Neutron momentum distribution

The KOTO experiment at J-Parc has been simulating events from a long
(~20 m) beam line. The main interest is Klongs, but neutrons and
photons are also important.

Attached is a plot of the MC neutron momentum distribution from a 30-GeV
proton beam on a Ni and Pt target, after passing through a complicated
beam line and collimators.

My concern is the Fluka distribution for Ni. I have raised concerns
before, and been told that the code should be OK. I want to check again.

The extra yield for Ni comes from the PHYSICS/PEATHRES option. It looks
like a broad resonance at ~10 GeV on top of a more typical exponential
decrease. It goes away with the Pt target and also with the PEATHRES
option removed. How sure is it that the PEATHRES option is properly
handled, such as normalized (e.g., an A-dependence)?

BTW, for the integrated Klong yield, Fluka gives 20.7 vs. our
(preliminary) measured yield of 19.4 (local units). Very good! G3 gives
15.1 and G4 gives 6.8 (perhaps not the best physics list).

Thank you,
Joe Comfort