Geant4—a simulation toolkit

Abstract

Geant4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of functionality including tracking, geometry, physics models and hits. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from $\text{250}\text{eV}$ and extending in others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting software engineering and object-oriented technology and implemented in the C++ programming language. It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical physics.

Introduction

Modern particle and nuclear physics experiments pose enormous challenges in the creation of complex yet robust software frameworks and applications. Of particular importance is the ever-increasing demand for large-scale, accurate and comprehensive simulations of the particle detectors used in these experiments. The demand is driven by the escalating size, complexity, and sensitivity of the detectors and fueled by the availability of moderate-cost, high-capacity computer systems on which larger and more complex simulations become possible. Similar considerations arise in other disciplines, such as: radiation physics, space science, nuclear medicine and, in fact, any area where particle interactions in matter play a role.

In response to this, a new object-oriented simulation toolkit, Geant4, has been developed. The toolkit provides a diverse, wide-ranging, yet cohesive set of software components which can be employed in a variety of settings. These range from simple one-off studies of basic phenomena and geometries to full-scale detector simulations for experiments at the Large Hadron Collider and other facilities.

In defining and implementing the software components, all aspects of the simulation process have been included: the geometry of the system, the materials involved, the fundamental particles of interest, the generation of primary particles of events, the tracking of particles through materials and external electromagnetic fields, the physics processes governing particle interactions, the response of sensitive detector components, the generation of event data, the storage of events and tracks, the visualisation of the detector and particle trajectories, and the capture for subsequent analysis of simulation data at different levels of detail and refinement.

Early in the design phase of the project, it was recognised that while many users would incorporate the Geant4 tools within their own computational framework, others would want the capability of easily constructing stand-alone applications which carry them from the initial problem definition right through to the production of results and graphics for publication. To this end, the toolkit includes built-in steering routines and command interpreters which operate at the problem setup, run, event, particle transportation, visualisation, and analysis levels, allowing all parts of the toolkit to work in concert.

At the heart of this software system is an abundant set of physics models to handle the interactions of particles with matter across a very wide energy range. Data and expertise have been drawn from many sources around the world and in this respect Geant4 acts as a repository that incorporates a large part of all that is known about particle interactions; moreover it continues to be refined, expanded and developed. A serious limitation of many previous simulation systems was the difficulty of adding new or variant physics models; development became difficult due to the increasing size, complexity and interdependency of the procedure-based code. In contrast, object-oriented methods have allowed us effectively to manage complexity and limit dependencies by defining a uniform interface and common organisational principles for all physics models. Within this framework, the functionality of models can be more easily seen and understood, and the creation and addition of new models is a well-defined procedure that entails little or no modification to the existing code.

Geant4 was designed and developed by an international collaboration, formed by individuals from a number of cooperating institutes, HEP experiments, and universities. It builds on the accumulated experience of many contributors to the field of Monte Carlo simulation of physics detectors and physical processes. While geographically distributed software development and large-scale object-oriented systems are no longer a novelty, we consider that the Geant4 Collaboration, in terms of the size and scope of the code and the number of contributors, represents one of the largest and most ambitious projects of this kind. It has demonstrated that rigorous software engineering practices and object-oriented methods can be profitably applied to the production of a coherent and maintainable software product, even with the fast-changing and open-ended requirements presented by physics research.

In the following sections we present a detailed overview of Geant4 and its features and capabilities, including the design and implementation of the various categories of physics models. Many new physics models have been developed, and others have been refined or extended. They have been created to support a growing range of applications for the software, including particle, nuclear, medical, accelerator and space physics. The code and documentation, as well as tutorials and examples, are available from our Web site [1].

The origin of Geant4 development can be traced back to two studies done independently at CERN and KEK in 1993 [2]. Both groups sought to investigate how modern computing techniques could be applied to improve what was offered by the existing GEANT3 program [3], which was a benchmark and source of ideas and valuable experience. These two activities merged and a proposal was submitted to the CERN Detector Research and Development Committee (DRDC) [4] to construct a simulation program based on object-oriented technology. The resulting project was RD44, a worldwide collaboration that grew to include the efforts of 100 scientists and engineers, drawn from more than 10 experiments in Europe, Russia, Japan, Canada and the United States.

