Laser-Based Photomultiplier Gain Monitoring for the ITER Radial Neutron Camera scintillator detectors

Le résumé fourni par la source

Abstract The ITER Radial Neutron Camera (RNC) is a diagnostic system designed to provide time resolved measurements of the neutron source profile through the application of reconstruction techniques to the line-integrated neutron fluxes. It is composed of two subsystems, the In-Port RNC and Ex-Port RNC located, respectively, inside and outside the Plug of Equatorial Port #01. The Ex-Port RNC features 16 Lines of Sight (LoS) along which the neutron flux is measured by a set of 3 detectors, two of which are scintillators (EJ-276G plastic and 4 He gas) coupled to photomultiplier tubes (PMTs). PMTs suffer gain variations (due to change of the incoming neutron flux, temperature drifts and PMT aging) that need to be monitored and corrected via software reconstruction. This is done using a light source of well-known characteristics (pulse amplitude, shape, duration), typically LED signals. However, the use of LEDs in the ITER environment is problematic: they do not tolerate high radiation levels and must be therefore located hundreds of meters away from the PMTs and their signal routed through optical fibers. The net result is that, given the expected radiation-induced reduction of the optical fiber transmission throughout the ITER lifetime, the LED power is insufficient. This work presents the design of a novel system of PMT gain monitoring that overcomes this problem by means of a 520 nm laser diode as replacement of the traditional LED-based system. The laser diode can generate high-power, customizable, tuneable pulses with stable intensities suitable for PMT monitoring in the ITER environment. Moreover, it is shown that a single laser diode is sufficient for monitoring the whole set of 32 RNC PMTs thus simplifying the system architecture and improving maintainability.

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