Irradiation tests of prototype self-powered gamma and neutron detectors

In the framework of the SCK•CEN-CEA Joint Instrumentation Laboratory, we are developing and optimizing a self-powered detector for selective in-core monitoring of the gamma field. Several prototypes with bismuth emitters were developed and tested in a pure gamma field (the PAGURE gamma irradiation facility at CEA) and in mixed neutron and gamma fields (in the OSIRIS reactor at CEA and in the BR2 reactor at SCK•CEN). Detailed MCNP modelling was performed to calculate the gamma and neutron sensitivities. Apart from a few failing prototypes, all detectors showed equilibrium signals proportional to the gamma field with a good long-term stability (under irradiation during several weeks). A tubular geometry design was finally selected as the most appropriate for in-core gamma detection, coupling a larger sensitivity with better response characteristics. In the same experiment in BR2 six prototype Self-Powered Neutron Detectors (SPNDs) with continuous sheaths (i.e. without any weld between the sensitive part and the cable) were extensively tested: two SPNDs with Co emitter, two with V emitter and two with Rh emitters, with varying geometries. All detector responses were verified to be proportional to the reactor power. The prompt and delayed response contributions were quantified. The signal contributions due to the impact of gamma rays were experimentally determined. The evolution of the signals was continuously followed during the full irradiation period. The signal-to-noise level was observed to be well below 1% in typical irradiation conditions. The absolute neutron and gamma responses for all SPNDs are consistent.