MCNP-Based Characterization of the High-Energy Background in Gamma Ray Spectrometers for Fusion Power Measurements in DT Plasmas

A new method for measuring fusion power in DT plasmas using high-energy gamma spectroscopy was recently demonstrated at JET. Besides a 105 times more intense background at lower energies, a gamma background in the signal energy region was observed but not identified, contributing to higher uncertainty on the measurements. Due to the rarity of the event considered and the complex geometry of the tokamak, a new method to simulate the gamma background at high energy using MCNP was required. In this work, a combination of variance reduction techniques was developed by analyzing the nature of the background and validated by comparing simulation results to experimental data. The library comparison showed that ENDF/B-VIII is the library that better describes the shape of the high-energy gamma background, although the simulated component is 2.5 times higher than experimental data. Results using Fendl 3.1d nuclear library presented reduced discrepancy in magnitude, albeit with a distinct spectral distribution.