neutron diagnostics are the instruments that measure a fusion plasma's performance directly, by counting and characterizing the neutrons its reactions produce.
Their special status among diagnostics comes from what they measure. Every other instrument reads the conditions for 📝fusion — temperature, density, confinement — while neutron diagnostics read the fusion itself: each 📝D-T reaction releases one 📝neutron, 14 MeV, so the total neutron rate is a direct, absolute measure of fusion power, and with it the energy gain factor 📝Q (fusion energy gain factor). Because neutrons carry no charge, they escape the magnetic field untouched, streaming out of the 📝plasma to detectors stationed around the machine.
The family divides by what it extracts from that stream. Fission chambers and calibrated activation foils establish the absolute neutron yield — the fusion power number itself. Neutron cameras view the plasma through collimated channels to map where the reactions occur, profiling the burning core. Neutron spectrometers resolve the energy distribution, whose Doppler broadening reveals the temperature of the reacting ions. Absolute calibration is the discipline's central challenge, since claiming a fusion power figure is only as credible as the calibration behind the count.