Study of mass spectra of light-actinide fission isotopes Kr and Xe in the framework of a new statistical approach

Results on light-actinide fission fragments Kr and Xe yields calculated within the framework of a new statistical approach are presented. The proposed statistical method is derived from the equilibrium condition of the ensemble of all possible fission fragments. Using ²³⁷Np isotope as an example, we investigate the influence of excitation energy (nuclear temperature) and nuclear particle emission on the formation of mass fission channels. The results obtained are compared with the experimental data on Kr and Xe isotope yields at the fission of Th, U, Np, and Pu.     

Evidence for Coulomb-field induced light-particle emission during scission

Energy and angular correlations of protons and α particles were measured in coincidence with fission fragments in the reactions 200 MeV, 254 MeV ³⁷Cl + ¹²⁴Sn and 318 MeV ²⁸Si + ¹⁴¹Pr. The light-particle yield is dominated by statistical emission from composite nucleus and from fission fragments, but α spectra also display the characteristics of evaporation during scission. We give Coulomb-field induced light-particle emission as a novel and hitherto only consistent explanation of this emission process.     

Photonuclear reactions of actinides in the giant dipole resonance region

Photonuclear reactions at energies covering the giant dipole resonance (GDR) region are analyzed with an approach based on nuclear photoabsorption followed by the process of competition between light-particle evaporation and fission for the excited nucleus. The photoabsorption cross-section at energies covering the GDR region is contributed by both the Lorentz-type GDR cross-section and the quasi-deuteron cross-section. The evaporation-fission process of the compound nucleus is simulated in a Monte Carlo framework. Photofission reaction cross-sections are analyzed in a systematic manner in the energy range of ∼ 10-20 MeV for the actinides ²³²Th , ²³⁸U and ²³⁷Np . Photonuclear cross-sections for the medium-mass nuclei ⁶³Cu and ⁶⁴Zn , for which there are no fission events, are also presented. The study reproduces satisfactorily the available experimental data of photofission cross-sections at GDR energy region and the increasing trend of nuclear fissility with the fissility parameter Z ²/A for the actinides.     

Fission Energy Surfaces and Ternary Fission

A three-center phenomenological model, able to explain qualitatively the recently obtained experimental results concerning the quasimolecu-lar stage of a light-particle accompanied fission process is presented. It was derived from the liquid drop model under the assumption that the aligned configuration, with the emitted particle between the light and heavy fragment, is reached by increasing continuously the separation distance, while the radii of the heavy fragment and of the light particle are kept constant. In such a way, a new minimum appears in the deformation energy at a separation distance very close to the touching point. This minimum allows the existence of a short-lived quasi-molecular state, decaying into the three final fragments. The influence of the shell effects is discussed. The half-lives of some quasimolecular states which could be formed in the ¹⁰Be and ¹²C accompanied fission of ²⁵²Cf are roughly estimated to be of the order of 1 ns and 1 ms, respectively.     

Deformation parameter changes in fission mass yields within the systematic statistical scission-point model

The fission fragment mass-yields are evaluated for pre-actinide and actinide isotopes using a systematic statistical scission point model. The total potential energy of the fissioning systems at the scission point is presented in approximate relations as functions of mass numbers,deformation parameters and the temperature of complementary fission fragments. The collective temperature, Tcoll, and the temperature of fission fragments, Ti, are separated and the effect of collective temperature on mass yields results is investigated. The fragment temperature has been calculated with the generalized superfluid model. The sum of deformation parameters of complementary fission fragments has been obtained by fitting the calculated results with the experimental data. To investigate the transitions between symmetric and asymmetric modes mass yields for pre-actinide and heavy actinides are calculated with this model. The transition from asymmetric to symmetric fission is well reproduced using this systematic statistical scission point model. The calculated results are in good agreement with the experimental data with Tcoll= 2 Me V at intermediate excitation energy and with T_(coll)= 1MeV for spontaneous fission.Despite the Langevin model, in the scission point model, a constraint on the deformation parameters of fission fragments has little effect on the results of the mass yield.     