The design choices faced by RD44 and the decisions arrived at are described in later sections, but key to its success was a careful design adapting object-oriented methodology and an early decision to use the practical C++ language.

The R&D phase was completed in December 1998 [1] with the delivery of the first production release. Subsequently the Geant4 Collaboration was established in January 1999 to continue the development and refinement of the toolkit, and to provide maintenance and user support.

A Memorandum of Understanding (MoU) [5] signed by all participating parties governs the formal collaboration. It is subject to tacit renewal every 2 years and sets out a collaboration structure composed of a Collaboration Board (CB), a Technical Steering Board (TSB) and several working groups. The MoU also defines the way in which collaboration resources—money, manpower, expertise, and key roles and activities (such as program librarian and documentation manager)—are measured in Contribution Units (CU), and it further delineates how the boards are constituted depending on the CU count for each signatory. Participating groups include experimental teams and collaborations, laboratories and national institutes.

It is the CB's mandate to manage these resources and to monitor the agreed responsibilities among the affiliates. This body is also charged with the evolution of the MoU. The TSB, on the other hand, is the forum where technical matters, like software engineering details and physics model implementation issues, are discussed and decided and where priorities are given to user requests. Its primary tasks are the supervision of the production service and the user support and the overseeing of ongoing further development of the program. The TSB is chaired by the spokesperson of the Collaboration, who is appointed by and reports to the CB. The spokesperson is (re)elected every two years.

Every domain of the Geant4 software that corresponds to a releasable component (library) is individually managed by a working group of experts. In addition, there is a working group for each of the activities of testing and quality assurance, software management and documentation management. A coordinator who is selected by the TSB heads each group. There is also an overall release coordinator. This clean overall problem decomposition makes the distributed software design and development possible in a worldwide collaboration. Every group can work in parallel, allowing an optimal use of manpower and expertise.

The Collaboration provides documentation and user support for the toolkit. The support model is described in more detail in Section 3.6.

Documentation [6] includes installation, user and reference guides, and a range of training kits (see also Section 1.4). It is intended to cover the need of the beginner through to the expert user who wishes to expand the capabilities of Geant4.

User support covers help with problems relating to the code, consultation on using the toolkit and responding to enhancement requests. A user may also expect assistance in investigating anomalous results.

A Web-based reporting system and a list of frequently asked questions (FAQs) are available on the Geant4. Web site [1]. The Collaboration also runs a Web-based user forum [7], with sub-forums according to areas of different interest.

Regular releases of the source code and documentation are freely available on the Web.

The toolkit includes examples at three levels:

• Novice: for understanding basic functionalities;

• Extended: focused on specific domains of application (they may also need additional third party libraries);

• Advanced: full programs created to utilise Geant4 in HEP experiments, and for space and medical applications.

They are intended to develop the user's understanding in many areas. Initial emphasis is on the classes describing the user's setup, which are required by the toolkit. These classes are explained in Section 2.4.

Geant4 also provides a training kit. It consists of a modular set of units, each covering a specific domain. The units contain descriptive material and examples, such as code excerpts, or plots with performance results. They are modular in themselves, providing different levels of coverage and complexity.

For the reader who wishes to obtain a broad overall view of the project from inception to realisation we describe basic principles of the design in Section 2.

Details that are needed to understand how to extend the toolkit, tailor it for special use and obtain optimal performance, are postponed to 4 The kernel, 5 Physics processes.

In between, we devote Section 3 to the important issue of the software process as it applies to a large, dispersed collaboration. It is here that the exploitation of modern software engineering techniques and object-oriented methods are discussed.

The basic algorithms and capabilities of the kernel are described in Section 4 and an overview of available physics processes and models is presented in Section 5. The latter also includes a sample of results and comparisons with GEANT3 and experimental measurements.

Additional capabilities are discussed in Section 6 and interactivity (user interfaces, visualisation and analysis) in Section 7.