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Fission Cross-Section for Near Lead Nuclei

#1309


Measurements and Comparison of Proton- and Neutron-Induced Fission Cross Sections of Lead and Neighboring Nuclei in the 20-200 MeV Energy Region

Tech Area / Field

  • PHY-ANU/Atomic and Nuclear Physics/Physics

Status
8 Project completed

Registration date
24.07.1998

Completion date
23.04.2003

Senior Project Manager
Malakhov Yu I

Leading Institute
Khlopin Radium Institute, Russia, St Petersburg

Collaborators

  • European Commission / Research Directorate-General / International Scientific Cooperation, Belgium, Brussels\nForschungszentrum Karlsruhe Technik und Umwelt, Germany, Karlsruhe\nUniversity of Uppsala / Department of Neutron Research, Sweden, Uppsala

Project summary

Nuclear data, as quantitative characteristics of nuclear reactions, have become particularly important in the intermediate energy region. They are necessary for development of new concepts of nuclear energy production and transmutation of radioactive waste with the use of accelerators, as well as for design of shielding for accelerators and space apparatus, neutron and proton therapy, medical isotope production and many other applications. They are important also for the development of theory of nuclear interactions, nuclear structure and nuclear matter properties.

Predictive power of available theoretical models and computational codes in the intermediate energy region is estimated as ±50% in average, although calculations often differ from each other and from experimental data by 2-3 orders of magnitude. Theoretical description becomes especially complicated in the energy region from a few tens of MeV to about 200 MeV, because of interference of collective and single-particle effects. Thus, it has been proposed to create files of experimental and evaluated data for the given energy region, similarly to the existing reactor-oriented databases for neutron-induced reactions below 20 MeV.

Lead target is considered as the most perspective for accelerator-driven energy production and/or waste transmutation systems. Another material under consideration is lead-bismuth eutectics, which loses to the pure lead because of higher induced activity but possesses better heating engineering characteristics. For calculations of key parameters of the lead targets (number and spectrum of outcoming neutrons, prompt and residual radioactivity and heat release, radiation stability – in the case of solid target), nuclear data are needed for reactions induced in lead by intermediate energy protons and neutrons. In particular, fission reaction data are needed. In spite of comparatively low cross section (a few percents of total inelastic cross section), fission reaction leads to products with high energy release and, often, with long half-lives. It is estimated that the contribution of the fission products to the overall residual activity of a lead target irradiated by 1.6-GeV protons for cooling time of about year amounts in 10-15%. Due to the above mentioned uncertainties in model calculations, as well as inaccurate benchmark experimental data, the estimation of the residual activity is very uncertain, in particular, the contribution to the activity from the fission process can be essentially different and, possibly, greater.

To obtain more reliable estimations, one needs new data on fission cross sections of lead and bismuth. Furthermore, the Bi(n,f) cross section is adopted as an important standard for neutron flux measurements in the energy region above 20 MeV, in particular for the fulfillment of the data requests in the mentioned accelerator-driven technologies. Besides the natPb and 209Bi (monoisotopic element) it is proposed to measure the fission cross sections of the enriched isotopes of lead 206, 207 and 208 and 205Tl (the neighbor of lead with lower Z) that is needed to develop the theoretical model for fission cross section calculations. The last one is of important to raise the quality of estimated data and to create the prerequisites for cross section estimations of any other nuclei in this region without carrying out the expensive experiments.

It is considered essential to perform the proton- and neutron-induced fission cross section measurements within a common experimental project and, as much as possible, with the use of the same experimental technique. Our earlier comparison of the proton and neutron cross sections showed interesting properties of the cross section ratio dependence on incident nucleon energy. The analysis of these ratios was performed in terms of fission probability as a general physical basis that defines the fission process induced by protons and neutrons. Such analysis consolidates the experimental data base taken as a whole and allows to obtain the most realistic results.

The up-to-date status of the proton- and neutron-induced fission cross section data for Pb and Bi in energy range from 20 up to 200 MeV is quite unsatisfied. There are only two studies of the natPb(p,f) cross section and the last of them was performed 25 years ago. Moreover, these measurements were carried out only at four energy points of the considered energy region. Since 1956 there are about 10 studies on the 206, 207, 208Pb and 209Bi(p,f) cross sections. The results disagree by factors for the energy regions where a comparison is possible. For example, there are discrepancies as large as almost one order of magnitude for 209Bi at 75 MeV. Only one study for the 205Tl(p,f) cross section at energies up to 70 MeV is published.

Neutron induced fission cross section measurements have only started recently for all considered nuclides, if one does not take into account two very early works (1950 and 1955) carried out for natPb and 209Bi respectively with very wide and poorly known neutron spectrum with FWHM of about 40 MeV. No measurements of the 205Tl(n,f) reaction have been made so far. Recently the natPb(n,f) and 209Bi(n,f) cross sections were measured by Staples et al. in the energy region about 30-450 MeV at the "white" neutron source at LANL. The absolute uncertainties are from 8 to 20% depending on the neutron energy. As we know these data have not yet been published. Practically the first experimental data on the Pb(n,f) cross section were the ones reported by KRIUU collaboration (V.G.Khlopin Radium Institute and Uppsala University). The 208Pb isotope data are obtained at the quasimonoenergetic neutron source of The Svedberg Laboratory at neutron energies of 45, 73, 96 and 162 MeV. The measurements were performed relative to the 238U(n,f) cross section with uncertainties of 12-15%. The first data on the 209Bi(n,f) cross section were obtained by the same KRIUU group and at the same time for a few energy points with relative uncertainties of about 10%. Such a work seems to be necessary because measurements at the quasimonoenergetic neutron beam and the "white" spectrum are carried out under different experimental conditions connected with different corrections for background events, time resolution of experimental setups and other experimental characteristics. The measurements supplement each other, because the results are obtained by fully independent measurements and thus increase the reliability of the data. A scanty information, disagreement or full absence (as it is occurred in the case of neutron data for 205Tl) of data on proton- and neutron-induced nuclear fission need the carrying out of new systematic measurements.

Presented project is the development of ISTC Project No 540 “Measurements of neutron induced fission cross sections in energy region 15 < En < 160 MeV for basic and applied researches” and preceding ISTC Project No 17 in the frame of which the above mentioned first results were obtained. In the frame of Project No 540 the study in the wide range of nuclei (from Au to Am) is carried out, among the above discussed nuclei only natPb(n,f), 208Pb(n,f) and 209Bi(n,f) cross sections at 4 energy points are measured and no measurements with use of protons are not provided.

More detailed (on energy and number of isotope targets) study performed with neutrons and protons under the same conditions is the special feature of this project that can be done the results needed to create the file of evaluated proton and neutron data which will be satisfied to requirements of ADS calculations in energy region 20-200 MeV.


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