Dear experts, for some studies I need to extract with FLUKA the angle in which certain particles traverse a volume, which would be the recommended scoring to use for this purpose?
I’d suggest the USRBDX and USRYIELD cards, see their description in the manual and note that they refer to different angles: both score particles on a boundary between two regions, but the first considers the particle angle wrt the normal to the surface at the crossing point, while for the second the angle is defined wrt the beam axis or any other axis the user specifies in another dedicated USRYIELD card with SDUM=BEAMDEF.
These would give you the particle angular distribution on the aforementioned region boundary. While crossing a region volume, the particle angle is in principle continuously modified by interactions and/or magnetic field, so it’s redefined at the end of each tracking step.
If for some good reasons you need an event by event scoring, then you shall personally modify the mgdraw.f user routine in order to write the desired information (see user routines and mgdraw.f). However, this latter solution is more advanced and worth implementing at your expenses only if the USRBDX/USRYIELD angular distributions are really not enough to your purposes.