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Signal-to-Background Ratio Calculations for ITER Upper Vertical Neutron Camera
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DOI：10.1080/15361055.2026.2642537.png)
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Abstract
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The vertical neutron camera (VNC) is an ITER diagnostic aimed at reconstruction of the time- and space-resolved neutron and alpha source profile in the plasma poloidal cross section as well as the ion temperature profile. It is composed of 11 neutron collimators: 6 collimators in the upper vertical neutron camera (UVNC) and 5 collimators in lower VNC, each having one detector unit, consisting of two fission chambers and two diamond detectors. High flux of background neutrons is a serious problem for the UVNC. During design optimization, the collimators were rearranged and broadened. In this work we analyze optimized UVNC model shielding efficiency using the signal-to-background (S/B) ratio parameter. We describe the methodology of the S/B ratio calculations for the VNC, which splits the background neutrons into two terms. The first one is indirect, i.e., neutrons that entered the detectors not through the collimators (either by flying through the VNC structure uncollided or by scattering on the surrounding materials). The second term is backscattered neutrons, i.e., neutrons that scattered on the elements of the blanket or divertor opposite to the collimator that enter the collimator and reach the detector. We show that broadening of the collimators in the toroidal direction led to a better S/B ratio but drastically increased the backscattered neutron term fraction. We prove that the contribution of neutrons, which undergo scattering in the collimator channel, to the direct neutron term cannot be ignored. Neutron transport calculations are done in C-MODEL for deuterium-tritium neutrons, for deuterium-deuterium (DD) neutrons born in plasma reactions, and for DD neutrons born in reactions between plasma ions and beam ions, taking into account anisotropy (both in energy and intensity) of the latter type of neutron source.
Keywords:
ITER
vertical neutron camera
neutron plasma diagnostics
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