Neutron Induced Total Fission Cross Sections
Measurement of Neutron Induced Fission Cross Sections for 238U, 232Th, 209Bi, natPb, 197Au and natW in the Energy Range 20-175 MeV
Tech Area / Field
- PHY-ANU/Atomic and Nuclear Physics/Physics
- PHY-PFA/Particles, Fields and Accelerator Physics/Physics
3 Approved without Funding
Nuclear Physics Institute, Russia, Leningrad reg., Gatchina
- FEI (IPPE), Russia, Kaluga reg., Obninsk
- Physikalisch-Technische Bundesanstalt / Braunschweig Branch, Germany, Braunschweig\nUniversity of Uppsala, Sweden, Uppsala\nUniversity of Uppsala / Svedberg Laboratory, Sweden, Uppsala
Project summaryFission is the most important nuclear reaction for society at large today due to its use in nuclear power plants. Much attention has been recently paid to fission at intermediate energies (20-200 MeV) due to its role in accelerator-driven systems (ADS) for nuclear waste transmutation and energy production, as well as in beam monitoring and dosimetry for radiation protection, cancer therapy, studies of particle-induced effects in electronics (single event effects, SEE), etc.
In spite of extensive experimental and theoretical efforts, intermediate-energy fission remains insufficiently understood in many aspects. Theoretical calculations cannot give consistent description even for basic observables, e.g., total fission cross sections, let alone a more difficult task to predict these quantities for an arbitrary unknown fission reaction. The experimental data, especially for neutrons, are very scarce and often inconsistent beyond the quoted uncertainties.
The goal of the project is to measure the absolute neutron-induced fission cross sections for heavy nuclei: 238U, 232Th, 209Bi, natPb, 197Au, and natW in the energy range 20-175 MeV. The measurements will be performed in collaboration with groups of physicists from Uppsala University (Sweden) and Physikalisch-Technische Bundesanstalt Braunschweig (PTB) (Germany), using the high-energy neutron beam facility in The Svedberg Laboratory (TSL) of Uppsala University . The TSL facility with quasi-monoenergetic neutron spectrum is one of the few facilities of this kind in the world. Its research potential is being further increased by ongoing upgrade , to be completed in 2003, aimed at the major increase of the neutron flux and the improvement of the time structure of the beam. No facilities of this type are available in Russia or other former Soviet Union Republics.
A novelty of the proposed experiment is the experimental technique for detection of fission fragments. Gas parallel plate avalanche counters (PPAC)  will be employed for intermediate-energy neutron-induced fission studies for the first time, which will ensure significant advantages in comparison with earlier studies:
1) the two fragments produced in the fission process will be detected in coincidence, which will allow us to achieve unprecedentedly good separation of fission from other competing reactions,
2) an additional criterion for fission event selection will be provided by pulse height of PPACs,
3) the efficiency of the detection systems will be reliable determined and model-independent,
4) the absolute fission cross sections will be obtained using neutron flux measurements via np-scattering.
Correlated fission fragments will be detected by two identical PPACs situated upstream and downstream of the target to be studied close to it. This provides a large solid angle acceptance near to 4. The targets will be prepared by vacuum evaporation on thin nickel backings with thickness of 150-200 g/cm2. The time signals from PPACs (time resolution about 300 psec) together with the signal phase-locked to the cyclotron RF will allow us to select the fission events induced by peak neutrons. As a result of the off-line analysis, fission cross sections for 232Th, 209Bi, natPb, 197Au and natW will be measured relative to the fission cross section of 238U. Then the data will be converted to the absolute fission cross sections using the results of absolute fission cross section measurement for 238U obtained by measuring elastic np scattering with the MEDLEY/DIFFICILE facility .
The suggested method of fission fragment registration was elaborated at Petersburg Nuclear Physics Institute, and we have a long-term experience in using PPACs at various accelerator beams. In particular, we successfully use PPACs in proton-induced fission studies in the framework of the current ISTC project No. 1405 , to be completed in the beginning of 2003.
The main experimental part of this Project including the design and manufacturing of a new kind of PPAC, manufacturing new reaction chamber, system of gas filling as well as target preparation will be performed at the PNPI (St.Petersburg) and IPPE (Obninsk). New experimental setup will be tested in the experiments on the proton beam with energy 200 MeV at the PNPI synchrocyclotron before using on the TSL neutron beam.
The following main results will be achieved in the project:
1) new experimental total fission cross sections of 238U, 232Th, 209Bi, natPb, 197Au and natW nuclei induced by neutrons with energies 20-175 MeV will be obtained with an accuracy of better than 10%,
2) a neutron monitoring system based on PPAC will be developed and commissioned that will give important contribution to the program for upgrade of the neutron facility at TSL, as the PPAC-based monitor is expected to play an important role at the new facility.
Via the involvement of the foreign partners from the Uppsala University (Sweden) and Physikalisch-Technische Bundesanstalt, Braunschweig (Germany) the project will be run in close connection with EC-sponsored collaborations in the field of nuclear data, in particular with the ongoing HINDAS project  and the proposed TREND project . In addition, a broad involvement of the foreign groups of physicists in the practical work is foreseen, including joint accelerator runs, data acquisition and processing, seminars and workshops, and, in a later stage of the project, co-supervision of Ph.D. students and preparation of joint publications.
1. J. Klug et al., Nucl. Instr. and Meth. A (accepted for publication).
2. TSL Progress Report 2000-2001, Uppsala University (2002).
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4. S. Dangtip et al., Nucl. Instr. and Meth. A452, 484 (2000).
5. L.A.Vaishnene et al., J. Nucl.Sci. Tech., Suppl.2, Aug.2002 v.1, p.323.
6. J.-P. Meulders, J. Blomgren, and N. Olsson, representing the HINDAS collaboration, Proc. Int. Conf. on Accelerator-Driven Transmutation Technologies and Applications, Reno, Nevada, Nov 12-15, 2001 (to be published).
7. J.-P. Meulders, TREND (Transmutation Relevant Nuclear Data). Presented in Int.Meeting on Basic Studies for Transmutation, Uppsala, Sweden, Sep. 12-14, 2002.