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Question Unable to generate arbitrary angular distributions using GPS 

Forum: Particles
Date: 02 Aug, 2018
From: Philipp Roser <Philipp Roser>

Hello fellow users and developers,

I am currently working on combining Geant4 with a simulation tool that generates trajectories of a moving particle source.
At each sample point of the trajectory, a Geant4 macro file will be written for later evaluation using Geant4. 
The file contains the position of the general particle source and the angular distribution of emitted particles using min/max phi/theta.

While getting familiar with how phi and theta are evaluated, I found the GPS behaving quite strange, when pointing into positive z-direction.

For example

/gps/pos/centre 0 -80 0 cm
/gps/ang/mintheta 260 deg
/gps/ang/maxtheta 280 deg
/gps/ang/minphi 260 deg
/gps/ang/maxphi 280 deg

produces a completely fine divergent pyramid-like beam towards positive y-direction.

Trying to do the same for positive z-direction resulted in the following macro code:

/gps/pos/centre 0 0 -80 cm
/gps/ang/mintheta -40 deg
/gps/ang/maxtheta 40 deg
/gps/ang/minphi 260 deg
/gps/ang/maxphi 280 deg

However, only the half of the previously mentioned pyramid is covered by particles. 
The remaining other half is truncated and no primary particles are emitted. 
Inverting the angular distribution to point towards negative z-direction works. 

I suspect, that negative values are not permitted. 
If that is the case, is there any possibility to still achieve my goal? 
Of course, I could transform my scene such that the GPS may remain static, but there are a lot of objects that are both mobile and immobile.

Any thoughts (and solutions) are highly appreciated.
Thanks in advance!

Best regards

PS: I could also implement my own ParticleGenerationAction and just do the angular sampling on my own, but I'd rather reuse what Geant4 offers already.

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