Microscopic analysis of the fission barriers in <Superscript>256</Superscript>Fm and <Superscript>258</Superscript>Fm

The fission barriers of ²⁵⁶Fm and ²⁵⁸Fm have been analyzed in the HFB theory. The potential energy and the effective inertia parameter have been calculated in the two-dimensional deformation space of quadrupole and octupole moments. Fission paths for various octupole moments of the exit point from the fission barrier have been determined. The half-life along each path has been calculated. The shortest half-lives have been obtained for the paths with reflection symmetric shapes of nuclei in both the considered isotopes.     

Diabolic pair transfer and oscillating behavior of backbending in rotating nuclei

Quantum effects generate “channels” which guide a nucleus from ground state to scission. We present here the channels of²⁵⁸Fm and relate them to the measured kinetic energies of the fission products, to the mass asymmetry and to the relative abundance of the various fission components. The microscopic reason is discussed.     

Bimodality and charge splitting in fission of actinides

Within the scission point model the bimodality in fission of actinides is demonstrated to be related to different neighboring charge and mass splittings. This phenomenon is peculiar not only for the fission of heavy nuclei like ^256,258Fm and ^256,258,262No but also for fission of lighter actinides like ²³⁶U, ²⁴⁰Pu and ²⁵²Cf. The experiments are suggested to prove our interpretation of bimodality.     

Spontaneous fission and alpha-decay properties of neutron deficient isotopes 257−253104 and 258106

Spontaneous fission and α-decay of^253?257104 and ²⁵⁸106 were investigated in irradiations of ^204,206,208Pb with ⁵⁰Ti and ²⁰⁹Bi with ⁵¹V, respectively. New spontaneous fission activities were identified and assigned to ²⁵³104, ²⁵⁴104, and ²⁵⁸106. The half-lives were measured as T_1/2 = (48 _?10 ⁺¹⁴ ) μs for ²⁵³104, T_1/2 = (23 ± 3) μs for ²⁵⁴104, and T_1/2 = (2.9 _?0.7 ^+1.3 ) ms for ²⁵⁸106. No indication for α-decay of any of these isotopes was found. For the α-decay branching ratios bα limits corresponding to bα ≤ 0.1 for ²⁵³104, bα ≤ 0.015 for ²⁵⁴104, and bα ≤0.2 for ²⁵⁸106 were obtained. These results prove a reduced fission probability for nuclei with neutron numbers N=152 up to Z=106 and a steep decrease of the fission half-lives for neutron numbers N < 152 up to element 104. α-decay data of ²⁵⁷104 and ²⁵⁵104 have been improved. An isomeric state decaying by α-emission was identified in ²⁵⁷104 and attributed to a low lying 11/2- [725] state. A small α-decay branch for the even — even nucleus ²⁵⁶104, indicated in an earlier experiment at SHIP, was confirmed, allowing a better founded extrapolation of experimental masses for even — even nuclei up to ²⁶⁴Hs (Z=108), the heaviest even — even nucleus identified so far.     

Cluster approach to the structure of nuclei with <Emphasis Type="Italic">Z</Emphasis> ≥ 96

The properties of alternating parity bands in heavy nuclei ²³⁴Th, ^239–242U, ^241–245Pu, ^243–248Cm, ^245–250Cf, ^248–251Fm, ^249–254No, ^253–256Rf, and ²⁵⁸Sg are analyzed within the dinuclearsystem model. The model is based on the assumption that the cluster-type shapes are produced by the motion of the nuclear system in the mass-asymmetry coordinate. The energies of the low-lying states whose parity is opposite to the parity of the ground state are predicted for the first time. The text was submitted by the authors in English.     

The spontaneous fission of 224Fm and 246Fm

The average total kinetic energies for the fission products of the neutron-deficient isotopes ²⁴⁴Fm and ²⁴⁶Fm have been measured as 196 ± 4 and 199 ± 4 MeV, respectively. The values agree well with those expected on the basis of systematics.     

Some dynamical aspects of the fission process

The properties of the nuclear mass parameter and its influence on the fission process are discussed. In particular, correlations between dynamical and statical paths to fission are considered. The least action trajectories idea is used for finding the dynamical path to fission. Ritz method for the action integral minimalization is applied. An example of the nucleus ²⁵²Fm is considered.     

