Impact of Gas Pressure on Fission Chamber Sensitivity in Campbelling Mode

The study presented in this paper aims at demonstrating the feasibility of operating miniature fission chambers in Campbelling mode for in core neutron Ωux monitoring in Zero Power Reactors (ZPR). Experiments were conducted in the MINERVE facility (CEA Cadarache, France) so as to optimize the neutron sensitivity of standard miniature fission chambers. In Campbelling mode, the detector sensitivity is highly dependent on the filling gas mixture and pressure which determine the Mean Fission Product Charge (MFPC). Thus, an investigation has been carried out to determine optimum gas parameters. Measurements were obtained in both pulse mode and Campbelling mode. Three gas mixtures were tested (pure Ar, and ) with pressure ranging from 1 to 9 bar. Campbelling mode signal was acquired with the Fast Neutron Detector System (FNDS) recently developed by CEA and in the frame of the Joint Instrumentation Laboratory. Interesting results were obtained which improve our knowledge of the detector operation. Firstly, it was found that the measurements obtained in both modes are very consistent. The MFPC as a function of the gas pressure was found to be non monotonic. Instead, it features a maximum between 3 and 4 bar. Secondly, experimental data show that the measuring range in Campbelling mode with this detector starts from fission rates as low as a few thousand counts per second. This allows for monitoring the neutron flux on two decades of reactor power (between 1 Wand 100 W). Moreover, the so called overlapping range, in which both pulse and Campbelling modes are usable, is about one decade with spectroscopy modules and more than two decades with fast counting electronic modules. This feature is of high interest to test and/or calibrate Campbelling mode electronic systems using the MINERVE facility.