A neutron spectrometer measures the kinetic energy (KE) of received free neutrons. As such, it is a type of particle spectrometer. Some types are used in labs when probing molecular and subatomic structure with neutron scattering and some types are used in planetary probes to detect water at or below the body's surface, through the changes in KE due to neutron scattering by hydrogen. (Free neutrons only last a few minutes before decaying so this requires a source to be nearby with a known KE distribution: cosmic ray interaction with matter produces them.) Neutron spectrometers are sometimes combined with gamma-ray spectrometers, I believe because they have related scientific goals: detection of various specific elements at or below the surface.