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Fragments of Fission by Intermediate Energy Neutrons

#3363


Developing the Experimental Database on Fragment Yield in Fission of the Main Nuclear Fuel Isotopes and other Minor Actinides Induced by Intermediate Energy Neutrons

Tech Area / Field

  • PHY-ANU/Atomic and Nuclear Physics/Physics
  • FIR-NOT/Nuclear and Other Technical Data/Fission Reactors
  • FIR-REA/Reactor Concept/Fission Reactors

Status
8 Project completed

Registration date
20.09.2005

Completion date
27.07.2010

Senior Project Manager
Malakhov Yu I

Leading Institute
Khlopin Radium Institute, Russia, St Petersburg

Collaborators

  • Oregon State University, USA, OR, Corvallis\nNRG an ECN KEMA company, The Netherlands, Petten\nTohoku University / Cyclotron and Radio Isotope Center, Japan, Sendai\nUniversity of Uppsala / Department of Neutron Research, Sweden, Uppsala

Project summary

The concept of accelerator-driven power reactor systems allows for the existence in the reactor of neutrons with energies higher than the boundary of neutrons of the fission spectrum, i.e., neutrons with energies higher than 14-20 MeV arriving from an external source – a neutron-producing target of an accelerator. However, in contrast to a conventional reactor range, there are no libraries of nuclear data even on one of the basic characteristics of the fission reaction, i.e., fission yield, for neutrons of such energies. The goal of this project is to bridge the gap for basic fuel elements (232Th, 238U) and minor actinides (237Np, 243Am, 246Cm) in the region of intermediate energies, i.e., 20-200 MeV (a small fraction of neutrons with higher energies that can be formed at an accelerator energy of 1 GeV does not play an important role). To this end, it is planned that missing experiments will be performed, all the available data on fission product yields will be systematized, and empirical dependencies of yields on nucleon structure of a fissioning nucleus and its excitation energy that can be used as a basis for development of libraries of evaluated nuclear data will be established. Experimental data on mass distributions of fragments (both primary, prior to emission of prompt neutrons, and secondary - after the emission) for fission by neutrons with energies below 14 MeV available for 232Th, 238U and other nuclides, such as 229Th, 233,235U, 237Np, 239,240,242Pu, 257Fm [A.C.Wahl, IAEA TECDOC-1168, International Atomic Energy Agency Nuclear Data Section (2000); L.E. Glendenin et al, Phys. Rev C 22(1980)152; V.М. Gorbachev et al. «Interaction of radiation with nuclei of heavy elements and nuclear fission». Reference book. Atomizdat, 1976] have shown that the shape of the distribution and, in particular, the ratio between yields of symmetric and asymmetric fission (often characterized by the peak-to-valley ratio) varies as the charge, Z0, mass, A0, and excitation energy, Е*, of a fissioning nucleus. The peak-to-valley ratio decreases as Z0 increases, А0 decreases for a given Z0, and Е* increases. To our knowledge, only one work on neutrons with energies above 20 MeV has been published [C.M. Zöller, Ph.D. thesis, TH Darmstadt, 1995]. The authors obtained the mass distributions of fission fragments of 238U in the range of Еn from 2,0(±0,5) MeV to 450(±50) MeV. The results of this work revealed the same trend of growth of symmetric fission with increasing excitation energy, but, because of insufficient statistics and specific features of the experimental procedure used, they do not allow the required quantitative regularity to be established for 238U, to say nothing of other nuclides. To solve the problem for neutrons of intermediate energies, new experimental data should be obtained.

Attention should be paid here to the data obtained in experiments with protons. The fact is that more than 50 years ago it was found that asymmetry of fission, i.e., peak-to-valley ratio, varies with the nucleus excitation energy, no matter how the nucleus is excited – by charged particles, neutrons, or photons [E.K. Hyde, The Nuclear Properties of the Heavy Еlements, vol. III: Fission Phenomena, Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1964]. The insensitivity to the agent that induces fission is a direct consequence of the model of a compound nucleus in which the way of fission is independent of the fashion in which the nucleus is formed (possible deviations from the model will be discussed below). However, we shall restrict our consideration to the case of nuclear fission by protons, which are most close to neutrons by induced angular moments.

