Angular distributions of light charged particles from 252Cf fission in the ranges 0–46° and 134–180°

Alpha particles, tritons, deuterons and protons accompanying ²⁵²Cf fission were registered in coincidence with both fission fragments by means of a system containing two-dimensional position-sensitive silicon detectors. Angular distributions, kinetic energy spectra of light charged particles as well as mass distributions of fission fragments in coincidence with light charged particles were measured. The experimental results are compared with some theoretical models.     

Multiparameter studies of polar emission in 236U fission

The energy distributions and relative intensities of protons, deutrons, tritons and α-particles emitted along the fission axis during thermal neutron fission of ²³⁵U were measured simultaneously with both fission fragment energies. The mass distributions of fragments, the total kinetic energy (TKE), the dependence of the mean TKE on the fragment mass, as well as the mean kinetic energy dependence of polar particles on the fragment mass and energy were subsequently deduced from these data. Although some experimental results agree remarkably well with the hypothesis that polar particles are evaporated in flight from fission fragments, the general conclusion is that these particles are emitted according to some other mechanism.     

Comparison of the spontaneous fission of 244Cm and 252Cf: (I). Fragment masses and kinetic energies

The energy of both fission fragments of ²⁴⁴Cm and ²⁵²Cf, respectively, was measured in coincidence with the prompt neutrons (see part II). Energy calibration of the surface-barrier detectors was done after the method of Schmitt et al. with the ²⁵²Cf fragments. Mean values and rms widths of the mass and energy distributions of both isotopes are calculated and compared with the results of other authors. The total kinetic energy of ²⁴⁴Cm fragments is at least as high as that of ²⁵²Cf fragments.     

Prescission neutrons in the fission of 235U and 252Cf nuclei

Experimental data obtained previously for the energy-angular distribution of neutrons originating from the fission of ²⁵²Cf (spontaneous fission) and ²³⁵U (thermal-neutron-induced fission) nuclei are analyzed, the angle being measured with respect to the direction of fission-fragment motion. A regularity common to all independent experiments is revealed: at an angle of about 90°, there exists an excess of neutrons (30% for ²⁵²Cf and 60% for ²³⁵U) that does not admit explanation within the model of neutron emission from fully accelerated fragments. Two possible explanations of this experimental fact—neutron emission during the acceleration process and the existence of an additional source of neutrons (predominantly, prescission neutrons)—are considered. It is shown that the latter conjecture describes the observed features for both nuclei more adequately. The total yield of prescission neutrons and their energy and angular distributions are determined.     

Emission properties of fission fragments and their isomeric ratios

The properties of neutron emission from fragments formed in the spontaneous fission of ²⁵²Cf and in the thermal-neutron-induced fission of ²³⁵U are analyzed on the basis of the statistical model of nuclear reactions. Upon extracting the mean excitation energies of fission fragments from experimental data on the mean multiplicities of neutrons, the observables of neutron emission can be described over wide ranges of total kinetic energies and masses. The observed values of mean fragment spins are also reproduced. A method for calculating the isomeric ratios of the independent yields of fission fragments that is based on the cascade-evaporation model of excited-nucleus decay is employed to describe experimental data on ²³⁵U fission induced by thermal neutrons and on ²³⁸U fission induced by alpha particles. The effect exerted on the isomeric ratios for fission fragments by two different assumptions on the spin distributions of primary-fragment populations—the assumption of the distribution associated with rotational degrees of freedom and the assumption of the distribution associated with the internal degrees of freedom of fully accelerated fragments—is investigated.     

Scission configuration in quaternary fission

Trajectory calculations have been carried out to obtain information about scission configuration in quaternary fission on the basis of observed angular-correlation and energy correlations between two alpha particles in the spontaneous quaternary fission of²⁵²Cf. A number of plausible hypotheses for the scission configuration were tested against the experimental observations on the two alpha particles. The role of mutual repulsion between two alpha particles at scission in deciding the final energy angular correlations has been examined. It was found that only one hypothesis regarding scission configuration is consistent with the experimental data.     

