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 View the MathML source 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

Abstract. In this paper we discuss the methods developed for the production of the INTEGRAL/SPI instrument response. The response files were produced using a suite of Monte Carlo simulation software developed at NASA/GSFC based on the GEANT-3 package available from CERN. The production of the INTEGRAL/SPI instrument response also required the development of a detailed computer mass model for SPI. We discuss our extensive investigations into methods to reduce both the computation time and storage requirements for the SPI response. We also discuss corrections to the simulated response based on our comparison of ground and inflight calibration data with MGEANT simulations.

Abstract. On Oct. 17, 2002, the ESA INTEGRAL observatory was launched into a highly elliptical orbit. SPI, a high resolution Ge spectrometer covering an energy range of 20-8000 keV, is one of its two main instruments. We use data recorded early in the mission (i.e. in March 2003) to characterize the instrumental background, in particular the many gamma-ray lines produced by cosmic-ray interactions in the instrument and spacecraft materials. More than 300 lines and spectral features are observed, for about 220 of which we provide identifications. An electronic version of this list, which will be updated continuously, is available for download at CESR. We also report first results from our efforts to model these lines by ab initio Monte Carlo simulation.

Intense and complex instrumental backgrounds, against which the much smaller signals from celestial sources have to be discerned, are a notorious problem for low and intermediate energy γ-ray astronomy (∼ 50 keV -10 MeV). Therefore a detailed qualitative and quantitative understanding of instrumental line and continuum backgrounds is crucial for most stages of γ-ray astronomy missions, ranging from the design and development of new instrumentation through performance prediction to data reduction. We have developed MGGPOD, a userfriendly suite of Monte Carlo codes built around the widely used GEANT (Version 3.21) package, to simulate ab initio the physical processes relevant for the production of instrumental backgrounds. These include the build-up and delayed decay of radioactive isotopes as well as the prompt de-excitation of excited nuclei, both of which give rise to a plethora of instrumental γ-ray background lines in addition to continuum backgrounds. The MGGPOD package and documentation are publicly available for download from http://sigma-2.cesr.fr/spi/MGGPOD/.We demonstrate the capabilities of the MGGPOD suite by modeling high resolution γ-ray spectra recorded by the Transient Gamma-Ray Spectrometer (TGRS) on board Wind during 1995. The TGRS is a Ge spectrometer operating -2in the 40 keV to 8 MeV range. Due to its fine energy resolution, these spectra reveal the complex instrumental background in formidable detail, particularly the many prompt and delayed γ-ray lines. We evaluate the successes and failures of the MGGPOD package in reproducing TGRS data, and provide identifications for the numerous instrumental lines.

Abstract. The INTEGRAL Mass Model (TIMM) was started in 1995 and aimed to create a detailed geometrical model of the whole INTEGRAL satellite on computer. In parallel, a comprehensive Monte Carlo simulation code (called GGOD) has been developed. The mass model and the Monte Carlo code together enable the in-flight operation of INTEGRAL to be simulated at the individual event level. Thus TIMM can be used to provide an independent evaluation of the performance of the individual instruments, to study the interference and complementarity between instruments, to generate test data for software development, and as a powerful tool for post-launch diagnosis. In this paper TIMM is briefly reviewed, some examples from ground calibration are presented, and preliminary comparison to flight data is shown. The future use of TIMM to flat field flight data is also briefly discussed.