Abstract
The research that was conducted in the framework of the present Ph.D thesis dealt with the study of neutron-induced fission cross-sections, namely the 240Pu(n,f) and the 237Np(n,f)ones. Both reactions were studied at the newly commissioned experimental area (EAR2) of the CERN’s n_TOF facility using the time-of-flight technique to determine the incident neutron energy. A Micromegas detector assembly was employed to detect the high kinetic energy fission fragments and calculate the fission cross-sections, whose detection efficiency was estimated through the means of Monte-Carlo simulations. The aforementioned reactions were the first ones to be studied at the new vertical flightpath, which although being a privilege, was followed by an extensive and complicated analysis since there was no previous experience available to support the research. In addition to the lack of experience, the high activity of the samples and the high neutron flux delivered at EAR2,resulted in a plethora of chall ...
The research that was conducted in the framework of the present Ph.D thesis dealt with the study of neutron-induced fission cross-sections, namely the 240Pu(n,f) and the 237Np(n,f)ones. Both reactions were studied at the newly commissioned experimental area (EAR2) of the CERN’s n_TOF facility using the time-of-flight technique to determine the incident neutron energy. A Micromegas detector assembly was employed to detect the high kinetic energy fission fragments and calculate the fission cross-sections, whose detection efficiency was estimated through the means of Monte-Carlo simulations. The aforementioned reactions were the first ones to be studied at the new vertical flightpath, which although being a privilege, was followed by an extensive and complicated analysis since there was no previous experience available to support the research. In addition to the lack of experience, the high activity of the samples and the high neutron flux delivered at EAR2,resulted in a plethora of challenges that had to be faced. First of all, the detection set-up had to be properly configured in order to reduce the high frequency noise that was present in EAR2, which back then was in a commissioning phase therefore many electrical grounding issues were encountered. In addition, the high activity of the samples, could potentially cause irreversible radiation damage to the detection set-up, therefore a careful installation was deemed mandatory. During the data taking, numerous problems occurring from the commissioning of the facility, were faced all of which were properly addressed in-citu, experimentally. The solutions that were improvised were crucial for the efficient operation of the facility, therefore they were adopted during its 4-year operation, so far. To proceed to the data analysis, the provided by n_TOF reconstruction routine had to be examined for its efficient operation. This important task was part of the early phase of the analysis and was successfully completed. As a result an efficient and properly working signal reconstruction routine was provided to the n_TOF collaboration and up to the present day is part of the standard analysis framework within the facility. In the same context, two specialized routines were developed in the framework of the thesis to address special pulse cases, which are provided to n_TOF and are used by many users of the facility. The analysis itself revealed a high fraction of counting losses of the order of 50-60%, which could not be accounted for, by incorporating in the analysis methodologies found in literature. To address these counting losses, an innovative methodology was developed, published and successfully applied to the experimental data thus yielding cross-sections in an impressively broad energy range that spanned from thermal to fast neutron energies, covering 9orders of magnitude. The 240Pu(n,f) is the first data-set in such a wide energy range in literature, thus providing a high resolution cross-section that is useful for future evaluations and will be provided in a parametrization trough the R-Matrix formalism, since it was theoretically investigated, so that it can be directly and easily available to reactor physicists. The 237Np(n,f) cross-section will be reported with a high statistical accuracy in the MeV region and addressed discrepancies that were observed in published data-sets. Auxiliary nuclear model calculations were performed through the means of the Hauser-Feshbach formalism which indicated that the present nuclear models require refinements in the case of sub-threshold fission.
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