  In an antiproton production (Pbar) target station of the Fermi National
Accelerator Laboratory (FNAL), the secondary particles produced by bombarding a
target with 120-GeV protons are shielded by a thick iron shield. In order to
obtain experimental data on high-energy neutron transport at more than
100-GeV-proton accelerator facilities, we indirectly measured more than 100-MeV
neutrons at the outside of the iron shield at an angle of 50{\deg} in the Pbar
target station. The measurement was performed by using the Au activation method
coupled with a low-background {\gamma}-ray counting system. As an indicator for
the neutron flux, we determined the production rates of 8 spallation nuclides
(196-Au, 188-Pt, 189-Ir, 185-Os, 175-Hf, 173-Lu, 171-Lu, and 169-Yb) in the Au
activation detector. The measured production rates were compared with the
theoretical production rates calculated using PHITS. We proved that the Au
activation method can serve as a powerful tool for indirect measurements of
more than 100-MeV neutrons that play a vital role in neutron transport. These
results will be important for clarifying the problems in theoretical
calculations of high-energy neutron transport.