Generation of angular momentum of fission fragments in a cluster model

Based on the dinuclear system concept, the role of bending vibrations in creation of the angular momentum of primary fission fragments is investigated. For ²⁵²Cf spontaneous fission, the angular momenta of the fragments are calculated as a function of the neutron multiplicity and compared with available experimental data. Different cluster compositions of the ²⁵²Cf fission modes at the scission point are considered.     

Coaxial Fission of Nuclei to Three Commensurate Fragments

Trajectory calculations of the angular distribution of fragments originating from the spontaneous true ternary fission of ²⁵²Cf nuclei are performed with allowance for the rotation of the fissile-nucleus axis. This rotation arises upon scission because of the formation of fragment spins despite the conservation of zero total angular momentum at all fission stages. The calculations in question lead to the conclusion that a collinear tripartition is the most probable in spontaneous true ternary fission of ²⁵²Cf nuclei. This confirms experimental data, thereby creating the basis that is necessary for obtaining deeper insight into them.     

Angular momenta of fission fragments in the α-accompanied fission of 252Cf

For the first time, average angular momenta of the ternary fission fragments ^{100, 102}Zr, ¹⁰⁶Mo, ^{144, 146}Ba and ^{138, 140, 142}Xe from the α-accompanied fission of ²⁵²Cf were obtained from relative intensities of prompt γ-ray transitions with the use of the statistical model calculation. Average values of the angular momenta were compared with the corresponding values for the same fission fragments from the binary fission of ²⁵²Cf. Results indicate the presence of a decreasing trend in the average values of angular momenta induced in ternary fission fragments compared to the same binary fission fragments. On the average, the total angular momentum extracted for ternary fission fragments is ∼1.4 ℏ lower than in binary fission. Consequently, results indicate that the mechanism of the ternary α-particles emission may directly effect an induction of angular momenta of fission fragments, and possible scenarios of such mechanisms are discussed. Further, the dependence of the angular momenta of ¹⁰⁶Mo and ¹⁴⁰Xe on the number of emitted neutrons from correlated pairs of primary fragments was obtained also showing a decreasing dependence of average angular momenta with increasing number of emitted neutrons. Consequences are briefly discussed.     

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.     

Ternary fission fragmentation of <Superscript>252</Superscript>Cf for all possible third fragments

The ternary fragmentation of ²⁵²Cf for all possible third fragments has been investigated using the recently proposed three-cluster model within a spherical approximation and satisfying the condition A ₁ 3 \geq A ₂ 3 \geq A ₃ . The most probable ternary configurations in the fission of ²⁵²Cf accompanied with all possible third fragment mass numbers from A ₃ = 1 to 84 are predicted and their independent and overall relative yields are calculated. The calculations of the properly charge minimized potential energy surface (PES) and yield reveal that even-mass third fragments are more favored than odd ones. In the most probable configuration having the minimum in the potential energy and the maximum in yield, among the three fragments, at least one (or two) of the fragment(s) associates itself with the neutron (or proton) closed shell and in some cases even with the doubly closed shell. The calculated relative yields imply that next to ¹⁴C (the heaviest third fragment observed in the spontaneous ternary fission of ²⁵²Cf , ^{34, 36, 38}Si , ^{46, 48}Ar , and ^{48, 50}Ca are presenting themselves as the most favoured cases to be observed as the third particle in the spontaneous ternary fission of ²⁵²Cf .     

True ternary fission and the rotation effect

Path calculations for fragments of the spontaneous true ternary fission of a ²⁵²Cf nucleus are performed. The results are considered from the standpoint of symmetries underlying the generalized nuclear model, and substantiation of the physical picture of the coaxial emission of the fragments during true ternary fission. The calculations are performed with allowance for the ROT effect, which is regarded as a major disturbance in this scenario. The obtained results are in agreement with experimental data.     

Coherent bremsstrahlung and GDR width from Cf cold fission

The energy spectrum of the high energy γ-rays in coincidence with the prompt γ-rays has been measured for the spontaneous fission of ²⁵²Cf. The nucleus–nucleus coherent bremsstrahlung of the accelerating fission fragments is observed and the result has been substantiated with a theoretical calculation based on the Coulomb acceleration model. The width of the giant dipole resonance (GDR) decay from the excited fission fragments has been extracted for the first time and compared with the thermal shape fluctuation model (TSFM) in the liquid drop formalism. The extracted GDR width is significantly smaller than the predictions of TSFM.     

