Neutrons are electrically neutral subatomic particles. Atomic nuclei consist of protons and neutrons.
Because of their properties (mass, magnetic moment, ...), neutrons can be applied for microscopy. In 2152, the starship Enterprise had a neutron microscope aboard. (ENT: "Stigma")
Starships tend to emit quantities of neutron radiation as a property of their warp cores, detectable by any other starships close enough to scan for neutrons. In 2293, Captain Spock noted a large neutron radiation that resulted from a Klingon Bird-of-Prey commanded by General Chang moving into position at extreme close-quarters to the hull of Spock's ship, the USS Enterprise-A. (Star Trek VI: The Undiscovered Country)