[en] A study is made of the influence of angular momentum and excitation energy on the mean kinetic energy (bar E_k) and dispersions of the mass and energy distributions of the fission fragments (σ²_m, σ²_E) of excited compound nuclei of Po and Ku formed in various ion-target combinations and at two energies of the incident ion for each combination. The measurements were taken with a DEMAS two-arm time-of-flight spectrometer, placed in the extracted-beam channel of the U-400 cyclotron of the G.N. Flerov Laboratory of Nuclear Reactions at JINR, Dubna. 59 refs., 16 figs., 4 tabs

[en] The authors have developed a spinning wheel analyzer of millisecond isotopes (SWAMI), an instrument to measure the kinetic energy of fission-fragment pairs from the spontaneous fission (SF) of millisecond isotopes. This instrument employs a continuous band of thin, aluminum foils mounted at the perimeter of a spinning wheel to stop and retain recoil products produced during continuous, heavy-ion bombardment of heavy, actinide targets. The rotation of the wheel moves the implanted reaction products past four pairs of surface-barrier detectors, where the energies of correlated fission fragments are measured. With this instrument, they can determine the mass and total kinetic energy (TKE) distributions for isotopes with half-lives between 0.5 and 300 ms. To date, they have used SWAMI to measure the mass and TKE distributions for two nuclides, 20-ms ²⁶⁰[104] and 1.2-ms ²⁵⁸No

[en] We measured the mass and kinetic energy partitioning in the spontaneous fission (SF) of ²⁵⁸Fm, ²⁵⁹Md, ²⁶⁰Md, ²⁵⁸No and ²⁶⁰[104]. All these fissions have mass distributions that are symmetric. Total kinetic energy distributions peaked near either 200 or 235 MeV. Surprisingly, the total kinetic energy distributions were skewed upward or downward from the peak energy in each case, except for ²⁶⁰[104], indicating a composite of two energy distributions. Such unique distributions have not been observed in the low-energy fission or SF of other nuclides. We interpret this as a mixture of liquid-drop-like and fragment-shell directed fission, although theory had not predicted this phenomenon. (orig.)

[en] We have measured the mass and kinetic-energy partitioning in the spontaneous fission of five heavy nuclides: ²⁵⁸Fm, ²⁵⁹Md, ²⁶⁰Md ²⁵⁸No, and ²⁶⁰[104]. Each was produced by heavy-ion reactions with either ²⁴⁸Cm, ²⁴⁹Bk, or ²⁵⁴Es targets. Energies of correlated fragments from the isotopes with millisecond half lives, ²⁵⁸No and ²⁶⁰[104], were measured on-line by a special rotating-wheel instrument, while the others were determined off-line after mass separation. All fissioned with mass distributions that were symmetric. Total-kinetic-energy distributions peaked near either 200 or 235 MeV. Surprisingly, because only a single Gaussian energy distribution had been observed previously in actinide fission, these energy distributions were skewed upward or downward from the peak in each case, except for ²⁶⁰[104], indicating a composite of two energy distributions. We were able to fit accurately two Gaussian curves to the gross energy distributions from the four remaining nuclides. From the multiple TKE distributions and the shapes of the mass distributions, we conclude that there is a low-energy fission component with liquid-drop characteristics which is admixed with a much higher-energy component due to closed fragment shells. We now have further evidence for this conclusion from measurements of the neutron multiplicity in the spontaneous fission of ²⁶⁰Md. 25 refs., 9 figs

No abstract available

[en] Investigations of mass and kinetic-energy distributions from spontaneous fission have been extended in recent years to an isotope of element 104 and, for half-lives, to an isotope of element 108. The results were surprising in that spontaneous fission half-lives turned out to be much longer than expected and mass and kinetic-energy distributions were found to abruptly shift away from those of the lighter actinides, showing two modes of fission. These new developments caused a re-evaluation of the understanding of the fission process, bringing an even deeper appreciation of the role played by nuclear shell effects upon spontaneous fission properties. (author) 22 refs.; 14 figs

[en] The present work reveals the applicability of an analytical model to evaluate the alpha decay of neutron deficient isotopes as well as spontaneous symmetric fission of superheavy elements

[en] Short note. 8 refs

[en] Theoretical results for masses and half-lives of heavy and superheavy nuclei, obtained within a macroscopic-microscopic model, are discussed. Concerning half-lives, main attention is paid to α-decay mode, but spontaneous fission is also considered. Even-even nuclei with proton number Z 82 - 124 and neutron number N = 126 - 190 are analysed. The theoretical analysis is closely connected with recent experiments on superheavy elements. In particular, predictions for nuclei, synthesis of which is planned in forthcoming experiments, are presented. (orig.)

[en] In recent years, the author has measured the mass and kinetic-energy distributions from the spontaneous fission of ²⁵⁸Fm, ²⁵⁹Md, ²⁶⁰Md, ²⁵⁸No, ²⁶²No, and ²⁶⁰[104]. All are observed to fission with a symmetrical division of mass, whereas the total-kinetic-energy (TKE) distributions strongly deviated from the Gaussian shape characteristically found in the fission of all other actinides. When the TKE distributions are resolved into two Gaussians, the constituent peaks lie near 200 and near 233 MeV. He concludes two modes or bimodal fission is occurring in five of the six nuclides studied. Both modes are possible in the same nuclide, but one generally predominates. He also concludes the low-energy but mass-symmetrical model is likely to extend to far heavier nuclei; while the high-energy mode will be restricted to a smaller region, a region of nuclei defined by the proximity of the fragments to the strong neutron and proton shells in ¹³²Sn