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For Application Developers
Tracking and Physics

5.4 Production Threshold versus Tracking Cut

5.4.1 General considerations

We have to fulfill two contradictory requirements. It is the responsibility of each individual process to produce secondary particles according to its own capabilities. On the other hand, it is only the Geant4 kernel (i.e., tracking) which can ensure an overall coherence of the simulation.

The general principles in Geant4 are the following:

  1. Each process has its intrinsic limit(s) to produce secondary particles.
  2. All particles produced (and accepted) will be tracked up to zero range.
  3. Each particle has a suggested cut in range (which is converted to energy for all materials), and defined via a SetCut() method (see Section 2.4.2).

Points 1 and 2 imply that the cut associated with the particle is a (recommended) production threshold of secondary particles.

5.4.2 Set production threshold (SetCut methods)

As already mentioned, each kind of particle has a suggested production threshold. Some of the processes will not use this threshold (e.g., decay), while other processes will use it as a default value for their intrinsic limits (e.g., ionisation and bremsstrahlung).

See Section 2.4.2 to see how to set the production threshold.

5.4.3 Apply cut

The DoIt methods of each process can produce secondary particles. Two cases can happen:

BEFORE being recopied to the temporary stack for later tracking, the particles below the production threshold will be kept or deleted according to the safe mechanism explained hereafter.

5.4.4 Why produce secondaries below threshold?

A process may have good reasons to produce particles below the recommended threshold:

These secondary particles are sent to the ``Stepping Manager'' with a flag GoodForTracking to pass the filter explained in the previous section (even when ApplyCut is ON).

5.4.5 Cuts in stopping range or in energy?

The cuts in stopping range allow one to say that the energy has been released at the correct space position, limiting the approximation within a given distance. On the contrary, cuts in energy imply accuracies of the energy depositions which depend on the material.

5.4.6 Summary

In summary, we do not have tracking cuts; we only have production thresholds in range. All particles produced and accepted are tracked up to zero range.

It must be clear that the overall coherency that we provide cannot go beyond the capability of processes to produce particles down to the recommended threshold.

In other words a process can produce the secondaries down to the recommended threshold, and by interrogating the geometry, or by realizing when mass-to-energy conversion can occur, recognize when particles below the threshold have to be produced.

5.4.7 Special tracking cuts

One may need to cut given particle types in given volumes for optimisation reasons. This decision is under user control, and can happen for particles during tracking as well.

The user must be able to apply these special cuts only for the desired particles and in the desired volumes, without introducing an overhead for all the rest.

The approach is as follows:

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