Monte-Carlo simulation of prompt gamma-ray background in the ITER radial gamma-ray spectrometer

The ITER Radial Gamma-Ray Spectrometer (RGRS) is a high-performance diagnostic instrument designed to measure gamma radiation emitted by the plasma in the ITER tokamak fusion reactor. RGRS will have three dedicated radial lines of sight plus one shared with the Radial Neutron Camera, and uses LaBr 3 scintillator detectors to record gamma-ray spectra for the study of fast ions, the monitoring of runaway electrons, and the measurement of fusion power. A critical challenge in the operation of RGRS will be the characterization of the gamma-ray background reaching the detectors, which is required to improve diagnostic design, correctly interpret detector spectra and assess diagnostic performance. This work presents MCNP Monte Carlo simulations of the prompt gamma-ray background in RGRS, employing a two-step simulation approach to isolate and quantify its dominant contributions. Results indicate that neutron-induced gamma-rays from the central column will be the primary prompt background source, exceeding by nearly one order of magnitude all other prompt neutron-induced contributions along the line of sight, with direct implications for the detector count rate and the signal-to-background ratio during full-power ITER operation.