Radiative neutron capture on $^{242}$Pu: addressing the target accuracies for innovative nuclear systems (Cross section measurements at the CERN n_TOF and BRR neutron beams from thermal to 500 keV)

A low-carbon energy oulook to mitigate the impact of climate change requires the progressive replacement of fossil fuel technologies by sources with low CO$_{2}$ emissions. In this context, nuclear energy is expected to play a relevant role. Ensuring the long-term sustainability of nuclear energy points to the use of innovative nuclear systems, such as Accelerator Driven Systems and Generation-IV reactors and new fuel compositions, such MOX fuels aimed at the reduction of nuclear waste. The design and operation of these nuclear innovative systems requires a better knowledge of the capture and fission cross sections of the Pu isotopes. For the case of $^{242}$Pu, a reduction of the uncertainty in the fast region (2-500 keV) from the current 35% down to 8-12% is required. Moreover, aiming at improving the evaluation of the fast energy range in terms of average parameters, the OECD NEA “High Priority Request List”, requests high-resolution capture measurements with improved accuracy below 2~keV. The uncertainties also affect the thermal point, where previous experimental results deviate from each other by 20%. In this context, a series of experimental campaigns were proposed to improve the accuracy of the $^{242}$Pu(n,$\gamma$) cross section in the different neutron energy ranges. This thesis presents the new measurement of the $^{242}$Pu(n,$\gamma$) cross section from thermal to 500 keV combining different neutron beams and techniques. In collaboration with JGU Mainz and HZ Dresden-Rossendorf, we produced a sample consisting of a stack of seven fission-like targets making a total of 95(4) mg of 99.95% pure $^{242}$Pu electrodeposited on thin (11.5~$\mu$m) aluminium backings. The high quality in terms of actinide to backing mass ration, large surface and thickness have been crucial to overcome the limitations of previous similar capture experiments. The radiative capture on this $^{242}$Pu sample has been measured using two complementary neutron beam facilities and different experimental techniques. The thermal point of the cross section was determined from a measurement carried out at the Budapest Research Reactor, where four out of the seven $^{242}$Pu targets were irradiated in the cold neutron beam of the PGAA facility. From this measurement, the thermal capture cross section was determined from the direct detection of the prompt $\gamma$-rays, the so-called Prompt Gamma Activation Analysis; and by means of Activation Analysis, i.e. the detection of the number of neutron captures produced in the sample from the detection of the subsequent decay of the product nuclei. The experimental set-ups for both techniques consist of high-resolution HPGe detectors. A second measurement was carried out at the CERN n_TOF facility, featuring a spallation-based pulsed neutron source with a white spectrum. In this experiment, the cross section was determined by means of the time-of-flight technique at the EAR1 measuring station (flightpath of 184~m) using a set of four C$_6$D$_6$ Total Energy detectors. The experimental set-up, data acquistion and analysis are described in detailed throughout this manuscript, focusing on the innovative methods developed in this work. From this measurement, the individual resonance parameters of 251 resonances have been extracted from the R-Matrix analysis of the Resolved Resonance Region (RRR) (1 eV - 4 keV), and the averaged cross section in the Unresolved Resonance (URR) Regions (1 - 500 keV) has been described in terms of average resonance parameters by means of a Hauser-Feshbach calculation allowing width fluctuations. This manuscript deals with a detailed discussion on the final results and the comparison with the previous experiments and current evaluations, with special emphasis on the achieved accuracies in each energy region. In summary, the measurements presented in this work improve the knowledge of the $^{242}$Pu(n,$\gamma$) cross section, reduce the current uncertainties and should help to clarify previous discrepancies among the experimental data. The work aims at contributing to the global effort to narrow the gap between the current status of nuclear data and the target accuracies required to design and operate nuclear innovative systems.