This will emit particles inward from the surface of a sphere with size "WorldRadius". Note -- WorldRadius need not be the same size as world_solid, and world_solid need not necessarily be a sphere. However, a sphere of radius WorldRadius must be completely enclosed within the world volume.
Added omnidirectional background source. To enable, use a BeamSize of -3:
quantity name="BeamSize" type="coordinate" value="-3" unit="mm"
Then, to specify the size, use:
quantity name="WorldRadius" type="coordinate" value="15" unit="cm"
This will emit particles inward from the surface of a sphere with size "WorldRadius". Note -- WorldRadius need not be the same size as world_solid, and world_solid need not necessarily be a sphere. However, a sphere of radius WorldRadius must be completely enclosed within the world volume.