Testing of compton-emission type detectors of neutrons and of triaxial fission chambers in a pulsed reactor

Abstract

Compton-emission type neutron detectors with an emitter containing hafnium or gadolinium and small triaxial fission chambers are promising detectors for monitoring the energy liberation in nuclear reactors. By contrast to the B-emission type neutron detectors which are widely employed in power reactors and have an emitter of rhodium, silver, or vanadium [1-3], the Compton-emission type neutron detectors with an emitter containing hafnium or gadolinium are almost without the inertia 14, 5] which is characteristic of fission detectors. Therefore the Compton-emission type neutron detectors and the fission chambers are preferably employed in automatic monitoring and controlling systems and in safety systems of reactors where the detectors must meet increased requirements in regard to fast response. A technology which is used in analogy in the manufacture of coaxial and triaxial cables with magnesia insulation is used in the manufacture of Compton emission type neutron detectors and fission chambers. We tested three Compton-emiss~on type neutron detectors in cable form with a hafniumcontaining emitter, three Compton-emission type neutron detectors of the same form with a gadolinium-containing emitter, and four triaxial small-scale KtV fission chambers [6]. In regard to their design, the Compton-emission type neutron detectors resembled the previously tested G-emission type neutron detectors with a silver emitter [7]. Measurements which were made during the manufacture of the Compton-emission type neutron detectors and during their operation in a research reactor have shown that the mean-square spread of the emitter diameters of these detectors and of their initial sensitivity amount to about 1.5%. The sensie tive element of the Compton-emission type neutron detector with the hafnium-containing emitter has an outer diameter of 3.0 m~n and a length of 0.83 m; the detector with the gadolinium-containing emitter has an outer diameter of 3.4 mm and a length of 0.85 m. Thedesign of the fission chambers hardly differs from that described in [8]. The characteristics of the small axial fission chambers with an outer diameter of 6.0 mm and one to two 50-mm-long sensitive elements were studied. In the case of two sensitive elements, the collecting electrode of one of them was without a fissile cover, i.e., it was used to monitor the dose rate of the y radiation. The gap between the collecting electrode and the housing of the fission chamber was about 0.7 mmwide. The operative volume of the chamber was filled with argon ofanalysis-type purity under a pressure of 0.101 MPa (760 mm Hg) at a temperature of 20~ The operating voltage applied to the electrodes of the triaxial chamber was changed from 200 to 300 V; this is much higher than in the operation and testing of the chambers in power reactors and research reactors [6]. The increased operating voltage results from the irradiation intensity which in the pulsed graphite reactor is higher than in the reactors indicated above. Magnesium oxide powder of analysis-type purity corresponding to GOST 452675 was the insulation of the Compton-emission type neutron detectors; magnesium oxide powder