I want to know how I can create a volumetric source, in order to simulate a real life source, which has a particular volume. The geometry will be cylindrical, and I want to make it using G4GPS, that emits photons in a 4pi distribution.

You can set in macro file as following:
/gps/pos/type Volume
/gps/pos/shape Cylinder
The default setting of angle type is isotropic.

For more details, please refer to the following:

Chia-Wei Huang

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Ok, but what else do I need to specify to basically define its dimensions. Only half_z and inner & outer radius?

I was thinking, the thickness of my cylindrical source is very small with respect to detector (.06 mm), should I make it ‘surface’ source?
I also saw, in G4 Manual, that there are ‘plane’ sources too and which one to implement in my problem? What is the actual difference between these two? Notice that my source emits 4pi.