[Gate-users] Positron travel prior to annihilation

[Martin Schain]

Dear GATE users,

I am new to this community and have not really figured out an efficient
way to search the archives, so I apologize if this matter is already
discussed.

We would like to simulate PET data in which the effect of positron
travel prior to annihilation is absent. It seems from the user manual
that this effect is hardcoded into geant4 and is therefore not easy to
alter, but as it would be very convenient for our work I wonder if
anyone has any experience or knowledge in how to accomplish this. 

If I recall the physics properly, the range which the positron travels
before it is annihilated depends on the energy it receives in the decay.
If it is possible to design a ligand which decays into positrons with
511 keV, causing immediate annihilation of the positrons and therefore
forcing their travelled distance to 0, then that could be an artificial
way to circumvent the problem. Is it possible to design such a ligand?
However it is of course preferred if the effect could simply be turned off.
 
I am grateful for all hints, suggestion or ideas in this matter. 
Best Regards
Martin Schain