-rays. The simulation of PIXE concerns a variety of experimental applications, that require the capability of calculating ionisation cross sections over an extended energy range: from a few MeV typical of material analysis applications to hundreds MeV or GeV range of astrophysical applications. Various theoretical and empirical models are available in literature to describe ionisation cross sections for different interacting particles, as well as compilations of experimental data. The current software prototype, developed by the NANO5 collaboration, has adopted the strategy of providing an extensive collection of ionisation cross section models as a function of element, atomic (sub-)shell, and incident particle kinetic energy. The cross sections for ionization of K, L and M shell by protons and α particles have been tabulated and assembled in an extensive data library (PAIX); the values at a given energy are calculated by interpolation. Cross sections have been derived from theoretical calculations based on the EPCSSR method, also including variants like Hartree-Slater, United Atom and high energy corrections, and from a variety of empirical models based on experimental data collections. The tabulations corresponding to theoretical calculations span the energy range between 10 kev and 10 GeV; empirical models are tabulated consistently with their energy range of validity. The adopted data-driven approach optimizes performance speed and offers flexibility for chosing a cross section model.

6 A collaboration with RSICC (Radiation Safety Information Computational Center) of Oak Ridge National Laboratories, is actually in progress, under the direct supervision of Frascati, in order to assemble and distribute to the whole scientific community the PAIX data libraries. 3 Conference Talks 1. L. Quintieri, Inter-Comparison and Validation of Geant4 Photon Interaction Models, Poster Session on Computing and Software for Experiments, (CHEP 2009) Orlando, Florida. 2. L. Quintieri, Research in Geant4 electromagnetic physic design and its effects on computational performance and quality assurance, (CHEP 2009), Orlando, Florida. 4 Publications in 2009 Journals 1. M. G. Pia et al., IEEE Trans. Nucl. Sci., 56, 3614 (2009). Conference Proceedings 1. M. Augelli et al., Inter-Comparison and Validation of Geant4 Photon Interaction Models Proceedings of the Nuclear Science Symposium and Medical Imaging Conference 2009, Orlando, Florida. 2. M. Augelli et al., Geant4-related R&D for new particle transport methods, Proceedings of the Nuclear Science Symposium and Medical Imaging Conference 2009, Orlando, Florida. 3. M. Augelli et al., Research in Geant4 electromagnetic physics design, and its effects on computational performanc and quality assurance, Proceedings of the Nuclear Science Symposium and Medical Imaging Conference 2009, Orlando, Florida. 4. M. G. Pia et al., New models for PIXE simulation with Geant4, Proceedings of CHEP (Computing in High Energy Physics) M. G. Pia et al., Design and performance evaluations of generic programming techniques in a R&D prototype of Geant4 physics, Proceedings of CHEP (Computing in High Energy Physics) M. G. Pia et al., R&D on co-working transport schemes in Geant4, Proceedings of CHEP (Computing in High Energy Physics) M. G. Pia et al., R&D for co-working condensed and discrete transport methods in Geant4 kernel, Proc. Int. Conf. on Mathematics, Computational Methods & Reactor Physics (M&C 2009), New York, References 1. S. Agostinelli et al., Nucl. Instr. and Meth. A 506, 250 (2003). 2. J. Alliso, et al, IEEE Trans. Nucl. Sci. 53, 270 ( 2006). 3. A. Alexandrescu, Modern C++ Design: Generic Programming and Design Patterns Applied, Addison-Wesley (E), February S. R. Klein, Radiation Physics and Chemistry, 75, 696 (2006). 5. M. G. Pia et al., IEEE Trans. Nucl. Sci. 56, 3614 (2009).