Modelling of fission chambers in current mode—Analytical approach

A comprehensive theoretical model is proposed to explain the functioning of fission chambers operated in current mode, even in very high neutron fluxes. The calibration curves are calculated as a function of basic physical parameters as fission rate, gas pressure and geometry of the chambers. The output current at saturation is precisely calculated, as well as the maximum voltage to be applied in order to avoid avalanche phenomena. The electric field distortion due to the space charge phenomena is also estimated. Within this model, the characteristic responses of fission chambers are correctly reproduced, in agreement with the experience feedback obtained at the ILL/Grenoble High-Flux Reactor.