Correlation experiments in photon-induced nuclear fission

The nuclear fission process is highly complex and proceeds in heavy actinide and trans-actinide nuclei either spontaneously or induced by various reactions. Photon-induced reactions are particularly well understood due to the well-known electromagnetic interaction and because only dipole and electric quadrupole excitations are possible. This dissertation presents three developments for correlation experiments in photon-induced nuclear fission and demonstrates the feasibility of such correlation experiments with bremsstrahlung and monochromatic photons in the entrance channel, measuring the full fission-fragment mass distribution with ionization chambers. First, to increase the experimental luminosity for in-beam experiments a multi-target Frisch-grid ionization chamber (FGIC) has been developed. Second, the pulse-height defect (PHD) in different gas mixtures of Ar and CF4 has been determined relative to the reference gas P-10. Third, a positionsensitive FGIC has been constructed that allows the azimuthal fragment emission angle to be determined. The performance of the newly constructed multi-target FGIC, holding up to three targets simultaneously, was tested by an experiment utilizing bremsstrahlunginduced fission on 238U and 232Th at E0 = 8.5MeV, performed at the Darmstadt High-Intensity Photon Setup (DHIPS). Information on the mass, total kinetic energy (TKE) and polar angular distribution of the fission fragments was determined by means of the double kinetic energy technique and the drift-time method, whereas the average TKE of the fission fragments was calibrated relative to established data. The extracted pre-neutron mass distributions for 238U(γ,f) and 232Th(γ,f) are in good agreement with literature data. For the 232Th data an excellent agreement of the shape of TKE distribution with the shape of the literature data is observed. The measured angular distributions were fitted and parametrized by a function which describes the theoretically expected angular distribution. For 238U considerable E2 contributions are detected, whereas for 232Th a clear dipole pattern is evident. The assessment of Ar+CF4 mixtures as a counting-gas in ionization chambers was conducted by using a twin FGIC and fission fragments emitted in 252Cf(sf). As fission fragments emitted in 252Cf(sf) are well studied, a reliable comparison with established data as a basis for a PHD calibration procedure was possible. A universal function describing the PHD in different mixtures of Ar+CF4 was found and was used to calculate pre-neutron mass and TKE distributions. An excellent agreement between average pre-neutron fission-fragment masses measured in all counting-gases and literature is demonstrated with deviations smaller than 0.25 amu. The TKE distributions are in good agreement with established data, and calculated ⟨TKE⟩ values are, within uncertainties, in good agreement with the recommended value of 184.15MeV. To build a more compact multi-target FGIC, one may profit from the high stopping power of the Ar+CF4 mixtures. However, a pressure dependence in the pulse-height data showed that with regard to stopping power no benefit is gained by using Ar+CF4 instead of P-10. The performance of the position-sensitive FGIC was studied by investigating measured fission-fragment mass and TKE distributions as well as angular distributions from 238U(γ,f) at Eγ = 11.2MeV and Eγ = 8.0MeV excitation energy with linearly and circularly polarized γ-ray beams. The experiment was performed at the High Intensity γ-ray Source (HIγS), at the Triangle Universities Nuclear Laboratory (TUNL). Fission-fragment mass and TKE distributions were studied by applying the double kinetic energy technique, and angular distributions were extracted by applying the drift-time method and the read-out of the position-sensing anode structure. Calculated pre-neutron mass and TKE distributions were compared to literature data yielding good agreement. The presented fission-fragment yield as a function of TKE and pre-neutron mass number was used to extract information of fission-mode weights for the standard modes, super-long and super-short mode. A predominant standard-2 mode contribution as expected from theory is evident. The extracted super-short mode contribution of 0.1% has not been observed before in reference data and might be the first evidence of the existence of the super-short mode in light actinides. The fission-fragment polar angular distribution for 238U(γ⃗cir,f) at Eγ = 11.2MeV was analyzed for various mass splits in the fragment pre-neutron mass distribution and is in very good agreement to literature data. The evidence for a working position-sensitive structure is provided by the successful measurement of the fission-fragment azimuthal angle φ, which was measured in coincidence to the polar angle θ. For nearly 100% polarized photons, a distinct anisotropic distribution is observed, with a minimum at φ = 90◦. Normalized values for the contribution of the dipole fission channels are calculated as σγ,f (1−, 0) = 0.484 ± 0.007, σγ,f (1−, 1) = 0.439 ± 0.019 and σγ,f (1+, 1) = 0.078 ± 0.019, using the measured angular distribution coefficients. Prompt fission neutrons (PFN) were measured in coincidence with fission fragments from 238U using four liquid scintillator neutron detectors arranged around the FGIC. Neutron and γ-ray signals are distinguished by means of pulse-shape discrimination and time-of-flight information. The total neutron number detected in each liquid scintillator detector, respectively, is in the order of magnitude of previous estimations. This demonstrates the feasibility to measure prompt-neutron observables in a thin-target in-beam experiment