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A neutron spectrometer, a specific type of particle spectrometer, measures the kinetic energy (KE) of received free neutrons. They are used in labs when probing molecular and subatomic structure with neutron scattering and are also common in planetary probes, for detecting water at or below the body's surface. The detection is through the changes in KE due to neutron scattering by hydrogen, which is very effective at slowing neutrons. Free neutrons decay in a few minutes, so the neutron source with a predictable KE distribution must be nearby, and cosmic-ray interaction with a planet's atmosphere and/or surface matter is such a source. The free neutrons do travel sufficiently that a neutron spectrometer in low orbit (or nearer) can sense the effects of water, which is very useful both for planet exploration and Earth monitoring. In planetary science missions, 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.