An experimental study of mass asymmetry in subbarrier photofission of 238U

A measurement of symmetric and asymmetric photofission yields of²³⁸U in the energy region from 4.5 MeV to 6.5 MeV has been performed. As γ-source the bremsstrahlung from a microtron has been used. The fission yields are obtained from β-counting of chemically separated isotopes ¹¹¹Ag, ¹¹⁵Cd and ¹¹⁷Cd for symmetric fission and ¹³⁹Ba for asymmetric fission. Some structures in the yield curves at about 5.3 MeV and a pronounced maximum at about 6.0 MeV in the valley-to-peak ratio in the fission fragment yield distribution are observed. It is suggested that these effects are connected with the double-hump fission barrier concept and indicate differences between fission barriers for symmetric and asymmetric fission.     

Evidence on the influence of fission channels on fission phenomena

Partial fission channel widths can be extracted from fission cross-section analysis only up to a rotation in the space of fission vectors. We propose to obtain a physically significant basis for fission vectors taking into account information from other fission characteristics such as the valley-to-peak ratio of the mass distribution and the number of prompt neutrons emitted per fission event. Fission widths and other channel characteristics are calculated for 0⁺ resonances in low energy neutron induced fission of ²³⁹Pu.     

Description of true and delayed ternary nuclear fission accompanied by the emission of various third particles

The mechanisms and the features of the main types of nuclear ternary fission (that is, true ternary fission, in which a third particle is emitted before the rupture of the fissioning nucleus into fragments, and delayed ternary fission, in which a third particle is emitted from fission fragments going apart) are investigated within quantum-mechanical fission theory. The features of T-odd asymmetry in true ternary nuclear fission induced by cold polarized neutrons are investigated for the cases where alpha particles, prescission neutrons, and photons appear as third particles emitted by fissioning nuclei, the Coriolis interaction of the spin of the polarized fissioning nucleus with the spin of the third particle and the interference between the fission amplitudes for neutron resonances excited in the fissioning nucleus in the case of projectile-neutron capture being taken into account. For the cases where third particles emitted by fission fragments are evaporated neutrons or photons, T-odd asymmetries in delayed ternary nuclear fission induced by cold polarized neutrons are analyzed with allowance for the mechanism of pumping of large fission-fragment spins oriented orthogonally to the fragment-emission direction and with allowance for the interference between the fission amplitudes for neutron resonances.     

The quantum and thermodynamical characteristics of fission taking into account adiabatic and nonadiabatic modes of motion

In the framework of the quantum theory of spontaneous and low-energy induced fission, the nature of quantum and thermodynamical properties of a fissioning system is analyzed taking into account adiabatic and nonadiabatic modes of motion for different fission stages. It is shown that, owing to the influence of the Coriolis interaction, the states of the fissile nucleus and of primary fission products are cold and strongly nonequilibrium. The important role of superfluid and pairing nucleon-nucleon correlations for binary and ternary fission is demonstrated. The mechanism of pumping of high values of relative orbital momenta and spins of fission fragments for binary and ternary fission and the nonevaporation mechanism of formation of third particles for ternary fission are investigated. The anisotropies and P-odd, P-even, and T-odd asymmetries for angular distributions of fission products are analyzed. The text was submitted by the author in English.     

Possible Mechanisms of Ternary Fission in the 197Au+197Au System at 15 A MeV

Ternary fission in ^197Au＋^197Au collisions at 15 A MeV is investigated by using the improved quantum molecular dynamical （ImQMD） model. The experimental mass distributions for each of the three fragments are reproduced for the first time without any freely adjusting parameters. The mechanisms of ternary fission in central and semicentral collisions are dynamically studied. In direct prolate ternary fission, two necks are found to be formed almost simultaneously and rupture sequentially in a very short time interval. Direct oblate ternary fission is a very rare fission event, in which three necks are formed and rupture simultaneously, forming three equally sized fragments along space-symmetric directions in the reaction plane. In sequential ternary fission a binary division is followed by another binary fission event after hundreds of fm/c.     

