[Gate-users] RE: Oxygen15 source

[Marcin Balcerzyk]

Hi Ashley.

 

That happens when you have positron escaping your phantom without any
interaction and when it happens too often. The clean solution is to put a
medium around your phantom and attachSD to it. At first I would attachSD to
air (default) of world object. If your world dimensions are too small to
stop positrons, add more dimension to it. Then there are dirty solutions:
(i) run a simulation on a slower computer - I had run the same simulation on
two different machines with same gate and geant4, lower speed healed the
problem, (ii) add a shell of water or plastic around your phantom if you
may, (iii) add remote lead  box around your system and attachSD to it.

 

To check the problem, try to see the histogram
Positron_anihilation_distance_mm in the top structure of root file (above
singles and coincidences) - run: "root filename.root" and then "TBrowser t"
from the prompt. It will tell you how far your positrons passed from
creation. But for that you need a clean closed root file, so first overdo
the positron shielding.

 

I hope that helps 

 

Marcin Balcerzyk

Univesrsitad Complutense de Madrid

 

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