neutron spectrometer is an instrument that measures the energy distribution of the neutrons a fusion plasma emits rather than only their number, reading ion temperature, fuel mix and fast-ion behavior out of the spectrum's shape.
The difference from the rest of the 📝neutron diagnostics family is the difference between counting and weighing. A flux monitor or fission chamber reports how many neutrons crossed it, which gives 📝fusion power and 📝Q (fusion energy gain factor); a spectrometer resolves what energy each one carried, which reports on the 📝plasma that made it. The underlying physics is Doppler shift again, at nuclear energies. A 📝D-T reaction between two ions at rest would emit its 📝neutron, 14 MeV at a single sharp energy; real reacting ions are in motion, so the emitted energies spread into a peak whose width grows with the square root of the 📝ion temperature — roughly 177 📝keV times the square root of the temperature in keV for a thermal D-T plasma, and about 83 keV times that root for the 2.45 MeV neutrons of the 📝D-D reaction. That relation assumes a Maxwellian distribution, and when external heating pushes the ions away from thermal equilibrium it breaks down — a failure that is itself informative.
Two further quantities live in the spectrum. The relative strength of the 14 MeV and 2.45 MeV peaks tracks the 📝tritium-to-📝deuterium ratio of the fuel, a number almost nothing else can reach inside a 📝burning plasma, though recovering it demands the reaction rates, the instrument's calibration, and a scattered-neutron background near 2.5 MeV low enough not to swamp the smaller peak. Above the thermal peak, energetic tails betray ions accelerated far past the bulk by 📝ion cyclotron resonance heating or neutral beams. 📝JET fielded the field's two reference instruments: TOFOR, a time-of-flight spectrometer on the D-D peak whose data showed deuterons carrying at least a megaelectronvolt under second-harmonic ICRF acceleration, and the 📝magnetic proton recoil spectrometer that measured the D-T spectra from 1997 onward.
📝SPARC's planned set includes both a spectrometric radial neutron camera and a high-resolution magnetic proton recoil core spectrometer, described in a 2024 overview of the machine's neutron diagnostic systems. Those are designs, backed by modeling and detector bench work; SPARC has produced no neutrons.