Ternary fission within statistical approach

The ternary system with a light nucleus between two heavy fragments is assumed to appear from the binary configuration near scission. The formation of a third light nucleus in the binary system is considered. The calculated charge distributions in spontaneous ternary fission of ²⁵²Cf and in induced ternary fission of ⁵⁶Ni are compared with the available experimental data. The neutron multiplicity from the fission fragments is described. The fine structures of the TKE-mass distribution are predicted.     

New indications of collinear tripartition in 252Cf(sf) studied at the modified FOBOS setup

An attempt has been made to search for collinear tripartition in spontaneous fission of ²⁵²Cf by using the FOBOS setup coupled with the neutron detector belt. A group of rare events were detected characterized by reduced total kinetic energy and total nuclear charge gated by the large neutron multiplicity measured. This fact is considered to be an experimental indication of collinear tripartition in ²⁵²Cf. The theoretical indication of the possible existence of the collinear cluster tripartition valley was obtained for the first time.     

Semi-microscopical description of the scission configuration in the neutronless fission of 252Cf

The neutronless fission of ²⁵²Cf is studied in the frame of a molecular model in which the scission configuration is described by two aligned fragments interacting by means of Coulomb (+ nuclear) forces. The study is carried out for different distances between the fragments tips and excitation energies. For a given deformation, the fragment's total energy is computed via the constrained Hartree-Fock + BCS formalism. The total excitation energy present in the fragments is supposed to contribute only to the fragments deformation and the asymptotic value of the kinetic energy is equated to the inter-fragment potential at scission. These two constraints are yielding a few fission channels for a fixed tip distance and excitation energy. Discarding those fission channels corresponding to a disequilibrium in the sharing of the excitation energy between the two fragments, we establish the most likely scission configurations for a specified excitation energy. Received: 24 September 1999     

Multiplicity distribution of prompt gamma rays in spontaneous ternary fission of252Cf

The first and the second moments of the multiplicity distribution of prompt gamma rays in spontaneous ternary fission of²⁵²Cf have been measured by the multiple coincidence technique. While both these moments were found to be nearly independent of the energy of the light charged particle accompanying the fission fragments, the width of the multiplicity distribution was larger than that in the case of normal binary fission by about 20%.     

Role of energy cost in the yield of cold ternary fission of 252 Cf

The energy costs in the cold ternary fission of ²⁵²Cf for various light charged particle emission are calculated by including Wong’s correction for Coulomb potential. Energy cost is found to be higher in cold fission than in normal fission. It is found that energy cost always increases with decrease in experimental yield in all the light charged particle emissions. The higher ground state deformation of the fragments, the odd–even effect and the enhanced yield in the octupole region observed in cold fission are found to be consistent with the concept of energy cost.     

Geometrical and dynamical state of the 252Cf nucleus at the scission point as determined from the experimental results of ternary fission

Resuits on the geometrical and dynamical states of fission fragments and α-particles are presented for the most probable mode of ternary fission, at the instant of release of the light particle. The distributions of the relevant physical quantities are obtained by means of a program calculating the trajectories of three point charges. A total of about 2 × 10⁵ combinations of the parameters, derived by subdividing the quantities into physically significant intervals, are used. The computed values are compared with experimental results and the distributions of the parameters are weighted by the spectrum of the α-parlicles. The dynamics of the system undergoing fission at scission point are characterized by the following quantities: (i) The total kinetic energy of fragments. (ii) The kinetic energy of the α-particle. (iii) The distance between the centres of charge of the fragments, (iv) The abscissa and Ordinate of the α-particle emission point, (v) The emission angle of the α-parlicle with respect to the line of flight of the light fragment. The distributions of above quantities are presented for the most probable model of spontaneous ternary fission of ²⁵²Cf The results obtained are discussed and relevant information on the dynamical properties of the last fission stage is presented.