Cold events in thermal-neutron-induced fission of heavy nuclei

An interpretation of the cold fission events in thermal-neutron-induced fission of heavy nuclei is given. The descent from the saddle point is considered as a dynamical process with reversible coupling between collective and intrinsic degrees of freedom. The distribution function for the collective variables is expressed as a product of two terms: the adiabatical and the dynamical factors. A simple model for symmetric fission to study the mass distribution is presented. As example, the calculations are performed for the nucleus ²⁶⁴Fm. Gross features of the cold fission are discussed as well as the dependence of the theoretical mass distribution on the parameters of the model. Received: 29 April 1998 / Revised version: 11 September 1998     

Effect of pre-equilibrium emission on probing postsaddle nuclear dissipation with neutrons

Using the stochastic Langevin model coupled with a statistical decay model, we study the influence of pre-equilibrium(PE) emission on probing postsaddle friction(β) with neutrons. A postsaddle friction value of(14-16.5) ×10~(21)s~(-1) and(11-13) ×10~(21)s~(-1) is obtained from comparing calculated and measured prescission neutron multiplicities of heavy fissioning systems248 Fm and256Fm in the absence and presence of the deformation factor.Moreover, it is found that a larger β is required to fit multiplicity data after the PE effect is accounted for, and that the effect becomes stronger when more energy is removed by PE particles. Our findings suggest that, to more accurately determine the postsaddle friction strength through the measurement of prescission neutrons, in addition to incorporating the contribution of PE evaporation source into the experimental multi-source analysis for particle energy spectra in coincidence with fission fragments, on the theoretical side, it is very important to make a precise evaluation of the energy that PE emission carries away from excited compound systems produced in heavy-ion fusion reactions.     

Neutron multiplicity at spontaneous fission of 246Fm

The neutron-deficient isotope ²⁴⁶Fm , produced in the complete fusion reaction ⁴⁰Ar$ + $²⁰⁸Pb , was investigated. The main goal of the experiment was to determine the neutron multiplicity at spontaneous fission of this isotope. For experiments aimed at the study of spontaneous fission of transfermium nuclei improvements in the focal plane detector system of recoil separator VASSILISSA have been made. A neutron detector consisting of 54 ³He -filled counters has been mounted around the focal-plane detector chamber. From the experimental data the average number of neutrons per spontaneous fission of ²⁴⁶Fm was determined ( $ \bar{{\nu}}$ = 3.55±0.5) .     

Spontaneous fission of <Superscript>256</Superscript>Rf,new data

Spontaneous fission properties of the short-lived neutron-deficient ²⁵⁶Rf nucleus produced in the complete fusion reaction with a beam of multiply charged heavy ⁵⁰Ti ions from the U-400 cyclotron (FLNR, JINR) are experimentally investigated. Its half-life and decay branching ratio are measured. The average number of neutrons per spontaneous fission of ²⁵⁶Rf (\(\bar v = \;4.47 \pm \;0.09\)) is determined for the first time.     

Study of fusion cross-section in heavy-ion fusion-fission reactions at around fusion barrier energies using the Langevin dynamical approach

The neutron detector with ³He -filled counters placed in the focal plane of the VASSILISSA separator is used for measuring the average number and determining the multiplicity distribution of prompt neutrons from the spontaneous fission of heavy short-lived isotopes. The test reaction $\ensuremath \mathrm{{}^{48}Ca}+\mathrm{{}^{206}Pb}=2{\rm n}+\mathrm{{}^{252}No}$ is used for tuning the separator settings and calibrating the detector system with the spontaneous fission of the ²⁵²No . The average neutron number per ²⁵²No spontaneous fission event is as large as $\ensuremath \bar{\nu}=4.06 \pm 0.12$ . The short-lived heavy isotope ²⁴⁴Fm , produced in the complete fusion reaction ⁴⁰Ar + ²⁰⁶Pb , is investigated. The average number of neutrons per spontaneous fission of ²⁴⁴Fm from the experimental data ( $\ensuremath \bar{\nu}=3.3 \pm 0.3$ is determined for the first time.     

Is triple fission predominant in 114298Z?

A comparison of HF-calculations for Fm, No and ₁₁₄Z indicate triple clustering at the fission barrier of ₁₁₄²⁹⁸Z getting more pronounced beyond the saddle. Neutron multiplicities and fragment kinetic energies are predicted assuming super clustering plus valence nucleons as indicated by the HF densities.     

Nuclear molecules in ternary fission

Experimental results on particle accompanied cold fission of ²⁵²Cf suggest the existence of a short-lived quasi-molecular state. Within our three-center phenomenological model, described in this paper, we found a possible explanation based on a new minimum in the deformation energy at a separation distance very close to the touching point. Half-lives of some quasimolecular states which could be formed in ¹⁰Be accompanied fission of ²³⁶U, ²³⁶Pu, ²⁴⁶Cm, ²⁵²Cf, ^252,256Fm, ^256,260No, and ²⁶²Rf are roughly estimated.     

