An innovating active neutronics interrogation and time-dependent method for calibrating fission chambers in the dedicated CELINA facility using a pulsed neutron generator

Fission chambers are often used in nuclear facilities to perform absolute measurements such as spectral indices i.e. ratio of microscopic fission cross section of different actinides, or fission cross sections or neutron flux. The precision of those measurements, and thus of the related nuclear data, directly depends on the ability to accurately calibrate those detectors. Existing methods for calibrating fission chambers need the use of a nuclear reactor to obtain standard spectra, i.e. either a fission spectrum or a thermal spectrum obtained in a thermal column. In every case, it needs to operate a reactor so that it involves complex and expensive experiments. This report presents an innovating method for calibrating miniature fission chambers fabricated at CEA Cadarache in the so called CELINA measurement cell. This method does not necessitate anymore the use of a nuclear reactor. Indeed, CELINA (CELI of Active Neutronics Interrogation) is a facility using a neutron generator based on Deuterium-Tritium reactions (14.1 MeV neutrons) operated in a pulsed mode (125 Hz), with an isotropic neutron emission of about 2.10⁹ neutrons/cm²/s. A single person is enough to perform the experiment. The fission chambers are positioned close to the generator. A fast neutron spectrum is obtained in this position using a time discrimination of the neutron energy plus a special device made of boron, polythene and cadmium in order to cut the remaining thermal neutrons. The method is based a) on a primary calibration using a matrix treatment of the counting rates obtained on reference fission chambers, in order to determine the macroscopic cross sections for all studied isotopes in CELINA, b) on the determination of the calibrated masses of the studied fission chambers. The results are compared with the results obtained in a ²³⁵U fission spectrum in the BR1 reactor of SCK-CEN Mol (Belgium). It allows validating the measurements performed in the CELINA f- - acility for fission chambers with diameters of 4 mm and 8 mm. The following uncertainties (at 1 standard deviation) are obtained on the calibrated masses: about 3% for ²³⁵U, ²³⁸U, ²³⁹Pu and ²³⁷Np fission chambers, about 4% for ²⁴⁰Pu chambers, and 5% for ²⁴¹Am chambers. In the near future, the method will be extended to ²³³U, ²³⁸Pu, ²⁴¹Pu, ²⁴²Pu and ²⁴³Am fission chambers. In the end, the more accurate calibration of fission chambers will permit to improve the knowledge on integral fission cross sections of actinides in relevant fission spectra for current and future concepts of reactors.