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Background: The sharing of excitation energy between the fission fragments is one of the key issues in studying nuclear fission. One way to address this is by studying prompt-fission neutron multiplicities as a function of other fission observables such as the mass, ¯ν(A). These are vital benchmark data for both fission and nuclear deexcitation models, putting constrains on the fragment excitation energy and hence on the competing prompt neutron/γ-ray emission. Despite numerous detailed studies, recent measurements done at JRC-Geel with the SCINTIA array in the epithermal region show surprisingly strong discrepancies to earlier thermal fission data and the Wahl systematics. Purpose: The purpose was to perform measurements of the prompt-fission neutron multiplicity, as a function of fragment mass and total kinetic energy (TKE), in 235U(nth,f) and 252Cf(sf), to verify and extend the SCINTIA results. Another goal was to validate the analysis methods, and prepare for planned investigations at excitation energies up to 5.5 MeV. Methods: The experiments were conducted at the former 7 MV Van de Graaff facility in JRC-Geel, using a Twin Frisch-Grid Ionization Chamber and two liquid scintillation detectors. A neutron beam with an average energy of 0.5 MeV was produced via the 7Li(p,n) reaction. The neutrons were thermalized by a 12 cm thick block of paraffin. Digital data acquisition systems were utilized. Comprehensive simulations were performed to verify the methodology and to investigate the role of the mass and energy resolution on measured ¯ν(A) and ¯ν(TKE) values. The simulation results also revealed that the ∂¯ν(A)/∂A and ∂¯TKE/∂¯ν are affected by the mass and energy resolution. However, the effect is small for the estimated resolutions of this work. Detailed Fluka simulations were performed to calculate the fraction of thermal neutron-induced fission, which was estimated to be about 98%. Results: The experimental results on ¯ν(A) are in good agreement with earlier data for 252Cf(sf). For 235U(nth,f), the ¯ν(A) data is very similar to the data obtained with SCINTIA, and therefore we verify these disclosed discrepancies to earlier thermal data and to the Wahl evaluation. The experimental results on ¯ν(TKE) are also in agreement with the data at epithermal energies. For 252Cf(sf) a slope value of ∂¯TKE/∂¯ν=(−12.9±0.2)MeV/n was obtained. For 235U(nth,f) the value is (−12.0±0.1)MeV/n. Finally, the neutron spectrum in the center-of-mass system was derived and plotted as a function of fragment mass. Conclusions: This work clearly proves the lack of accurate correlation between fission fragment and neutron data even in the best-studied reactions. The new results highlight the need of a new evaluation of the prompt-fission multiplicity for 235U(nth,f).

In this thesis, isomeric yield ratios of twenty nuclides produced in the fission of natU and 232Th by protons at 25-MeV and natU by high-energy neutrons were studied. The experiments were performed at the IGISOL-JYFLTRAP facility at the University of Jyväskylä. It is the first time that direct ion counting is used for the determination of the intensities of the states of interest, thus avoiding dependency on knowledge of nuclear decay schemes and properties. This was possible due to the superior resolution of a Penning trap which was utilized for this work. Two different techniques were employed, namely the sideband cooling technique and the phase-imaging ion-cyclotron-resonance technique. With the former, a mass resolving power of m/δm = 105 can be routinely achieved, while the latter, which was recently implemented at JYFLTRAP, offers an increase in the mass resolving power by a factor of ten. In addition, isomeric yield ratios were also determined by means of γ-ray spectroscopy. From a comparison of the same isomeric pair from two different reactions, a dependency on the fissioning system can be observed. This indicates an effect of the fission mode to the yield ratio. Moreover, the evolution of the odd-A isotopes of Cd and In in the mass range A = 119 - 127 exhibit two distinguishably different trends. The ratios for the isotopes of In decrease with increasing mass, while the ratios for the isotopes of Cd are almost constant until mass number A = 125, where an increase can be noticed. The origins of the angular momentum in the fission fragments is one of the long-standing questions regarding the fission process. Surprisingly, fission fragments have been observed to carry a considerable amount of angular momentum, even from fissioning systems with very low (or even zero) angular momentum. So far, the angular momentum can only be inferred from other fission observables, such as the isomeric yield ratios. In this work, a methodology was developed in order to deduce the root-mean-square angular momentum (Jrms) of the primary fragments by employing the nuclear reaction code TALYS. Lower values of Jrms for the more spherical nuclei, near the closed-shell neutron configuration at N = 82, and higher ones for fragments with odd proton number have been deduced, in agreement with other studies. Moreover, a correlation between the angular momentum of the primary fragments with the electric quadrupole moments of the products was observed for the isotopes of In. The data can be used to gain insight into scission configuration and as guide for models that propose mechanisms for the generation of the angular momentum. Furthermore, the observed correlation is an indication of the role that the repulsive Coulomb force, together with the shape of the nascent fragment, play in the generation of the fragments’ angular momentum.

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