Fission of Heavy Elements by 30-3000 MeV Protons
Measurement and Analysis of Fission Cross Sections of Heavy Targets Induced by 30-3000 MeV Protons
Tech Area / Field
- PHY-PFA/Particles, Fields and Accelerator Physics/Physics
3 Approved without Funding
Khlopin Radium Institute, Russia, St Petersburg
- ITEF (ITEP), Russia, Moscow
Project summaryThe Project is aimed at experimental determination of the cross sections for fission of 30-3000 MeV proton-irradiated heavy nuclei by the method of thin-film breakdown counters (TFBC) and solid-state nuclear track detectors (SSNTD) and computer-aided simulation of obtained results.
The hybrid technology based on subcritical nuclear reactors fed by external neutrons is among the very promising, inherently-safe, and ecologically-pure nuclear technologies. A 1-2 GeV proton accelerator is most often regarded as a source of external neutrons. An extensive set of nuclear data is required with a view to nuclear physics calculations of the accelerator-driven systems and, mainly, the neutron-producing targets of the latter. The cross sections of incident-proton interactions with the target nuclei within the entire energy range (from the reaction threshold to the incident proton energy) are necessary for the key target parameters (in particular the number of neutrons produced and the target activation) to be calculated. The fission process is notable among various types of interactions. Even in the case of comparatively light nuclei (Hg, for example), when the cross section for fission in the intermediate-energy range is only a few percent of the total reaction cross section, the fission product activity makes a significant contribution to the total target activation within a long cooling time. The fission reaction contribution to the total interaction cross section increases in the case of heavier nuclei (such as U and transuranium elements). Fission at intermediate energies becomes the most important process for the high-fissionable targets, as well as for the direct transmutation of minor actinides when affected by a proton beam.
We have analyzed all the published experimental data (both ours and published elsewhere) on the cross sections for fission of nuclei from 181Ta to 243Am by intermediate-energy protons. The experimental database on the (p,f) cross sections is resulted from thorough search for, and compilation of, the data on the experimental (p,f) cross sections at energies ranging from fission threshold to 30 GeV. The data were sought with the use of Nuclear Structure References database (NSR) and the bibliographic materials accumulated at V.G.Khlopin Radium Institute for many years. The data presented only as graphs were digitized by the method of scanning and subsequent computer-aided processing. The database includes some 180 data sets pided by their confidence degree into groups of experimental quality (reliability of determining the proton flux, target weight, detection efficiency for fission fragments, etc.). From the figure it is seen that the 238U, 232Th, and 209Bi data are most copious (more than 100 data sets). About a half of the data have been admitted to be sufficiently reliable and were used to approximate the energy dependencies of fission cross sections. The 235U, Pb, W, and Ta data are less copious (about 10 data sets or less for each nuclide). The 233U, 237Np, 239Pu, 243Am, and 241Am (p,f)-reaction cross sections were measured in as little as 2-3 works and still at energies of up to 80 MeV.
The fission cross sections of the heaviest nuclei from 232Th to 239Pu were found to increase rapidly with proton energy (due mainly to the increase in the Coulomb barrier penetrability), to reach their maximum of 1.5-2.0 barn at 40-50 MeV, and to decrease afterwards (due to the increased nuclear transparency and to nucleon knockout in intranuclear cascading). This purely-qualitative pattern of the energy dependence of cross sections cannot underlie practical calculation procedures. The following has to be specified:
— the positions of the cross section peaks at the energy scale (and their dependence on nucleonic composition of nuclei);
— the rates of the decrease of the cross sections after passing the maxima (and their dependence on nucleonic composition of nuclei);
— possible structurization in the region of the decrease, namely, slowdown of the decrease at 100-500 MeV and its speedup above 1 GeV.
Despite the about 10% mean statistical deviations from the monotone approximations of the cross sections above 50 MeV, some of the experimental results differ by a factor of 2-3 from the fitting curve. The database is much poorer in the case of Pb and Bi (which are the main pretenders to the materials of neutron-producing targets of accelerator-driven facilities). The Pb and Bi nuclear cross sections are about 10 times as small as in the first group of nuclei. Much fewer works treat Pb and Bi and, besides, their measurement accuracy is lower. For example, the data obtained by different groups on the 209Bi fission by 75 MeV protons differ from each other by a factor of almost 10. The data on W and Ta, whose fission cross sections are even lower, are but snatches.
It has been concluded that the present-day fission cross section database is insufficient. On the whole, the present-day accuracy of the nuclear data for ADS is about an order as low as the reactor data accuracy, thereby resembling the situation with the nuclear fission data as of 1940. Undoubtedly, like the case of reactors, the data requirements will rise as the practical interest in novel nuclear systems developments and, moreover, with approaching the technical computation of demo facilities.
To meet the above requirements, the Project will undertake systematic measurements of the cross sections for fission of nuclei from 181Ta to 243Am under proton irradiation from almost the reaction threshold to (3 GeV within a 10-20% accuracy throughout the energy range. An important feature of the Project is the fact that the measurements will be carried out by identical methods throughout the energy range. In such a way, the Project will eliminate one of the substantial defects of the present-day measurements, namely, the "piecewise" character of the measurements made by separate groups within but narrow energy ranges.
The results to be obtained will be not only of practical but also of fundamental interest. They will permit a deeper insight into nuclear fission at high excitation energies and into the mechanism of the intermediate-energy nucleon interactions with atomic nuclei. Thereby, the theoretical models can be developed, which have again to be used in calculating the quantities required by practical uses.
The following results will be obtained by the proton accelerator using the detectors both available and to be designed.
The natTa, natW, 206-208Pb, natPb, and 209Bi fission cross sections at 50-3000 MeV proton energy region will be measured to within an accuracy from 15% to 30% (depending on energy).
The 232Th, 235,238U, 237Np, 239Pu, and 243Am fission cross sections at 30-3000 MeV proton energy region will be measured to within an accuracy from 10% to 20% (depending on energy).
After approbation of mercury targets manufacturing technology, if the latter will be appropriate for measurements, the nat,198,200,202Hg targets will also be used in measurements.
The target measurements are expected to be taken after each 30 MeV proton energy interval within a 30-200 MeV proton energy range and after each 100 MeV interval within a 800-3000 MeV range.
The reconstruction of synchrotron extraction system is planned to obtain the beam with permanent energy of 30 to 3000 MeV. This will be made by manufacturing of low voltage converter for feeding synchrotron circular magnet to generate magnetic cycles with flat top below 800 MeV. Thereby, the 500-800 MeV proton extraction will be obtained in the framework of the project and used for measurements. The 200-500 MeV proton extraction may require additional investigations and technical development of low voltage converter.
Several measurements on 232Th and 243Am will be performed on low intensity secondary proton beam, which is equipped with magnetic optics and system of absolute monitoring. These measurements require a great amount of accelerator time but will give an absolute cross section values and their coordination with other targets and energies and with data of other authors.
The results of the above measurements will allow to analyze anew all the published data to get the cross section approximations within the given energy range for practical uses.
The results obtained will be computer-simulated at ITEP by the HETC, INUCL, CEM95, LAHET, and CASCADE codes. Besides, the codes used at LANL, CEA, Bruyeres-le-Chatel, and JAERI may be used.
The fission cross sections will be made absolute using an induction beam monitor calibrated with the use of the 27Al(p,E)24Na reaction.
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