I am using an iron, a carbon and a lead foil for my experiment, for which I
have to simulate background (about 1 mm of thickness for each) using the
detector CLAS at TJNAF. I have noticed using the debugger implemented in our
local package (gsim), that geant only takes 1 step inside each foil (even when
I made the lead 10 times thicker...). I have been wondering (not being a geant
specialist at all) if the number of steps that geant takes inside a volume had
any connection to the probability of interaction inside that volume... To test
the idea, I defined a new lead, new iron and new carbon taking a lot smaller
values for TMAXFD, STEMAX, DEEMAX, EPSIL, and STMIN in my GSTMED call for
these new materials, so as to force geant to take more steps inside them. The
autcome is, geant does still only takes 1 step inside each foil with AUTO 1 in
my ffread card, but DOES take at least 30 steps inside the foils with AUTO 0.
Then, I send 100000 bremstrahlung gammas on my targets and count the number of
hits in the DC, in the case of the old materials (1 step inside each foil,
used AUTO 1), and in the case of the new materials (at least 30 steps inside
each foil, with AUTO 0): The VERY PUZZLING outcome of that is: in the first
case I get about 5 times more hits in the DC than in the second case!!!
  If anyone had any clue on what is going on here, I would appreciate hearing
of it. Am I doing something wrong??? If the probability of interaction in a
volume is REALLY related to the number of steps taken inside it, what is then
an optimum number of steps inside a given volume???
  Please, ask me if you have any questions on the infornmation I have just
given you. You can find information about gsim on www.jlab.org/Hall-B (click
on GSIM: Monte Carlo Simulation link)...
Clarisse Tur