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Forum: Physics List
Re: Question Recombination (Maciej Sznajder)
Date: 11 Jan, 2011
From: Tom Roberts <Tom Roberts>

Hydrogen chemistry is very important in determining the rate that electrons are recombined into hydrogen. There are many important reactions:

    e- + p => H
    e- + H => H-
    e- + H2 => H2(-)
    e- + 2H2 => H3(-) + H
    ... etc. Plus others involving H- and H2(-), ...

The cross-sections are all highly dependent on the energy of the electron. And a small admixture (~0.01%) of a material such as SF6 can have a huge effect on the overall rate.

Given that in most materials the density of free protons is quite low, the other reactions are often more important. Note that Geant4 has no material that has any free protons at all; I don't know how one would create such a material (I believe it would require custom code).

Geant4 has no beam-beam processes. So having both p and e- in your beam will not generate any instances of the first reaction above. The overall design of Geant4 precludes such interactions, as each beam particle is tracked until it is killed, then the next beam particle is tracked, etc. So there is no opportunity for interactions between beam particles; due to low beam densities, rates are generally quite low, anyway.

My program, G4beamline, has the ability to model such interactions, and has computations for space charge and annihilation of e+ e- beams. This required the replacement of several Geant4 kernel routines such as RunManager, TrackingManager, SteppingManager, etc. The new routines track all of the members of a std::vector<G4Track> for a time step, and then permit collective interactions at each time step.

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