  Charged-current anti-neutrino interactions on hydrocarbon scintillator in the
MINERvA detector are used to study activity from their final-state neutrons. To
ensure that most of the neutrons are from the primary interaction, rather than
hadronic reinteractions in the detector, the sample is limited to momentum
transfers below 0.8 GeV/c. From 16,129 interactions, 15,246 neutral particle
candidates are observed. The reference simulation predicts 64\% of these
candidates are due to neutrons from the anti-neutrino interaction directly, but
also overpredicts the number of candidates by 15\% overall, which is beyond the
standard uncertainty estimates for models of neutrino interactions and neutron
propagation in the detector. Using the measured distributions for energy
deposition, time of flight, position, and speed, we explore the sensitivity to
the details those two aspects of the models. We also use multiplicity
distributions to evaluate the presence of a two-nucleon knockout process. These
results provide critical new information toward a complete description of the
hadronic final state of neutrino interactions, which is vital to neutrino
oscillation experiments.
