It would be nice if there were a way to produce a uniform external source using the GPS. This would be useful, for example, for simulating cosmic muon spectra or external gamma rays. The idea is as follows: the user would provide the radius R of a bounding sphere for his/her volume. When it is time to generate an event, first a momentum direction p^hat is simulated (according to whatever angular distribution the user prefers). Then the position of the event is chosen randomly on a disk of radius R whose normal points in the direction p^hat and whose center is at the vertex -R*p^hat. This gives a uniform flux of events impingent on the detector in the direction p^hat. Jason
All these are already possible in GPS,if I understood your requirments correctly. Please refer to the helps available at the command line or its user manual(http://reat.space.qinetiq.com/gps/new_gps_sum_files/gps_sum.htm). Further examples GPS usage can be found in the "exgps" extended example.
