Determination of fission fragment angular momentum from isomeric ratio measurement

This PhD was carried out at CEA in the Laboratoire d'Etudes de Physique of CEA Cadarache in collaboration with Laboratoire de Physique Subatomique et Corpusculaire (LPSC), Grenoble and Institut Laue-Langevin (ILL), Grenoble. Under this collaboration, a programme to measure fission yields from actinides of interest for current and innovative fuel cycles has been carried out since several years. These fission yields are involved in the fuel cycle studies mainly for the inventory calculation at the end of the cycle as well as for the determination of the prompt gamma-rays emitted by the fission fragments. Even though nearly eight decades have passed since the discovery of fission, a clear understanding of the entire process has still not been achieved. Some aspects such as the generation of angular momentum of the fission process is not clearly understood. A few theories have been developed to explain this process of the generation of angular momentum of fission products. This PhD thesis aims at improving the understanding of the generation of the angular momentum of the fission products. To achieve this, the PhD is broadly divided into 2 parts. The first part of the PhD work is to measure and analyse several isomeric yields as a function of fission product kinetic energy. The experiments were carried out at the LOHENGRIN recoil mass spectrometer located at the Institut Laue-Langevin using ²⁴¹Pu target. During the ten-day experimental campaign, the LOHENGRIN recoil spectrometer is used to select the fission products of interest at various kinetic energies. The detection area consists of two high purity germanium (HPGe) clover detectors and an ionisation chamber. The gamma rays arising from microsecond isomers are analysed using the coincidence gamma spectrum, where a coincidence is created between the ionisation chamber and the HPGe detectors. Whereas the gamma rays arising from the decay of the ground state analysed by using the full spectrum. In order to determine the isomeric ratios, gamma ray intensities (taken from literature) and the HPGe detector efficiencies are required. We also need to add few corrections before we can obtain our final isomeric ratios. To characterise the HPGe detectors and obtain the detector efficiencies, a second experiment was carried out using various point sources. Simulations were carried out using MCNP by creating this experimental setup. The simulated efficiencies were compared with experimental efficiencies obtained from the point source experiment and the HPGe detectors parameters were obtained. Using these detector parameters, the experimental setup from the isomeric ratios experiment was simulated using MCNP and the detector efficiencies for the isomeric ratio measurements were obtained. The second part of the PhD concerns the modelling of fission products using an existing Monte Carlo code; FIFRELIN developed at CEA. This code can simulate the fission process as well as fission fragment de-excitation process along with determining a large number of fission observables. By confronting results obtained with this code and those measured at ILL, the angular momentum of the fission products is calculated. Through this PhD, we also aim to create a databank of existing isomeric ratios in order to validate the FIFRELIN code. (author)