Geant4

Geant4 (for GEometry ANd Tracking) is a platform for "the simulation of the passage of particles through matter," using Monte Carlo methods. It is the successor of the GEANT series of software toolkits developed by CERN, and the first to use Object oriented programming (in C++). Its development, maintenance and user support are taken care by the international Geant4 Collaboration. According to its website, "application areas include high energy physics and nuclear experiments, medical, accelerator and space physics studies." The software is used by a number of research projects around the world.

The Geant4 software and source code have always been freely available from the project web site but, until version 8.1 (released June 30, 2006), no specific software license for its use existed. Geant4 is now provided under the Geant4 Software License.

Geant4 features
Geant4 includes facilities for handling geometry, tracking, detector response, run management, visualization and user interface. For many physics simulations, this means less time need be spent on the low level details, and researchers can start immediately on the more important aspects of the simulation.

Following is a summary of each of the facilities listed above:
 * Geometry is an analysis of the physical layout of the experiment, including detectors, absorbers, etc., and considering how this layout will affect the path of particles in the experiment.
 * Tracking is simulating the passage of a particle through matter. This involves considering possible interactions and decay processes.
 * Detector response is recording when a particle passes through the volume of a detector, and approximating how a real detector would respond.
 * Run management is recording the details of each run (a set of events), as well as setting up the experiment in different configurations between runs.
 * Geant4 offers a number of options for visualization, including OpenGL, and a familiar user interface, based on Bash.

Geant4 can also optionally perform histogramming, but it requires external software that implements the AIDA framework in order to do so.

Some high energy physics experiments using Geant4

 * BaBar and GLAST at SLAC
 * ATLAS and CMS at LHC, CERN
 * Borexino at Gran Sasso Laboratory
 * MINOS at Fermilab
 * EXO

Geant4 applications outside high energy physics
Because of its general purpose nature, Geant4 is well suited for development of computational tools for analysing interactions of particle with matter in many areas. These include:
 * Space applications where it is used to study interactions between the natural space radiation environment and space hardware or astronauts;
 * Medical applications where interactions of radiations used for treatment are simulated.