GEANT has five different parameters primarily responsible for
determining the step size (ie, the distance between successive points
at which GEANT calculates the attributes of the particle). They are
(as described in the section CONS200 of the GEANT manual):
TMAXFD - The maximum angular deviation due to the magnetic field
permitted in one step (degrees).
STEMAX - The maximum step size permitted (cm).
DEEMAX - The maximal fractional energy loss permitted in one step
(0<DEEMAX<=1).
EPSIL - The boundary crossing precision (cm).
STMIN - The minimum allowed value for the maximum step imposed by
energy loss, multiple scattering, Cerenkov or magnetic field
effects (cm).
I went through the code and checked the values put in, and found they
varied a lot by detector and material. The ranges:
TMAXFD: 0.3-5 degrees
STEMAX: 0.006-1 cm
DEEMAX: 0.05-0.5
EPSIL: 0.005-0.1 cm
STMIN: 0.003-0.1 cm
Some variation certainly makes sense, since the characteristics of the
components vary a lot. GEANT has some built in defaults for these
values:
TMAXFD: 20 degrees
STEMAX: 10,000,000 m
DEEMAX: 0.25 if the material is not `sensitive' and X0 (the radiation
length) is less than 2 cm.
0.25-0.2/sqrt(X0) otherwise.
EPSIL: No default.
STMIN: 2R/sqrt(X0) if the material is not `sensitive' (R is the range
of an electron of energy CUTELE+200kev).
5R/sqrt(X0) if the material is `sensitive'.
Other parameters which affect the code speed are those which determine
at what energy GEANT stops tracking the particles: CUTELE, CUTGAM,
CUTHAD, CUTMUON, and CUTNEU. In the code these are typically 1 MeV
for electrons and gammas and 1 or 10 MeV for the rest depending on
material.
The GSIM code is very slow, so most people run with the AUTO switch
set to 1. This has the effect of overriding the programmed in values
of STEMAX, DEEMAX, and STMIN with the defaults. This can be done in
individual cases by using a negative value for that particular
parameter in the code.
The next message will give some indications on how the code speed
depends on the settings of these parameters.
-David Rowntree