Neutron-induced fission fragment angular distribution at CERN n TOF: The Th-232 case

This thesis work was done in the frame of the study of the neutron-induced fission of actinides and subactinides at the CERN n TOF facility using a fast Parallel Plate Avalanche Counters (PPACs) setup. This experimental setup provide us with an intense neutron beam with a white spectrum from thermal to 1 GeV and with an outstanding high resolution provided by its flight path of 185 m. In our experiment, fission events were identified by detection of both fission fragments in time coincidence in the two PPAC detectors flanking the corresponding target. This technique allowed us to discriminate the fission events from the background produced by α disintegration of radioactive samples and by particles produced in spallation reactions. Because PPAC detectors are insensitive to the γ flash, it is possible to reach energies as high as 1 GeV. The stripped cathodes provide the spatial position of the hits in the detectors, so that the emission angle of the fission fragments can be measured. Inside the reaction chamber up to nine targets can be simultaneously measured, using two of them as references (235U and 238U in our case). The reliability of this method was established in earlier measurements of fission cross sections in actinides and subactinides up to 1 GeV neutron energy, with unprecedented resolution over the whole range. The novel aspect of this experiment is the geometrical arrangement of the targets and detectors, which were tilted 45◦ with respect to the neutron beam direction. This configuration covered a larger angular acceptance to polar angles, so that the full angular distribution could be measured. In previous experiments, the perpendicular placement of the detector setup limited the acceptance to angles smaller than 65◦. One main objective of this thesis is to demonstrate the suitability of such a geometrical configuration for measuring the angular distribution of the fragments emitted in fission. 122 Summary and conclusions 232Th was a good study case because, in spite of the high interest of this nucleus as a component of nuclear reactors fuel, its large anisotropies (both in forward- and side-peaked distributions) that change quickly with the neutron energy, have been scarcely measured out of the fission threshold region. All the experimental techniques used to obtain the final results of this work from the raw data are explained in the present manuscript. The first stage of the analysis involved identifying the fission events and rejecting the background signals. Analysis of the cathode signals to identify the position of the hits and, therefore, the trajectory of the fission fragments has also been explained. Finally, neutron energy was calculated using the time of flight technique and unambiguously associated to each fission event. Simulation work was done with Geant4 to study the angular acceptance of this new experimental setup, and compare the results with the previously achieved when the detector setup was placed perpendicular to the beam direction. A good agreement between the simulation and experimental data was obtained for the 235U(n,f) case at low energies. Finally, a study on the angular distribution of the fission events was presented, which made it possible to obtain the fission fragment angular distributions for the 232Th(n,f) reaction in the full energy range. The anisotropy parameter, defined as the ratio of the number of fragments emitted in the beam direction and at 90◦, was calculated as a function of the neutron energy. In addition to this, the fission cross section of 232Th(n,f) was also provided, relative to the standard 235U(n,f) cross section. At this moment, because the target masses have not been measured yet, the cross section has been normalized to the ENDF/B-VII.1 evaluation. It was shown how the symmetric and asymmetric fission modes can be identified with this experimental setup by using the time differences between both fission fragments. Its evolution with the neutron energy was also shown. Conclusions The main conclusions that should be highlighted from this work are the following: This wor