Symmetric and asymmetric modes of 232 Th photofission at intermediate energies
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CLEI Experiment
of
232
Symmetric and Asymmetric Modes Th Photofission at Intermediate Energies N. A. Demekhina* and G. S. Karapetyan1)
Yerevan Physics Institute, Alikhanian Brothers str. 2, AM-375036 Yerevan, Armenia Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia Received December 18, 2008; in final form, May 21, 2009
Abstract—Yields of fragments originating from 232 Th photofission were measured at bremsstrahlungphoton endpoint energies of 50 and 3500 MeV. Charge and mass distributions of fission fragments were analyzed. On the basis of the model of multimode fission, symmetric and asymmetric channels are singled out in 232 Th photofission at intermediate energies. This decomposition made it possible to estimate the contributions of various fission components and the fissility of 232 Th. DOI: 10.1134/S1063778810010047
INTRODUCTION
energies [18, 19] and about the properties and structure of hot fissile nuclei at relativistic energies [20, 21]. In systematizing fission products, one often invokes the hypothesis of multicomponent fission [22, 23], employing it to decompose the curve of the fragment mass yield and to extract symmetric and asymmetric components. At first, such an analysis was performed for fragments originating from spontaneous fission [24] and for reactions involving lowenergy charged particles and neutrons [9, 25, 26]. Later on, this hypothesis was applied to separate fission components in reactions induced by protons [10, 12, 13] and neutrons in the energy region extending up to 500 MeV [27–29]. In studying photofission in a beam of bremsstrahlung photons, the decomposition of the mass yields of fission fragments makes it possible to single out the high-energy contribution to the total yield of fission fragments. In the present study, the mass yield of fragments originating from 232 Th photofission in the region of low and intermediate energies is analyzed for the first time with an eye to exploring the relationship between the contributions of symmetric and asymmetric components as a function of energy. A comparison of experimental data obtained in reactions involving particles of different sorts makes it possible to reveal general regularities of the process being studied.
Fission is one of the main processes in which the structure and properties of heavy nuclei manifest themselves most clearly. The change in the character of fission in response to the change in energy reflects the dependence of the properties of fissile nuclei on the excitation of the nuclear system being considered. Asymmetric fission, which is dominant at low energies, is characterized by a clear-cut manifestation of shell effects [1–6]. Symmetric fission, which is described predominantly by the classic liquid-drop model, prevails at higher energies [7, 8]. As the projectile energy changes, the properties of the charge and mass distributions of fission fragments undergo changes along with the branching fractions of the symmetric and asymmetric fission modes [9–12]. A global rearrangement of
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