Quantitative comparison of symmetric component in fission (to the total yield of fragments) for protons and neutrons, at the energy of incident nucleons up to about 100 MeV, can be performed on the basis of the data obtained in the studies of fission of 238U by C.M. Zöller [Ph.D. thesis, TH Darmstadt, 1995] and C. Chung and J. Hogan [Phys.Rev. C 25(1982)899]. The comparison reveals that the difference is about 30%, the difference between determinations of the symmetric fission contribution by different methods (in the first work) being approximately half of this value. These numerical results can be regarded as the characteristic of the accuracy of available measurements and their processing (and also the degree of similarity of the curves mentioned above). Results of two recent works for 238U [V.A. Rubchenya et.al., Nucl. Instrum. Methods Phys. Res. A463 (2001)653, О.I.Batenkov et al., accepted for publication in Nuclear Physics, 2004] nearly coincide with the data for this nucleus in the case of neutrons. An important feature of these works is that the authors measured yields of prompt fission neutrons from both a fissioning nucleus and fission fragments simultaneously with fragment yields, which gives information on the nucleus excitation energy at different stages of the fission process. This information is vital for development the fission model that is able to describe mass distribution of fragments of excited nuclei [M.C. Duijvesijn, A.J. Köning, F-J.Hambsch, Phys. Rev. C 64(2001)014607].

It seems likely that the information obtained in experiments with protons will remain in the nearest future the only information; only basing on this information it will be possible to judge about changes in charge distributions with variation of nucleus’s excitation energy (caused by either protons or neutrons). This emphasizes once more that protons must be a valuable tool for determination of fragment yields in the region of intermediate energies. All the data on charge yields considered above were obtained by the radiochemical or mass spectrometric techniques. The on-line physical methods that have recently appeared, such as measurements of the КХ rays and gamma-rays of fragments, measurements of ionization losses of fragments of known masses and velocities (or bare nuclei in radioactive fluxes), numbering of the shape of the pulse caused by a fragment in a detector, have not been tested so far for beams of intermediate energies. The exception is experiments at a high-resolution gamma- spectrometer, GEANIE, at LANSCE, in which the function of excitation of a number of Xe isotopes in 1-100 MeV neutron fission of 238U isotopes was found [ T. Ethvignot, et. al., Journal of Nuclear Science and Technology, Supplement 2 (2002) 254]. This may be a special task which will not be solved in the framework of this project. This makes the role of systematization and analysis of all the available data on charge distributions of protons more important.

The problem of the composition of fragments produced at fission embraces nearly the entire physics of fission, beginning with cross sections and probabilities of fission (the ratio between the fission and neutron widths, Γf/Γn) and the influence of of the nuclear structure of a fissioning nucleus and fragments on the potential energy surface of the deformed nucleus and ending with de-excitation of excited fragments and their angular distributions. Intermediate energies create additional problems, such as dynamics of nuclear scission (neutrons before and after fission), multi-chance of fission, mechanisms of formation of spectra of fissioning nuclei in interaction of energetic protons and neutrons with nuclei (compound nucleus, pre-equilibrium processes, and fast cascade) and their relative role at different energies of incident particles. It is impossible to review here all experimental and theoretical works devoted to these problems. We briefly mention only the studies carried out at our Institute. The Institute was the place where the physics of fission in Russia came into being [N.А. Perfilov, К.А. Petrzhak, V.P. Eismont*, in “Essays on History of Development of Nuclear Physics in the USSR”, Kiev, 1982]. The first Soviet collected book on nuclear fission ["Physics of Fission of Atomic Nuclei" (edited by N.А. Perfilov and V.P. Eismont) М., Gosatomizdat, 1962] contained papers of scientists working at the Institute: А.N. Protopopov “Asymmetry of nuclear fission", V.П. Eismont "Angular anisotropy of fission", N.А. Perfilov "Nuclear fission by high-energy particles" etc. In the 1950s and during ensuing years, V.P. Eismont and his colleagues carried out investigations of a wide range of problems, such as symmetry of nuclear fission, nuclear shells and prompt fission neutrons, two types of nuclear fission, two types of fission and nuclear charge distribution, relation between mass distributions and kinetic energies of fragments, neutrons of fission of excited nuclei, characteristic radiation and charge of fragments, correlation of masses, kinetic energies and charges of fragments, energy balance of fission and shells in fragments.

