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Hello Daniel, > I need to use a sphere as a particle source with the gps class. > > and I want the particles to shoot just in the inside of the sphere GPS is indeed a good choice, it will make your life easy. For generating particles on the surface of a sphere, you can insert this in your macro: /gps/pos/type Surface /gps/pos/shape Sphere /gps/pos/radius 2.0 m /gps/pos/centre 0. 0. 0. cm > They also should be distributed even on the surface of the sphere. The vertices will be automatically uniformly distributed on the surface. For the angular distribution: During the run, for each point, randomly generated on the surface, GPS orients +z of the "generation frame" as normal to the surface, directed towards inside. Normally, then, for generating particles inside the sphere to reproduce an isotropic angular distribution, one has to specify a "cosine law" biasing, and let the theta go from 0 to 90 degrees: /gps/ang/type cos /gps/ang/mintheta 0 deg /gps/ang/maxtheta 90 deg I am not sure what you mean with > maximally tangential to the sphere. If you really want the particles to only start parallel to the surface, you can just modify the mintheta to 90 degrees: /gps/ang/mintheta 90 deg /gps/ang/maxtheta 90 deg If instead you mean only normal to the surface (i.e. directed to the centre of the generation sphere) you set maxtheta to 0 degrees. /gps/ang/mintheta 0 deg /gps/ang/maxtheta 0 deg I hope this helps Kind regards, Giovanni ESA / ESTEC Space Environments and Effects e-mail: Giovanni.Santin@esa.int