Multi-modal analysis for low energy neutron-induced fission of ~(235)U

Low energy neutron induced fission of ~(235)U is studied in the framework of the multi-modal fission model. The fission fragment properties, such as the yields, the average total kinetic energy distribution and the average neutron separation energy, are investigated for incident neutron energies from thermal to 6.0 MeV. The multi-modal fission approach is also used to evaluate the prompt fission neutron multiplicity and spectra for the neutron-induced fission of ~(235)U with an improved version of the Los Alamos model for incident neutrons below the (n, nf) threshold. The three most dominant fission modes are taken into account. The model parameters are determined on the basis of experimental data. The calculated results are in good agreement with the experimental data.     

Asymmetry of mass and charge division in spontaneous fission

The order-disorder model has been used to explain asymmetry of mass and charge division and related phenomena in fission. According to this model the fission process involves two steps consisting of charge polarisation into two ‘impending fragment clusters’ with beta stable neutron numbers and subsequent distribution of the balance neutrons between the two. Mode of elemental division of the fissioning nuclei is attributed to the charge polarisation in the first step. Theory of reaction rate has been applied to the system. The frequency term is obtained by applying the conditional stochastic process under charge polarisation constraint and the energy-dependent term is given by the condition of minimum in free energy of the system. Using this, the relative probability of polarisation into given charge pair is arrived at. The model uses stable neutron numbers for the charges as the only input. No explicit assumption or quantification on the preference of formation of shell closure species in fission is necessary. The statistics developed on the principle of equala priori probability of all charge polarisation is used. The shell effects come into play only in deciding a stable neutron number for the charges. The total isotopic yield distribution for a number of fission reactions shows asymmetry in the actinide region which reduces with increasing mass/charge of the fissioning nuclide and bunching of the higherz peaks. Although the mass yields obtained therefrom for a number of fission reactions agree with experimental results, the peaks of the distributions are slightly shifted away from the symmetric point and the distributions are somewhat narrower. Charge distribution parameters obtained from these results are also presented.     

A calculation of nuclear fissilities for medium-heavy nuclei

The results of a fission-spallation competition calculation for medium heavy nuclei are presented. The fission probabilities were calculated with different sets of fission barrier heights. A comparison with experimental fissility values for both photons and alpha-particles shows that a modified liquid drop model due to Krappe and Nix yields fission barrier heights in better agreement with experimental trends than are given by earlier versions of the liquid drop model. Quantitative determination of the fission barrier heights is prevented by uncertainties inherent in existing experimental data as well as in the fission-spallation competition calculations.     

Dynamic treatment of ternary fission

The new dynamic model of light charged particle (LCP) formation in ternary fission is presented. The model is based on the assumption that light particles are formed as a result of two random neck ruptures during the time interval about one single-particle period. The connection of the final stage of ternary fission and of the saddle point descent stage was obtained in the framework of the density moments method. The analysis of LCP formation has shown that LCP mass and charge distributions are strongly governed by statistical nucleon exchange in the LCP-light fragment double system. New semiclassical expressions for the calculations of LCP yields and relative ternary fission probability are given. The results of calculations are in satisfactory agreement with the experimental data.     

高温下复合核的热裂变速率

在核裂变的动力学描述中,考虑到质量随坐标的连续变化,提出了自由能有效势,导出了一个新的裂变速率公式.改进后的公式对于核温度与势能位垒高度相当的热核裂变同样适用,其结果比Kramers公式更接近于断点裂变速率的朗之万模拟值.     

