Abstract
Neutron-induced fission cross sections of 238U and 235U are used as standards in the fast neutron region up to 200 MeV. A high accuracy of the standards is relevant to experimentally determine other neutron reaction cross sections. Therefore, the detection effciency should be corrected by using the angular distribution of the fission fragments (FFAD), which are barely known above 20 MeV. In addition, the angular distribution of the fragments produced in the fission of highly excited and deformed nuclei is an important observable to investigate the nuclear fission process.In order to measure the FFAD of neutron-induced reactions, a fission detection setup based on parallel-plate avalanche counters (PPACs) has been developed and successfully used at the CERN-n_TOF facility. In this work, we present the preliminary results on the analysis of new 235 U(n,f) and 238 U(n,f) data in the extended energy range up to 200 MeV compared to the existing experimental data.
Highlights
The angular distribution of the fragments produced in the fission of an excited nucleus (FFAD) provides relevant information of the fission barrier structure and the transition levels
A good knowledge on the FFAD is required in the calculation of the detection efficiency for fission fragments (FF) to provide accurate measurements of fission cross sections
The FFAD experimental data for the (n,f) reactions of 235U and 238U published in the EXFOR [2] database reach 20 MeV, the only 238U data exceeding this energy up to 100 MeV are those of Ryzhov et al [3]
Summary
The angular distribution of the fragments produced in the fission of an excited nucleus (FFAD) provides relevant information of the fission barrier structure and the transition levels. A good knowledge on the FFAD is required in the calculation of the detection efficiency for fission fragments (FF) to provide accurate measurements of fission cross sections. The recently published n_TOF data of the 238U/235U fission cross section ratio [1] were obtained fitting the anisotropy parameter of the FFAD data available in the EXFOR database. The FFAD experimental data for the (n,f) reactions of 235U and 238U published in the EXFOR [2] database reach 20 MeV, the only 238U data exceeding this energy up to 100 MeV are those of Ryzhov et al [3]. The recently published data of 238U and 235U by Vorobyev et al [4] and the data of Leong [5] reach 200 MeV. In this work we present the preliminary results of the FFAD of 235U and 238U up to 200 MeV measured at the n_TOF facility (CERN)
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