Absolute measurement of the deuterium-tritium reaction gamma-ray emission in magnetic confinement fusion plasmas

The Joint European Torus performed its second deuterium-tritium (DT) campaign at the end of 2021. This unique opportunity was exploited to carry out the first absolute measurement in a magnetic confinement plasma of the total gamma-ray emission from the DT fusion reaction, namely the less probable (branching ratio = 2.4 10(-5)) electromagnetic counterpart of the main neutronic decay channel. A single line of sight gamma-ray spectrometer was employed for this purpose. Numerous challenges had to be addressed in order to pursue the goal: (1) characterise the detector beamline and its detection efficiency in absolute terms, (2) dealing with an extended and non-uniform source, (3) suppress the intense neutron background, (4) handle the high event rate at the detector and (5) discriminate between signal and background in the acquired energy spectrum. This paper describes the procedure adopted for measuring the total DT gamma-ray yield of 96 DT plasma discharges with event rates below hundred kHz. The results were validated through a comparison with the neutron yields provided by the Joint European Torus neutron monitors, revealing an outstanding 0.983 linear correlation. This work proves the feasibility to employ the gamma-ray emission of the DT fusion reaction as a secondary indicator for measuring the fusion power in magnetically confined DT plasmas.