Analysis of prompt fission neutron spectrum and multiplicity for 237Np（n,f） in the frame of multi-modal Los Alamos model

The improved version of Los Alamos model with the multi-modal fission approach is used to analyse the prompt fission neutron spectrum and multiplicity for the neutron-induced fission of 237Np. The spectra of neutrons emitted from fragments for the three most dominant fission modes (standard Ⅰ, standard Ⅱ and superlong) are calculated separately and the total spectrum is synthesized. The multi-modal parameters contained in the spectrum model are determined on the basis of experimental data of fission fragment mass distributions. The calculated total prompt fission neutron spectrum and multiplicity are better agreement with the experimental data than those obtained from the conventional treatment of the Los Alamos model.     

The study of prompt and delayed muon induced fission

Mass yield and total kinetic energy release (TKE) distributions of fragments from prompt and delayed muon induced fission, separately, have been measured for the isotopes²³⁵U,²³⁸U,²³⁷Np and²⁴²Pu. The distributions from prompt muon induced fission are compared with the corresponding distributions from hadronic reactions and from spontaneous fission (s.f.). The distributions from the delayed muonic fission processes are compared to the distributions for neutron and proton-induced fission. No mass distributions measured in the prompt muonic fission process show any signature, which can be attributed to the presence of the muon. Differences observed between the TKE distributions of prompt muon induced and hadron induced fission can be explained by the screening effect of the negative charge of the muon bound in the orbit of one of the fission fragments. The observed yield of symmetric muon induced fission was found to be defined merely by the value of the excitation energy.     

Mass and energy dependence of pion-induced fission

Data for fission induced by pi meson beams from 80 to 500 MeV are presented for nuclei from Fe through Pu as measured by solid state track detectors. The general trends for binary fission withπ ⁺ are reproduced fairly well by a calculation in the ‘high excitation’ limit with standard level density and fission barrier parameters, butπ ^? data are underpredicted. A universal dependence of the binary fission probabilities with the fissility (Z±1)²/A is found to be valid for both pion beam charges for all beam energies below the delta resonance. Probabilities for observing three fragments withπ ⁺ are not reproduced by a ternary fission application of the model found to work for binary fission.     

Mechanisms for emission of4He in the reactions of 334 MeV40Ar with238U

Emission of⁴He in the reaction 334 MeV⁴⁰Ar+²³⁸U has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the⁴He evaporative spectra from fusion fission the composite system is shown to be the predominant contributor; whereas, for sequential fission the dominant emission is from the fragments. This result demonstrates a correlation between evaporative emission probability and lifetime expectancy of the composite system. To account for the observed⁴He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the⁴He spectra from both fusion fission and sequential fission exhibit higher intensities and larger energies than those expected from purely evaporative processes. This forward-peaked component must be related to a very rapid or pre-thermalization stage of the reaction. At backward angles yet another component is observed for fusion fission. As it is sensitive to the fragment masses but does not carry the kinematic shift characteristic of their full acceleration, this component must originate near to the time of scission. The average⁴He energy for this component is approximately 17 MeV (c.m.), and its intensity is correlated with a plane perpendicular to the fission fragment separation axis. These signatures are similar to those for long range alpha particle emission in low energy fission. Alpha particles evaporated from the composite nuclei in fusion-fission reactions are shown to be preferentially associated with fission events which result in the more symmetric masses. This result is consistent with the notion that mass asymmetric fission is a faster process than symmetric fission. Such a correlation between mass asymmetry and lifetime is an essential part of the “fast fission” or “quasifission” idea, which has attracted much current attention.     

