Developpement and optimisation of gamma-ray spectrometry techniques : control and reduction of the associated uncertainties
In the frameworks of the neutronics calculation schemes improvement and qualification, this thesis consists of the conception, the optimization and the development of g-ray spectrometry techniques for the measurement of integral parameters, in the EOLE and MINERVE facilities. The work aims at correcting systematic errors and at controlling and reducing uncertainties on the measurement results. The achieved progresses allow a more precise comparison between calculations and experiments and a better knowledge of some nuclear data, especially integral capture and fission cross sections. Firstly, the former instrumentation - using an analog chain (2026 amplifier and PCA3 converter / analyser) - has been fully tested to estimate the experimental errors on the determination of neutronics parameters. This work shows a systematic bias up to 12% on the measurement of the power distribution scaling factor and up to 3% on the measurement of the axial buckling, due to the lack of pulse pile-up correction. Following a comparative study, these electronics have been replaced by a digital DSP2060 system with an adapted dead time correction. With the optimization of its parameters, a precision at better than 0.5% has been achieved until a 1.5'105 s-1 count rate. Consequently, this work allows new kinds of experiments, which could not have been done with a sufficient precision by the past. Secondly, the former methods applied to the treatment of measurement raw data and to the variance propagation, have been analysed and discussed. Different systematic errors, which were not taken into account by the past, are now corrected. They concern the influence of the activity decay on the dead time correction (about 2%), the true coincidence effect (about 5%) and the solid angle effect on the efficiency transfer corrections (about 8%). Besides, the description of more precise methods for the variance propagation leads to a 3 times better uncertainty on fundamental neutronics parameters - scaling factor on power distributions (about 0.6%) and axial buckling (about 0.7%) - and a 2 times better uncertainty on the measurement of the uranium 238 conversion ratio (about 1.5%). These progresses have been applied to the conception of experiments in support to the qualification of cross sections, for actinides (232Th, 236U, 237Np, 242Pu) and neutron absorbers (151Eu, 153Eu, 164Dy, 170Er, 180Hf). The comparison of capture cadmium ratio and capture index measurements, with APOLLO2 deterministic calculations or MCNP4C2 probabilistic calculations, with the JEF2.2 , ENDF/B-VI.8 ore JEFF3.1 libraries, shows systematic biases on the integral capture cross sections up to 50% for certain isotopes, with an experimental uncertainty better than 2%. Lastly, experiments dedicated to the qualification of decay data have been carried out for the first time in the Experimental Program Laboratory (LPE) of CEA Cadarache. A systematic error of about 5% has been identified on the JEFF3.1 evaluation of the half-life of strontium 92. This nuclear data has been revaluated with specific measurements, with a 2 times better uncertainty compared with the past. Fission yield fluctuations on the 235U(n,f) reaction have also been identified between a thermal and an epithermal neutronics spectra, from -0.8% to +2%. The study gives some recommendations on the calculation method to estimate the average fission yield, for under-moderated reactor concepts.