Spontaneous fission in actinides and heavy elements: A semiclassical view

The systematics of spontaneous fission in actinides and heavy elements are investigated within the framework of the semiclassical quantization. The interaction barrier is calculated by using the asymmetric two-centre shell model and its appropriate parameterization generates the analytical expression for the tunneling probability. The powerful semiclassical quantization technique lifts the degeneracy between the degenerate levels just after tunneling across the barrier. Our results clearly show that dissipation enhances the tunneling rate. It has also been seen that the sub-barrier fission from the excited state is a slow process. An appropriate condition for the fission isomeric state is also pointed out. The survival probability of heavy elements is a remarkable outcome of our model. We have tested our model for nuclei ₉₂ ²³⁶U , ₉₈ ²⁵²Cf , ₁₀₀ ²⁵⁴Fm and ₁₀₂ ²⁵²No .     

Two-dimensional sequence of shapes for cold-fission of264Fm

The neutron-rich²⁶⁴Fm should be the best cold-fissioning nucleus, owing to the strong shell effect of the doubly magic fragments¹³²Sn. The optimum cold fission path in the plane of two independent shape coordinates, the separation distanceR and of the radius of the light fragmentR ₂, is determined. Werner-Wheeler inertia tensor and Yukawa-plus-exponential deformation energy are used to calculate the action integral. Microscopic shell and pairing corrections are based on the asymmetric two-center single-particle model. Different variation lawsR ₂=R ₂(R), for the fission trajectory were tested: polynomials of 2nd, 3rd, and 4th degree and an exponential function. The partial half-life for cold fission is estimated to be several orders of magnitude shorter than that of α-decay.     

Microscopic description of fission in neutron-rich plutonium isotopes with the Gogny-D1M energy density functional

The most recent parametrization D1M of the Gogny energy density functional is used to describe fission in the isotopes ^232-280Pu . We resort to the methodology introduced in our previous studies (Phys. Rev. C 88, 054325 (2013) and Phys. Rev. C 89, 054310 (2014)) to compute the fission paths, collective masses and zero point quantum corrections within the Hartree-Fock-Bogoliubov framework. The systematics of the spontaneous fission half-lives t _SF , masses and charges of the fragments in plutonium isotopes is analyzed and compared with available experimental data. We also pay attention to isomeric states, the deformation properties of the fragments as well as to the competition between the spontaneous fission and α-decay modes. The impact of pairing correlations on the predicted t _SF values is demonstrated with the help of calculations for ^232–280Pu, in which the pairing strengths of the Gogny-D1M energy density functional are modified by 5% and 10%, respectively. We further validate the use of the D1M parametrization through the discussion of the half-lives in ^242–262Fm. Our calculations corroborate that, though the uncertainties in the absolute values of physical observables are large, the Gogny-D1M Hartree-Fock-Bogoliubov framework still reproduces the trends with mass and/or neutron numbers and therefore represents a reasonable starting point to describe fission in heavy nuclear systems from a microscopic point of view.     

Spontaneous fission of neutron-deficient fermium isotopes and the new nucleus 241Fm

Spontaneous fission of neutron-deficient fermium isotopes (N ≤ 144) was investigated. The nuclei were produced in fusion-evaporation reactions of ⁴⁰Ar beams with ^{204,206–208}Pb targets. Excitation functions of the two-and three-neutron evaporation channels were measured. The new isotope ²⁴¹Fm was identified, and its decay properties were determined. A previously made assignment of a 0.8 ms spontaneous-fission activity to the isotope ²⁴²Fm could not be confirmed. Isomeric states were not observed in any of the investigated isotopes. The experiments were performed at the velocity filter SHIP of GSI in Darmstadt.     

Cluster pre-existence probability

Pre-existence probability of the fragments for the complete binary spectrum of different systems such as ⁵⁶Ni, ¹¹⁶Ba, ²²⁶Ra and ²⁵⁶Fm are calculated, from the overlapping part of the interaction potential using the WKB approximation. The role of reduced mass as well as the classical hydrodynamical mass in the WKB method is analysed. Within WKB, even for negative Q -value systems, the pre-existence probability is calculated. The calculations reveal rich structural information. The calculated results are compared with the values of preformed cluster model of Gupta and collaborators. The mass asymmetry motion is shown here for the first time as a part of relative separation motion.     

The new isotopes247Md,243Fm,239Cf,and investigation of the evaporation residues from fusion of206Pb,208Pb,and209Bi with40Ar

The evaporation residues produced in heavy ion fusion reactions between⁴⁰Ar and²⁰⁶Pb,²⁰⁸Pb, and²⁰⁹Bi, respectively, were investigated. They were separated in-flight using a velocity filter (SHIP) and identified after implantation into an array of position sensitive surface barrier detectors by measuring their decay characteristics. Three newα-emitting isotopes were found:²⁴⁷Md,²⁴³Fm, and²³⁹Cf.²⁴⁷Md was identified by correlation to its known daughter decay²⁴³Es, and²⁴³Fm and²³⁹Cf were observed as correlated decays. Moreover, all previously known deexcitation channels, characterized by spontaneous fission or byα decay, could be observed for the first time in one experimental setup. Our results for formation cross sections and decay characteristics are in agreement with known data.