Sub-Coulomb transfer and fission in collisions of129Xe,132Xe and136Xe with238U

Integral cross sections for fission and for one- and two-neutron transfer reactions in the system¹³²Xe+²³⁸U were measured radiochemically in the energy range 0.7≦E/E _Coul≦1. The excitation functions for fission and transfer are found to be essentially parallel below 0.85×E _Coul. Even at the lowest energies the transfer cross sections exceed the fission cross section by more than one order of magnitude. With the other projectiles¹²⁹Xe and¹³⁶Xe different transfer cross sections illustrating their sensitivity for the ground stateQ-values,Q _gg , are observed while the fission cross sections are the same as in the¹³²Xe +²³⁸U reaction. The fission data are interpreted in terms of a continuous transition between Coulomb fission and several transfer-induced fission processes.     

Determination of fragment isotopic yields in the fission of 252Cf accompanied by light charged particles

High resolution measurements of prompt γ-rays emitted from fission fragments accompanied by light charged particles (LCP) and those emitted from normal binary fission fragments in ²⁵²Cf fission were simultaneously carried out using a 30 cm³ Ge(Li) detector. A “sandwich” type of arrangement in the source-detector assembly was used to eliminate Doppler broadening of the γ-ray lines in both cases. About thirty γ-ray lines were assigned to specific fission fragments by comparison with the published results for binary fission, and the ratios of intensities of these γ-ray lines in LCP fission and binary fission were determined. Assuming that 2⁺ → 0⁺ transition intensities of doubly even isotopes are a measure of their yields, the yields of several such isotopes in LCP accompanied fission relative to that in binary fission were obtained.     

Isotopic distributions of Rb and Cs in the proton and neutron induced fission of 238U

The statistical model for fission is used to calculate the isotopic distribution of Rb and Cs in the fission of ²³⁸U induced by high-energy protons and reactor neutrons. The double-humped behaviour of the isotope yield curve as observed in high-energy proton-induced fission and the experimentally observed hump on the neutron-rich side in neutron-induced fission are semi-quantitatively reproduced.     

Ternary fission induced by 540 MeV Fe ions on uranium

The ratio of ternary to binary fission for uranium irradiated with 540 MeV Fe ions was found to be (4.3 ±0.7)%, when fission fragments in the forward hemisphere of the laboratory system are investigated. The detector was mica, heated after irradiation for 1 hour at 500°C, so as to reveal only fission fragments as tracks. The relatively high yield of ternary fission is interpreted qualitatively on the basis of potential-energy curves for binary fission and prolate ternary fission. The two curves appear to be fairly similar for the two decay modes.     

Fission of 232Th, 238U and 235U induced by negative muons

The absolute yields of prompt and delayed fission induced by negative muons in ²³²Th, ²³⁸U and ²³⁵U have been measured. The delayed fission yields are much lower than could be predicted from Γ_n/Γ_f systematics for 15–20 MeV nuclear excitation. The systematics of prompt fission yields are compared with recently obtained photofission data. It is suggested that prompt fission can be used for investigating the channel structure of the fission barrier.     

Resonances in the isomeric and prompt fission probabilities of240Pu

The prompt and isomeric fission probabilities of²⁴⁰Pu have been studied using the²³⁹Pu(d,p) reaction. A number of resonances are observed in the subbarrier population of the 4 ns fission isomer for excitation energies between 4.0 and 6.2 MeV. Apart from a structure at 4.3 MeV, they do not coincide with transmission resonances found in prompt fission. Calculations with an extended doorway state model which simultaneously reproduce the measured delayed and prompt fission probabilities yield revised fission barrier parameters as well as spectroscopic information on fission and gamma widths of highly excited states in the second minimum.     

