  We find a new utility of neutrons, usually treated as an experimental
nuisance causing unwanted background, in probing new physics signals. They can
either be radiated from neutrons (neutron bremsstrahlung) or appear through
secondary particles from neutron-on-target interactions, dubbed "neutron beam
dump". As a concrete example, we take the FASER/FASER2 experiment as a
"factory" of high-energy neutrons that interact with the iron dump. We find
that neutron-initiated bremsstrahlung contributions are comparable to
proton-initiated ones, in terms of the resulting flux and the range of
couplings that can be probed. The neutron bremsstrahlung can be used to probe
dark gauge bosons with non-zero neutron coupling. In particular, we investigate
protophobic gauge bosons and find that FASER/FASER2 can probe new parameter
space. We also illustrate the possibility of neutron-induced secondary
particles by considering axion-like particles with electron couplings. We
conclude that the physics potential of FASER/FASER2 in terms of new physics
searches can be greatly extended and improved with the inclusion of neutron
interactions.
