Hello @liq, welcome to the FLUKA forum!
To answer your questions:
- In your USRYIELD card, when you select “EMERGING” you will score the distribution of neutrons when they are created, i.e., when the inelastic hadronic interaction occurs that produces it. On the other hand, when you select “ENTERING”, you score the distribution of particles which trigger the inelastic interactions, i.e., the particles that cause it to happen. If you choose neither but just leave it blank you will calculate the neutron yield on the surface boundary going from your TARGET to VOID regions. If you like you can further consult on this topic in this post on particle yields or in this post on a spallation neutron source.
- Considering you choose indeed the last case: in FLUKA results you obtain are always expressed per primary particle. So in your case you obtain a result in units of neutrons/sr/GeV/ion (so GeV, not MeV!).
Let us know if you run into any other issues,
Cheers,
Andreas