[en] Short communication

[en] The secondary-beam facility of GSI Darmstadt was used to study the fission properties of 70 short-lived radioactive nuclei. Most of them have not been accessible so far in conventional fission experiments. Relativistic secondary projectiles were produced by fragmentation of a 1 A GeV ²³⁸U primary beam and identified in nuclear charge and mass number. Using these reaction products as secondary beams, the giant resonances, mostly the giant dipole resonance, were excited by electromagnetic interactions in a secondary lead target, and fission from excitation energies around 11 MeV was induced. The fission fragments were identified in nuclear charge, and their velocity vectors were determined. Elemental yields and total kinetic energies have been obtained for a number of neutron-deficient actinides and preactinides. The characteristics of multimodal fission of nuclei around ²²⁷Th were systematically investigated. The proton even-odd effect was determined for all systems

[en] Mass distributions of fragments in the low-energy fission of nuclei from ¹⁸⁷Ir to ²¹³At have been analysed. This analysis has shown that shell effects in symmetric mode fragment mass yields from the fission of pre-actinide nuclei could be described if one assumes the existence of two strongly deformed neutron shells in the arising fragments with neutron numbers N₁ ∼ 52 and N₂ ∼ 68. A new method has been proposed for quantitatively describing the mass distributions of the symmetric fission mode for pre-actinides with A ∼ 180 / 220. (orig.)

[en] A model calculation is presented which predicts the complex nuclide distribution resulting from peripheral relativistic heavy-ion collisions involving fissile nuclei. The model is based on a modern version of the abrasion-ablation model which describes the formation of excited prefragments due to the nuclear collisions and their consecutive decay. The competition between the evaporation of different light particles and fission is computed with an evaporation code which takes dissipative effects and the emission of intermediate-mass fragments into account. The nuclide distribution resulting from fission processes is treated by a semi-empirical description which includes the excitation-energy dependent influence of nuclear shell effects and pairing correlations. The calculations of collisions between ²³⁸U and different reaction partners reveal that a huge number of isotopes of all elements up to uranium is produced. The complex nuclide distribution shows the characteristics of fragmentation, mass-asymmetric low-energy fission and mass-symmetric high-energy fission. The yields of the different components for different reaction partners are studied. Consequences for technical applications are discussed. (orig.)

[en] Isotopic series of 58 neutron-deficient secondary projectiles (^205,206At, ^205-209Rn, ^{208-212,217,218}Fr, ^211-223Ra, ^215-226Ac, ^221-229Th, ^226-231Pa, ^231-234U) were produced by projectile fragmentation using a 1 A GeV ²³⁸U beam. Cross sections of fission induced by nuclear and electromagnetic interactions in a secondary lead target were measured. They were found to vary smoothly as a function of proton and neutron number of the fissioning system, also for nuclei with large ground-state shell effects near the 126-neutron shell. No stabilization against fission was observed for these nuclei at low excitation energies. Consequences for the expectations on the production cross sections of super-heavy nuclei are discussed. (orig.)

[en] An analysis of fragment mass distributions in the low-energy fission of nuclei from ¹⁸⁷Ir to ²¹³At has shown that shell effects in symmetric mode fragment mass yields of these nuclei could be interpreted as an influence of two strongly deformed neutron shells in arising fragments with neutron numbers N₁ ∼ 52 and N₂ ∼ 68. A new method has been proposed for quantitative describing the mass distributions of a symmetric fission mode for pre-actinides with A ∼ 180-220

[en] Mass distributions of fragments in the low-energy fission of nuclei from ¹⁸⁷Ir to ²¹³At have been analysed. This analysis has shown that shell effects in symmetric-mode fragment mass yields from the fission of pre-actinide nuclei could be described if one assumes the existence of two strongly deformed neutron shells in the arising fragments with neutron numbers N₁∼52 and N₂∼68. A new method has been proposed for quantitatively describing the mass distributions of the symmetric fission mode for pre-actinides with A∼180-220. (orig.)

[en] A model calculation is presented which predicts the complex nuclide distribution resulting from peripheral relativistic heavy-ion collisions involving fissile nuclei. The model is based on a modern version of the abrasion-ablation model which describes the formation of excited prefragments due to the nuclear collisions and their consecutive decay. The competition between the evaporation of different light particles and fission is computed with an evaporation code which takes dissipative effects and the emission of intermediate-mass fragments into account. The nuclide distribution resulting from fission processes is treated by a semiempirical description which includes the excitation-energy dependent influence of nuclear shell effects and pairing correlations. The calculations of collisions between ²³⁸U and different reaction partners reveal that a huge number of isotopes of all elements up to uranium is produced. The complex nuclide distribution shows the characteristics of fragmentation, mass-asymmetric low-energy fission and mass-symmetric high-energy fission. The yields of the different components for different reaction partners are studied. Consequences for technical applications are discussed. (orig.)

[en] Isotopic series of 58 neutron-deficient secondary projectiles (^205,206At, ^205-209Rn, ^{208-212,217,218}Fr, ^211-223Ra, ^215-226Ac, ^221-229Th, ^226-231Pa, ^231-234U) were produced by projectile fragmentation using a 1 A GeV ²³⁸U beam. Cross sections of fission induced by nuclear and electromagnetic interactions in a secondary lead target were measured. They were found to vary smoothly as a function of proton and neutron number of the fissioning system, also for nuclei with large ground-state shell effects near the 126-neutron shell. No stabilization against fission was observed for these nuclei at low excitation energies. Consequences for the expectations on the production cross sections of super-heavy nuclei are discussed

[en] Nuclear fission from excitation energies around 11 MeV was studied at GSI, Darmstadt for 76 neutron-deficient actinides and pre-actinides by use of relativistic secondary beams. The characteristics of multimodal fission of nuclei around ²²⁶Th are systematically investigated and related to the influence of shell effects on the potential-energy and on the level density between saddle point and scission. A systematic view on the large number of elemental yields measured gave rise to a new interpretation of the enhanced production of even elements in nuclear fission and allowed for a new understanding of pair breaking in fission. (author)