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Question Need some help with G4StepLimitter, G4UserLimits, and other things 

Keywords: SteppLimiter; UserLimits; SetEnergyRange; step size
Forum: Event and Track Management
Date: 17 Mar, 2008
From: Miguel Couceiro <Miguel Couceiro>

Hi all,

I am performing a very simple simulation for computing the number
of primary ionization clusters produced in a gas when a pion passes
through it. I am using LowEnergyProcesses for Photons and Electrons,
with energy cuts of 100 eV, and range cuts of 1 nm.

G4ProductionCutsTable::GetProductionCutsTable()->SetEnergyRange(100.0*eV, 100.0*GeV);

To all particles that I am interested in setting a step limitation,
I had the line

pManager->AddDiscreteProcess(new G4StepLimiter());

at the end of the process manager. I thought that in this way all
the other processes would be treated first, and then the step
limitation would be applied.

In my detector construction I have inserted the line

m_pDetector_log->SetUserLimits(new G4UserLimits(fract*m_fDetectorThickness)); 

My gas volume has a 1 mm thick, and I am using a fixed seed for my
simulations. When I comment the above line, I get, in the first event,
two steps of 0.5 mm for which of them I get “msc”. When I activate
the UserLimits with fract < 0.5, I get SteppLimitter as the process,
instead of “msc”. Besides, if I use fract = 0.1 and fract 0.01, with
the same initial seed, the total energy deposit in the gas, the point
from where the pion leaves the gas, as well as it's momentum direction
will be different.

Since the cuts and seed are all the same, I would expect to obtain
the same results when I reduce “fact”, but with more precise local
energy deposit. Is this correct, or the step size will also influence
the physics results?

Another question concerns the step size on the boundary surfaces
to take into account more realistic backscattering effects. How can
I implement this step limitation, or were can I find some more
information about it? (The user manual is not clear to me in this point.)

At last, some papers discuss the possibility to use LowEnergyProceses
down to 100 eV. Since I am interested in electron avalanches, and since
gas ionization energies are of the order of 10 to 20 eV, is it possible
to use GEANT4 to study the primary ionization cluster statistics,
as well as electron avalanche statistics? Some papers also discuss the
fact that, since LowEnergyProcesses rely on EEDL data, they should be
applicable to energies down to 10 eV? However, cross sections and
stopping powers from EPDL97 and EEDL for 10 eV have large errors,
mainly EDL97. Since the command 

G4ProductionCutsTable::GetProductionCutsTable()->SetEnergyRange(100.0*eV, 100.0*GeV);

sets the energy cuts for both photons and electrons, then the results
for photons shouldn't be very reliable. Is this correct, or one can
in fact set the low energy cut down to 10 eV, without significant loss
of accuracy?

Thanks in advance,
Miguel

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1 None: Re: Need some help with G4StepLimitter, G4UserLimits, and other things   (Vladimir IVANTCHENKO - 18 Mar, 2008)
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