Principle

The simulation of the processes which accompany the propagation of a particle through the material of the detector (e.g. bremsstrahlung, $\delta$ -rays production, Compton scattering and so on) is performed by GEANT in the following steps:

1. Fetch a new particle to be tracked (often called track or history) from the stack supported by the link JSTAK (see [TRAK399]). This is done once at the beginning of each new track. The number of interaction lengths that the particle is going to travel, before undergoing each one of the possible discrete processes, is sampled at this point. These operations are done in the routine GLTRAC.
2. Evaluate the distance to the interaction point. This is done by the individual tracking routines (GTGAMA, GTNEUT, GTHADR, GTNINO, GTMUON, GTHION and GTCKOV) which control the tracking of particular particles. The number of interaction lengths remaining to travel before each of the possible processes (often called tracking mechanisms or simply mechanisms) is multiplied by the inverse of the macroscopic cross-section for that process in the current material (i.e. the interaction length). This gives the distances that the particle has to travel before each of the processes occurs in the current medium. The minimum among these numbers is the step over which the particle will be transported. In addition to the physics mechanisms, four pseudo-interactions are taken into account in the calculation of the step:
   1. boundary crossing. The crossing of a volume boundary is treated like a discrete process. A particle never crosses a boundary during a step but rather stops there ( NEXT mechanism);
   2. maximum step limit. For each tracking medium a value for the maximum step can be specified by the user. Process SMAX;
   3. maximum fraction of continuum energy loss, maximum angular deviation in magnetic field or maximum step for which the Molière formula, to simulate multiple scattering is valid. These are continuous processes, which introduce a limitation on the tracking step expressed by a single variable (see section [PHYS325] on GMULOF).
   4. energy and time cut. Charged particles in matter are stopped when their energy falls below their energy threshold or when their time of flight exceeds the time cut;
   More information is given in the individual sections explaining the implementation of the physical processes.
3. Transport the particle either along a straight line (if no magnetic field or for a neutral particle) or along a helicoidal path (for charged particles in magnetic field).
4. Update the energy of the particle if continuous energy loss was in effect (charged particles in matter).
5. If a physical discrete process has been selected, generate the final state of the interaction.
6. If the incident particle survives the interaction (Compton, $\delta$ -rays production, bremsstrahlung, direct pair production by $\mu$ and $\mu$ -nucleus interaction, hadronic elastic scattering), sample again the number of interaction lengths to travel before the next event of the same kind. This is generally done by specialised routines: GMUNU, GCOMP, GBREM, etc.
7. Update the number of interaction lengths for all the processes and go back to $(2)$ till the particle either leaves the detector or falls below its energy threshold or beyond its time cut or disappears in an interaction.