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Forum: Fast Simulation, Transportation & Others
Date: 27 Aug, 2015
From: Francois Mauger <Francois Mauger>

Hi

I use G4.9.6.

I'm interesting in tracking low energy Li6(2+) ions in vacuum with 
electric RF field.
(this was done previously through SIMION but out group want to integrate 
the full MC response
within the only G4 tool, using a detailed geometry).
I have carefully read the G4 source code and what I understand is
that I can pass the effective charge (2+) from my primary generator 
typically with:

<pre>
 
_particle_gun_->GSetParticleDefinition(4ParticleTable::GetParticleTable()->GetIon(3, 
6, 0));
  _particle_gun_->SetParticleCharge(2 * CLHEP::eplus);
  _particle_gun_->SetMomentum({the proper kinematics from some Li6(1+) 
beta decay generator});
</pre>

thanks to this, the underlying G4PrimaryTransformer object should
fill some atomic shell (only the K shell, whatever the number of atomic 
electrons is!)
of the G4ElectronOccupancy instance in the G4DynamicParticle
The G4ElectronOccupancy::AddElectron also takes care of updating the 
ion's dynamic charge
and mass (well, neglecting electron binding energy, but I don't care...).

At this stage, the primary event is ready to be tracked, taking into 
account the realistic
charge state of the Li6 recoil ion.

Now I need to transport the ion in the geometry (mainly vacuum but with 
E field everywhere)
together with the beta particle emitted simultaneously from my home-made 
decay.
The G4Transportation class seems to be the responsible of this task.

In the G4Transportation::AlongStepGetPhysicalInteractionLength method, any
dynamic particle is fetched form the processed G4Track.
The dynamic charge is extracted, as well as the magnetic moment
and *rest* mass ( ParticleDef->GetPDGMass()), NOT the dynamic mass.

Following the same scheme, the fFieldPropagator is given (through 
SetChargeMomentumMass):
  - dynamic charge
  - momentum mag
  - rest mass!!!

Q1: Why not using the dynamic mass here ? should this be fixed ?

of course I guess the only possible annoying case is for ions with 
special charge state
and the correction on the realistic mass is generally tiny (~ n(e-) * 
511 keV vs ~ A(ion) * GeV)...

G4FieldTrack object is also created with rest mass and charge is even 
NOT passed!!!
Note that in the G4CoupledTransportation class, a dedicated method 
(G4FieldTrack::SetChargeAndMoments)
is used to explicitely pass the dynamic charge... strange !

however I assume the fFieldPropagator has this information so probably 
there is no problem
because the G4PropagatorInField::ComputeStep should push the Q/P/M 
triplet in the ChordFinder
which in turn passes it to the fIntgrDriver and finally to the equation 
of motion.


Q2: I'd like to know at least if the effective charge of my Li6(2+) ions 
will be properly taken into account
within the equation of motion in the electric field ? (considering taht 
the rest vs dynamic mass bias should not be a big issue)

Q3: I'm not really interested in a detailed description of the Li6(2+) 
ion material other than in
vacuum where the E field drives it. However, for my information, what 
charge is effectively used to
compute energy losses (and other EM mechanisms) if the ion enters some 
solid volume ? It there some sort
of effective ion/medium atomic recombination effect taht is taken into 
account ?

Thanks a lot for comments, hints and help.

cheers

frc
-- 
Fran├žois Mauger
Unicaen/CNRS/IN2P3

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1 Idea: Re: G4Transportation and dynamic charge and mass   (Vladimir Ivanchenko - 04 Jan, 2016)
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