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Forum: Event and Track Management
Re: Question Particle artificially moved from one volume to another. (alexandre Creusot)
Date: 03 Dec, 2008
From: Gumplinger Peter <Gumplinger Peter>

Hi Alexandre,

> Via a created process, I move the postStepPoint back to the glass (position,
> momentum and time are changed).

I am surprised you can do this inside a process. I thought the step points cannot be changed directly, only through a 'ParticleChange'.

> I would like to know if this change of volume has any influence on the photon
> characteristics.

I am not surprised that you get this null-step. A similar null-step is done by the G4 navigator when a photon reflects on a surface in G4OpBoundaryProcess. Here, I am only changing the direction/polarization (albeit, as I said, through a ParticleChange object). Nothing else is changed about the photon.

What do you mean by 'photon characteristics'? You've already modified the position, time and direction. Certainly, nothing will change its energy.

> More generally, I would like to know if you can move freely a particle from
> one volume to another one.

Developers more familiar with the G4 navigator know better to answer this question. However, I perceive it as very dangerous. If you want to do something like this in your process, you are advised to kill the original photon in your process, and re(place) it (with itself) as a secondary generated by your process where you like. If however the secondary is no longer in the same volume; e.g. TheTouchableHandle would be different, you need to code:

 aSecondaryTrack->SetTouchableHandle((G4VTouchable*)0);

Cheers, Peter

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