activation analysis is the measurement of a neutron field through the radioactivity it induces: a sample of known composition is deliberately exposed, retrieved and counted, and the isotopes it has become reveal the neutron fluence that made them.
The method turns a liability into an instrument. 📝neutron activation is ordinarily a problem — the process that leaves a reactor's structure radioactive after shutdown — but the same physics applied to a sample chosen on purpose is a measurement of unusual directness. A foil of known mass and element sits in the neutron field; some of its nuclei are transmuted into a radioisotope of known half-life; the foil is then retrieved and counted on a high-purity germanium detector, whose gamma spectrum identifies the isotope while its decay rate says how many nuclei were made. With the reaction cross-section known, that count returns the neutron fluence the foil saw — the time-integrated companion to 📝neutron flux.
What makes it indispensable is that the answer is absolute and carried in the sample itself. Electronic detectors count continuously but drift, saturate and must be calibrated against something; an activated foil holds its result in its own nuclei, independent of whatever electronics were running at the time. Fusion facilities therefore treat activation as the yardstick that the fast counters are cross-calibrated to. The choice of reaction also buys crude spectral discrimination: aluminium-27 to sodium-24, or niobium-93 to niobium-92m, need neutrons of several MeV and so respond only to the 📝neutron, 14 MeV of 📝D-T burning, while the inelastic reaction on indium-115 peaks just below 2.5 MeV and sees 📝D-D neutrons instead. 📝JET's in-vessel activation system pneumatically fires capsules of foils to irradiation ends at the edge of the 📝vacuum vessel and back to a counting station; it supplied the absolute yield in the machine's 2017 in-vessel 14 MeV neutron calibration.
The technique predates 📝fusion energy. Georg von Hevesy and Hilde Levi reported in Nature in 1936 that irradiating rare earth elements with slow neutrons and watching the induced activity decay identified which elements were present, founding neutron activation analysis as a method of chemical assay. 📝SPARC's design carries two independent foil activation systems for fluence measurement, with capsules called rabbits, roughly 25 mm across and 86 mm long, driven pneumatically some thirty metres to shielded germanium counting stations, drawing on a foil menu of silicon, indium, niobium, zirconium, aluminium, chromium, copper, iron and nickel. Nothing has yet been irradiated: SPARC has not run.
Two independent activation systems is redundancy on purpose. The fluence number underneath SPARC's fusion power claim should not depend on a single instrument — or on any instrument that had to be powered on and working at the moment the neutrons arrived.