Experimental study of columnar recombination in fission chambers

The construction of the Jules Horowitz Reactor (JHR), the development of third generation reactors, and the development of Small Modular Reactors (SMR) have led to a need for more reliable and robust power monitoring. This, in turn, requires more substantial theoretical and experimental study of the physical phenomena at work in the detectors. This work focuses on fission chambers, and more specifically on electron-ion recombination, a disturbing effect that can bias calibration coefficients connecting the measured quantity to the fission rate in detectors. Here, columnar recombination that occurs in ionization traces formed by the passage of heavy ionizing particles is studied in detail. The dynamics of ionization column formation have been simulated to estimate the parameters of column radius and charge density, which have been compared to the assumptions of existing models. Furthermore, the applicability of recombination coefficients available in the literature to the weakly ionized non-equilibrium plasma conditions of ionization columns is discussed. Two experimental campaigns were designed to measure recombination in pure argon. The COREX modular device was designed to investigate columnar recombination and its angular dependence using the Frisch grid method and an alpha deposit. A simulation tool was developed to simulate the operation of this detector and select its parameters. A measurement campaign on the Belgian BR1 reactor was used to study the application of the model to the case of fission chambers, using average charge density parameters. As recombination coefficients are specific to each gas mixture, measurements with other mixtures used in fission chambers will enable this phenomenon to be added to the models describing the operation of these detectors. (author)