During recent 10 years V.P. Eismont and О.I. Batenkov, in cooperation with their collaborators and foreign colleagues - K. Aleklett, J. Blomgren, H. Condé, N. Olsson, P-U.Renberg (Sweden), W. Loveland (USA) - have devoted much effort to obtaining nuclear data at intermediate energies, and, with funding of Russian participants by ISTC grants No. 17, 540, 1145, 1309, and 2213, have published more than 50 methodological, experimental, and review papers. The papers were published in such journals as “Nuclear Instruments and Methods in Physical Research”, “Radiation Measurements”, “Physical Review C”, “Izvestiya RAS”, “Atomnaya Energiya” and proceedings of International conferences on nuclear data for science and technology: Gatlinburg, USA, 1994, Trieste, Italy, 1997, Tsukuba, Japan, 2001, 6 abstracts have been submitted to Santa Fe, USA, 2004.

The papers concerning the ratio between neutron- and proton-induced fission cross sections, and also angular distributions of fragments in proton- and neutron-induced fission have shown that there are no distinctions between characteristics of proton- and neutron-induced fission except those that can be easily explained by Coulomb effects and minor differences at the stage of direct interactions with the nucleus-target.

No doubt, acquisition of new experimental data, systematization, analysis, and derivation of new empirical relations for fragment yield at intermediate energies will contribute to the progress in the physics of fission, which is, as before, of great theoretical and practical interest, first, as the physics of nuclear states with extreme deformations, and, second, as widely discussed prospects of energy production and (or) transmutation of accumulated long-lived radioactive waste (that contaminates the environment) with the help of accelerator-driven (hybrid) nuclear reactors.

In view of the facts considered above, i.e., 1) the availability of a much greater amount of information on mass distributions for proton-induced fission than on neutron-induced fission, 2) the absence of data on charge distributions for neutron energies higher than 14 MeV, i.e., for the area of intermediate energies of interest, 3) a significantly higher intensity of proton sources as compared with neutron sources, which allows 2v measurements, 4) possibility of measurements of angular and energy distributions of prompt neutrons owing to the factor mentioned above, 5) the absence of fundamental physical distinctions in properties of fission of excited nuclei produced by irradiation by neutrons and protons, 6) the possibility of taking into account minor differences in the input channel of the reaction (pre-equilibrium decay, internal cascade) for protons and neutrons presented by modern models, and with the aim of developing the experimental database on fragment yield at intermediate energy neutrons, it seems reasonable

  1. to perform additional measurements for protons and, in particular, in the nearly “white” region of energies (above 100 MeV) to obtain more exact information;
  2. to perform test measurements for neutrons at several experimental points, for instance, at energies 60 MeV, 175 MeV, and 95 MeV (in the latter case, measurements are to be carried out under the conditions providing the most correct comparison, i.e., at high kinetic energies of fragments);
  3. to perform a detailed comparison of all available and new data for protons and neutrons that can be obtained in the framework of the project, to perform a quantitative estimation of the degree of coincidence and find empirical (or semi-empirical) relations for all parameters of mass and charge yields and their dependences on the nucleon structure of a fissioning nucleus and excitation energy.


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