Conceptual design of a Cherenkov based gamma-ray diagnostic for measurement of 17 MeV gamma rays from T(D, gamma)5He in magnetic confinement fusion plasmas

At present, the only method for assessing the fusion power throughput of a reactor relies on the absolute measurement of 14 MeV neutrons produced in the D-T nuclear reaction. [1] For ITER and DEMO, however, at least another independent measurement of the fusion power is required. The 5Henucleus produced in the D-T fusion reaction has two de-excitation channels. The most likely is its disintegration in a particle and a neutron, D+T->5He->?+n, by means of the nuclear force. There is however also an electromagnetic channel, with a branching ratio ~10-5, which leads to the emission of a 17 MeV gamma-ray, i.e. D+T->5He*-> 5He+?. [2] The detection of this gamma-ray emission could serve as an independent method to determine the fusion power. In order to enable 17 MeV gamma-ray measurements, there is need for a detector with some coarse energy discrimination and, most importantly, capable to work in a neutron rich environment. Conventional inorganic scintillators, such as LaBr3(Ce), have comparable efficiencies to neutrons and gamma rays and they cannot be used for 17 MeV gamma-ray measurements without significant neutron shielding. In order to overcome this limitation, we here propose the conceptual design of a gamma ray counter with a variable energy threshold based on the Cherenkov effect and designed to operate in intense neutron fields. The detector geometry has been optimized using Geant4 so to achieve a gamma-ray to neutron efficiency ratio better than 105. The design is based on a gas Cherenkov detector and uses a CsI coated scintillating GEM (Gas Electron Multiplier) as photon pre-amplifier, together with a wavelength shifter to minimize the sensitivity to neutrons. Photons produced in the GEM are collected by an optical window and a bundle of optical fibers, which guides them towards an array of silicon photomultipliers (SiPMs) located further away from the plasma, in a region at low nuclear radiation.