The method to be used for energy loss straggling is chosen in GFLUCT. If -rays are produced ( DRAY = 1) above the cut value DCUTE and the detailed PAI simulation for straggling in thin layers (see PHYS334) is not chosen ( STRA = 0, default), GLANDZ is called always as it samples from the restricted distribution (the energy loss of the -rays which are explicitly produced should not be taken into the energy loss distribution).
If -rays are not produced, the values of and are computed, and Urbán, Landau, Vavilov or Gaussian model is chosen accordingly. If Urbán model is used, GLANDZ will be called with infinite ( BIG) value for the -ray production.
When Landau model is chosen, it is possible to use a routine from the CERN Program Library to sample random numbers from the Landau distribution : GENLAN written by James and Hancock [] and copied into GEANT source file as GLANDG. This routine has been modified in order to reproduce the tail for large values of X. The original routine did not give values of X larger than 200.
CALL GENLAN(X)It should be noted that over the years considerable confusion has arisen over the precise form and features of the Landau distribution. As an example, a simpler form of had been proposed by Moyal [] and used on occasions which has little in common with the function defined in section
[].
For the Vavilov distribution we have used the function GVAVIV by Rotondi and Montagna [].