Speaker
Description
Collimated line-of-sight diagnostics such as the Radial Gamma Ray Spectrometer (RGRS) could be used for fusion power measurement at ITER. Compared with other approaches, such as fission chambers or activation foils, they offer the advantage of observing only the plasma section along a collimated line of sight, simplifying the calibration procedure by removing the need for complex neutronic calculations, such as the calculation of the adjoint flux for every point of the plasma. However, calculating neutron and gamma fluxes in regions located tens of meters from the plasma presents the unique computational challenge of low particle statistics.
This work presents MCNP-based Monte Carlo simulations aimed at characterizing the gamma background expected at the RGRS LaBr₃ detectors during full-power ITER operation. The ITER C-model was updated to reflect the latest RGRS design changes, including new collimator diameters, reoriented detectors aligned with the lines of sight, and an updated representation of Radial Neutron Camera. A two-step simulation method was developed to estimate the contribution of neutron-induced prompt gamma radiation originating from the central column, that was identified as the primary source of gamma background along the line of sight, contributing to a gamma current roughly one order of magnitude higher than that from all other sources combined.
The expected detector count rate under full-power conditions was estimated at approximately 1.4x107 counts per second. Model alignment was verified by comparing transport factors between MCNP and the Linalytic code, yielding agreement within 2.5%. These results provide an improved basis for the performance evaluation of RGRS and inform future background studies, including contributions from nearby lines of sight, such as that of the High-Resolution Neutron Spectrometer (HRNS).
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