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Forum: Event and Track Management
Re: Question Need some help with G4StepLimitter, G4UserLimits, and other things (Miguel Couceiro)
Date: 18 Mar, 2008
From: Vladimir IVANTCHENKO <>

On Mon, 17 Mar 2008, Miguel Couceiro wrote:

> *** Discussion title: Event and Track Management
> Email replies to must include:
>   In-Reply-To: <"/eventtrackmanage/771">
>   Subject: ...change this to be about your reply.
> 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


This question is shown in wrong forum (should be emprocesses).

First of all you need to compare simulation results to some data to 
similar gas mixture. It seems that there are two modles which may give 
you a reasonable agreement for gaseous detectors: standard and PAI.

Please, consult $G4INSTALL/examples/extended/electromagnetic/TestEm7, Em8 
for details.


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