Measurement of the $$^{232}$$Th(n,f) cross section with quasi-monoenergetic neutron beams in the energy range 2–18 MeV

The fission cross section of $$^{232}$$ 232 Th has been measured at fast neutron energies, using a setup based on Micromegas detectors. The experiment was performed at the 5.5 MV Van de Graaff Tandem accelerator in the neutron beam facility of the National Centre for Scientific Research “Demokritos”. The quasi-monoenergetic neutron beams were produced via the $$^{3}$$ 3 H(p,n), $$^{2}$$ 2 H(d,n) and $$^{3}$$ 3 H(d,n) reactions, while the $$^{238}$$ 238 U(n,f) and $$^{235}$$ 235 U(n,f) reactions were used as references, in order to acquire cross-section data points in the energy range 2–18 MeV. The characterization of the actinide samples was performed via $$\alpha $$ α -spectroscopy with a Silicon Surface Barrier (SSB) detector, while Monte Carlo simulations with the FLUKA code were used to achieve the deconvolution of the $$^{232}$$ 232 Th $$\alpha $$ α peak from the $$\alpha $$ α background of its daughter nuclei present in the spectrum. Special attention was given to the study of the parasitic neutrons present in the experimental area, produced via charged particle reactions induced by the particle beam and from neutron scattering. Details on the data analysis and results are presented.

Role of Polar vs Non-polar Configurations in the Decay of 268Sg* Compound Nucleus Within the Skyrme Energy Density Formalism

The effect of polar and non-polar configurations is investigated in the decay of 268Sg* compound nucleus formed via spherical projectile (30Si) and prolate deformed target (238U) using the dynamical cluster decay model. The SSK and GSkI skyrme forces are used to investigate the impact of polar and nonpolar (equatorial) configurations on the preformation probability P0 and consequently on the fission cross-sections of 268Sg* nucleus. For non-polar configuration some secondary peaks corresponding to magic shells Z=28 and N=50 are observed, whose magnitude is significantly suppressed for the polar counterpart. The effect of polar and non-polar configurations is further analyzed in reference to barrier lowering parameter ΔVB. The calculated fission cross-section find adequate agreement with experimental data for chosen set of skyrme forces.

RESONANCE EVALUATION OF 239Pu IN THE RESONANCE REGION UP TO 4 keV

Resolved resonance evaluation of the 239Pu cross section in the energy range up 4 keV has been carried out with the SAMMY computer code. Existing resolved resonance evaluation such as ENDF/B-VIII.0 and JENDL4 data libraries for 239Pu is limited to 2.5 keV whereas above this energy the unresolved resonance methodology is used. High resolution transmission and fission data taken at the Oak Ridge Electron Linear Accelerator (ORELA) permitted extending the resonance region up to 4 keV. The thermal and average fission cross section values calculated with the resonance parameters derived in the evaluation fall close to that indicated in the suggested IAEA fission standards.

Towards the experimental validation of a small Time-Projection-Chamber for the quasi-absolute measurement of the fission cross section

To accurately measure neutron-induced fission cross sections, to characterize neutron-beam lines or to make dosimetric investigations, it is necessary to have high accuracy measurements of neutron fluence. It is possible to perform independent and precise neutron flux measurements with respect to the 1H(n,n)p elastic scattering cross section. The use of a silicon detector is recommended from 1 to 70 MeV neutron energy. However, it has been observed that a high electrons background forbids its use below 1 MeV. Hence, a new gaseous proton-recoil telescope is developed and characterized to overcome this limit. It should provide quasi-absolute neutron flux measurements with an accuracy around 3% and is not sensible to gamma and electrons background. It consists in two ionization chambers read by a segmented micromegas technology detection plane. The gas pressure inside is adjustable to the proton range in the detector and therefore to the neutron energy. This detector is described in details below and the newest results of its characterization are presented. A special attention is paid to detection efficiency measurements.

238U FISSION NEAR THE THRESHOLD

Using the data on angular distribution of fission fragments, a threshold has been determined in a dipole fission channel with J = 1 spin projection to the nucleus symmetry axis. It has been shown that the peak observed in the 238U fission cross section is determined by the contribution from the quadrupole excitation.

A Model Study of a Homogeneous Light-Water Thorium Reactor

This work presents a computational study of a 232 Th -based homogeneous light-water reactor. Thorium reactors have been proposed as an alternative to the uranium fuel cycle since they exploit both the availability of thorium and its ability to afford fissile uranium isotopes by a sequence of neutron captures. Besides 233 U , as a result of the neutron captures, a significant amount of 234 U (36.3%) and 6.46% of 235 U are formed in the reactor under study. More importantly, the proposed simulation points out the possibility of a continuous withdrawal of the uranium isotopes without compromising the criticality and the power output of the reactor. This withdrawal affords the fissile material for the startup of reactors other than the first one, which requires a one-time only limited amount of fissile material. The significant molar fraction of the 234 U (0.17) in the extracted fuel does not pose a limitation on weapon proliferation, as a consequence of its high fission cross section for high-energy neutrons.

Preliminary results on the study of 237Np(n,f) at n_TOF/EAR2 in energies related to nuclear waste transmutation

The accurate knowledge of neutron-induced fission cross sections of isotopes involved in the nuclear fuel cycle is essential for the optimum design and safe operation of next generation nuclear systems. Such experimental data can additionally provide constraints for the adjustment of nuclear model parameters used in the evaluation process, resulting in a further understanding of the nuclear fission process. In this respect measurements of the 237Np(n,f) cross section have been performed at the n_TOF facility at CERN in the horizontal 185 m flight-path (EAR1) which were discrepant by 7% in the MeV region. The neutron-induced fission cross section of 237Np(n,f) was recently restudied at the EAR2 19.5 m vertical beam-line at CERN’s n_TOF facility, over a wide range of neutron energies, from 100 keV up to 15 MeV, using the time-of-flight technique and a modern set-up based on Micromegas detectors. This study was performed in an attempt to resolve the aforementioned discrepancies and to provide accurate data of a reaction that is frequently used as reference in measurements related to feasibility and design studies of advanced nuclear systems. Preliminary results with a high statistical accuracy that resolve the discrepancies will be presented along with a brief discussion concerning the facility and the analysis.