Angular distributions,relative orbital angular momenta,and spins of fragments originating from the binary fission of polarized nuclei

The angular distributions of fragments originating from the binary fission of odd and odd-odd nuclei capable of undergoing spontaneous fission that are polarized by a strong magnetic field at ultralow temperatures and from the low-energy photofission of even-even nuclei that is induced by dipole and quadrupole photons are investigated. It is shown that the deviations of these angular distributions from those that are obtained on the basis of the A. Bohr formula make it possible to estimate the maximum relative orbital angular momentum of fission fragments, this estimate providing important information about the relative orientation of the fragment spins. The angular distributions of fragments originating from subthreshold fission are analyzed for the case of the ²³⁸U nucleus. A comparison of the resulting angular distributions with their experimental counterparts leads to the conclusion that the maximum relative orbital angular momentum of binary-fission fragments exceeds 20, the fragment spins having predominantly a parallel orientation. The possibility is considered for performing an experiment aimed at measuring the angular distributions of fragments of the spontaneous fission of polarized nuclei in order to determine both the spins of such nuclei and the maximum values of the relative orbital angular momenta of fission fragments.     

Trap-assisted studies of odd, neutron-rich isotopes from Tc to Pd

We review the present and future of trap-assisted structure studies of odd, neutron-rich Tc, Ru, Rh and Pd isotopes at the limits of present experimental techniques. These nuclei of refractory elements are produced in light-particle induced fission and filtered by their mass number with the IGISOL mass separator. Further mass separation with the JYFLTRAP Penning trap system provides a clean, monoisotopic beam perfectly suited for precise nuclear spectroscopy. Connecting the IGISOL and the JYFLTRAP facilities to the recently installed MCC30/15 cyclotron opens new prospects for post-trap spectroscopy of very exotic, neutron-rich nuclei.     

The fission width of excited nuclei

An interpolation formula for the fission width is obtained valid for small and large values of the viscosity parameter and assumptions concerning the shape of the potential energy barrier. Some inaccuracy in the commonly used statistical model expression for Γ_f is discussed.     

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.     

On experimental potentialities of correlation analysis of the inhomogeneously broadened spectra

The potentialities of the correlation technique with respect to extracting the homogeneous line profile from an inhomogeneously broadened spectrum are analyzed by computer simulation. The signal-to-noise ratio for autoconvolution of the homogeneous line profile, obtained by statistical processing of an array of realizations of the inhomogeneously broadened spectrum for different experimental conditions and different values of parameters of the system is calculated. A general form of the dependence of the signal-to-noise ratio on the magnitude of the accumulated signal is established. An interpolation formula that expresses this dependence in an analytical form is obtained and a simple procedure is proposed that allows one to plot this dependence for specific experimental situations.     

P-odd, P-even, and T-odd asymmetries in true quaternary fission of nuclei

The coefficients of P-odd, P-even, and T -odd asymmetries for a third and a fourth prescission particle emitted in the true quaternary fission of nuclei that was induced by polarized cold neutrons were studied on the basis of quantum-mechanical fission theory. By using non-evaporation (nonadiabatic) mechanisms of light-particle emission, these coefficients were compared with the analogous coefficients for prescission third particles emitted in the ternary fission of nuclei.     

Influence of the shell effects on fission fragment angular anisotropies

The influence of the damping shell corrections with increasing excitation energy on the fission fragment angular anisotropies is considered. In the framework of the statistical approach to nuclear fission, experimental data on fission fragment angular anisotropies obtained in the ⁴He + ²³⁸U reaction is analyzed. Information about the energy dependence of the shell corrections is obtained from this analysis.     

Implications on the chiral quark bag from the deuteron asymptoticD-state

Fission yields of^124–132In in the thermal neutron fission of²³⁵U were determined for the first time. Charge displacementsΔZ=Z _p?Z _UCD were calculated for the corresponding mass chains. Both fission yields and charge displacement values were compared with those obtained by systematics by Wahl et al. and Wolfsberg. It was found that the fission yields of the In isotopes obey the gaussian distribution. The displacement function of Wolfsberg seems to give the better representation of the experimental results. Half-lives of^124–131In were determined from beta decay curves.