Neutron dosimetry, i.e., the measurement of neutron flux or fluence, is of essential importance in nuclear facilities. In nuclear power plants, neutron fluence is measured on the reactor pressure vessel, enabling the assessment of radiation damage to the vessel material (dislocation of atoms in the crystal lattice), which over time leads to increased embrittlement. Fast neutrons make the main contribution, while the contribution of epithermal neutrons is not negligible. Nuclear reactions sensitive to the epithermal energy range are few, and the shapes of reaction cross-sections are generally less known and unexplored. The objective of this master's thesis is to investigate the measurability of reaction rates for a new set of inelastic scattering nuclear reactions, specifically the (n,n') type, which could be useful for epithermal neutron dosimetry.
In the theoretical part of the master's thesis, we first focused on the history of radioactivity, defined atoms and their structure, and introduced radioactivity and nuclear reactions. We then described nuclear decays and presented neutron reactions. We described radioactive decay and rate and cross-section for the reaction. We also defined and described neutron activation analysis and reactor dosimetry. We briefly introduced the basics of probability theory and statistics.
The experimental part of the master's thesis focused on irradiating samples at the TRIGA research reactor at the Jožef Stefan Institute (JSI), measuring gamma ray spectra with high-purity germanium (HPGe) detectors, and calculating reaction rates based on the measured results. We independently calculated the reaction rates based on known neutron spectra and nuclear data from the IRDFF-II (International Reactor Dosimetry and Fusion File) library specialized for neutron dosimetry and the ENDF/B-VIII.0 library for general use. The calculated values were then compared with the reaction rates obtained from measurements. We found that measuring reaction rates for (n,n') type reactions was challenging but worth further research in the context of neutron dosimetry, and we highlighted some obstacles in this regard.
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