Investigation of the reaction <Superscript>208</Superscript>Pb(<Superscript>18</Superscript>O, <Emphasis Type="Italic">f</Emphasis>): Folding angular distributions of fission fragments and gamma-ray multiplicity

Correlations between folding angular distributions of fission fragments and the gamma-ray multiplicity are studied for ¹⁸O + ²⁰⁸Pb interactions at energies of the beam of ¹⁸O ions in the range E _lab = 78–198.5 MeV. The probabilities are determined for complete-and incomplete-fusion processes inevitably followed by the fission of nuclei formed in these processes. It is found that the probability of incomplete fusion followed by fission increases with increasing energy of bombarding ions. It is shown that, for the incomplete-fusion process, folding angular distributions of fission fragments have a two-component structure. The width of folding angular distributions (FWHM) for complete fusion grows linearly with increasing energy of ¹⁸O ions. The multiplicity of gamma rays from fission fragments as a function of the linear-momentum transfer behaves differently for different energies of projectile ions. This circumstance is explained here by the distinction between the average angular momenta of participant nuclei in the fusion and fission channels, which is due to the difference in the probabilities of fission in the cases where different numbers of nucleons are captured by the target nucleus.     

Symmetric and asymmetric fission of Ac isotopes near the fission threshold

Fission probabilities and fragment anisotropies were investigated for fission of ²²⁷Ac and ²²⁸Ac by means of direct reactions with a ³He beam on a ²²⁶Ra target. Triple-humped mass distributions are found also for excitation energies where second-chance fission is excluded. The fission barrier is higher for symmetric fission compared to that for asymmetric fission by 1.2 and 2.0 MeV, respectively, suggesting that the character of the mass split is already predetermined at the saddle point.     

Multimode approach to 241Am and 237Np fission induced by 660-MeV protons

The results obtained by measuring cross sections for the formation of fragments originating from ²⁴¹Am and ²³⁷Np fission induced by 660-MeV protons are presented. The charge and mass distributions of fragments are analyzed within the multimode-fission model, symmetric and asymmetric fission channels being separated. The contributions of various fission components are estimated, and the fission cross sections for the ²⁴¹Am and ²³⁷Np nuclei are calculated along with the fissilities of these nuclei.     

Fission reactions of 469-MeV56Fe+238U: Detection of4He/1H emission from pre- and post-fission sources

The emission of⁴He and¹H has been measured in coincidence with fission for reactions of 469-MeV⁵⁶Fe+²³⁸U. By using a gas-ionization telescope in kinematic coincidence with a position-sensitive avalanche detector, the folding angle between two fission fragments was determined in order to distinguish fusion reactions from fission following smaller-momentum-transfer collisions. In both fusion fission and sequential fission reactions, the⁴He/¹H energy spectra are relatively narrow with relatively flat angular distributions at backward angles and become broader in energy with enhanced cross-sections at forward angles. The extent of forward peaking is significantly greater for peripheral collisions than for central collisions. The light-charged-particle multiplicities are quite similar for⁴He and¹H, being much larger for fusion fission than for sequential fission. Detailed comparisons of the spectral shapes with Monte Carlo simulations of reaction kinematics impose strong constraints on the participation of different emission sources. We find important contributions to the observed⁴He/¹H emission both from accelerated fragments (FE) and from the composite system prior to fission (CE). For⁴He emission, the multiplicity of CE is much larger for fusion fission than for sequential fission, possibly as a consequence of the higher spins and shorter reaction times associated with deeply inelastic and quasi-elastic processes. For¹H emission, a corresponding but somewhat smaller difference is observed for the CE multiplicities. An excess of⁴He/¹H particles, found at forward angles in both fusion and sequential fission processes, cannot be attributed to evaporative emission from any fragments and therefore must originate in pre-thermalization emission.     

Fission-to-spallation ratio and fission dynamics manifestation

Fission of highly excited nuclei is affected by the viscous character of the system motion in deformation coordinates as is reported for very heavy nuclei with Z ⩾ 90. The long-time-scale fission was proved for such systems formed in heavy-ion-induced reactions. The overdamped diabatic motion may influence also fission of the spallation-residue products in reactions with protons at intermediate energy. In the present work, the experimental results on fission-to-spallation ratio are analyzed and the evidences for the long-time-scale fission are found in the fission excitation functions for medium-mass targets with Z = 70–75. The text was submitted by the authors in English.