Measurement of the ²³⁶U(n, f) Cross Section at n_TOF

R. Sarmento, I. F. Goncalves, P. Vaz, M. Calviani, N. Colonna and The n TOF Collaboration

J. Korean Phys.Soc. 59,1793 [doi: 10.3938/jkps.59.1793 | PDF Download]

A precise knowledge of the ²³⁶U neutron-induced fission cross-section is required for the development of accelerator-driven systems and reactors based on the Th-U cycle. The evaluated data presently stored in the nuclear data libraries rely on outdated experimental measurements and show large discrepancies in the energy region between 1 keV and 100 keV. More recent measurements made at LANSCE and GELINA yielded results which are in disagreement with the literature for the resonance region and below 10 eV. In order to improve the present knowledge of the ²³⁶U(n, f) cross-section, a new measurement was performed at the neutron Time-Of-Flight facility n_TOF at CERN. A Fast Ionization Chamber was used, in which four samples of ²³⁶U and two of ²³⁵U were mounted. The ²³⁶U(n,f) cross-section was determined relative to the standard ²³⁵U(n, f) reaction. The contribution from the ²³⁵U contamination in the samples was subtracted, together with the alpha-particle background. Finally, the data were corrected for dead-time and detection efficiency. The n_TOF results on the ²³⁶U(n, f) cross-section are reported in the neutron energy region from 200 meV to 2 MeV. The present high-resolution data point to several shortcomings in the current evaluated databases in the sub-threshold region. High accuracy cross-sections, important for applications to fast nuclear reactors as well as for fundamental Nuclear Physics, are also reported here.