A global view on the properties of fission channels is presented. Surprisingly, the positions of the two asymmetric fission channels are found to be constant in atomic number over the whole range of systems investigated.

The present work reports on a systematic study of fission-fragment element yields in the fission of 70 neutrondeficient actinides and pre-actinides between 205 At and 234 U within the concept of independent fission channels [1]. The fission channels are characterized by several parameters, e.g. the average mass or charge split, the mass or charge width, and the mean total kinetic energy, respectively the elongation of the scission configuration. We extract the values of these parameters for three fission channels from the measured data of 15 nuclides [2] which show features of multi-modal fission. The experiment was performed at GSI Darmstadt: Relativistic secondary projectiles were produced via fragmentation of a 1 A GeV primary beam of 238 U and identified in A and Z by the fragment separator FRS. Details of the experimental technique are given elsewhere [2]. In Fig. 1, we compare the parameters of the independent fission channels, determined in the present work, with the body of previously available data. This allows a systematic view on the variation of position () and width (σ) of the standard I (S1) and standard II (S2) channels, on the width of the super-long (SL) channel and on the relative yields of these three channels as a function of Z and A of the fissioning system. The data are restricted to spontaneous fission and to initial excitation energies up to a few MeV above the fission barrier. The widths of the S1 and S2 fission channels show fluctuations, which are appreciably larger than the reported statistical uncertainties of the fits. No systematic trend can safely be deduced over the whole mass range. One may only conclude that the width of the S1 fission channel amounts to about 3.5 mass units, while the width of the S2 fission channel is appreciably larger with about 5 mass units. The width of the SL fission channel is rather well determined for the light systems of the present experiment, for which the symmetric fission component dominates, to about 10 mass units over a large mass range, see ref. [2]. Thus, the SL fission channel is much broader than the S1 and S2 fission channels. The values deduced previously for heavier systems fluctuate enormously. These fluctuations are probably explained by the tiny yield of the super-long channel for the heavier systems. The positions of the S1 and S2 fission channels show a systematic variation as a function of mass number for a given element. This trend is clearly seen for all elements, in spite of some fluctuations. The average slope of the isotopic trend seems to be slightly larger than 0.5. This means that the positions of the S1 and S2 fission channels vary in neutron number and are rather stable in proton number. The relative yield of the symmetric SL fission channel shows an exponential decrease with increasing mass number. For systems with A > 234, the yield of this channel becomes so low that it could not be determined any more. At the same time, the complementary yield of the lumped asymmetric component increases with increasing mass. Deducing a systematic trend in the competition between the S1 and the S2 fission channels requires a more detailed analysis [3].

A global view on the properties of ssion channels is presented. Surprisingly, the positions of the two asymmetric ssion channels are found to be constant in atomic number over the whole range of systems investigated.

Electromagnetic-induced fission of several neutron-deficient actinides and pre-actinides was studied at GSI Darmstadt by use of relativistic secondary beams. The characteristics of multi-modal fission of nuclei around 226Th are systematically investigated and interpreted as the superposition of three fission channels. Properties of these fission channels have been determined for 15 systems. A global view on the properties of fission channels including previous results is presented. The positions of the asymmetric fission channels are found to be constant in atomic number over the whole range of systems investigated.

Isotopic series of 58 neutron-deficient secondary projectiles (205,206At, 205–209Rn, 208–212,217,218Fr, 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.

We report on an experimental programme to measure the nuclide production in fission in inverse kinematics. Using relativistic beams of primordial and radioactive nuclei, new experimental information on the influence of shell effects on fission has been obtained. The transition from symmetric to asymmetric fission around 227Th has systematically been mapped. The heavy asymmetric component was found to be astonishingly stable near Z = 54 for all systems investigated.

[Abstract] At GSI, Darmstadt, an experimental program on fission with relativistic heavy-ion beams is in progress. A large range of excitation energies, combined with low angular momentum and small shape distortion is accessible. Full nuclide identification of the reaction residues is achieved by applying inverse kinematics. The nuclide production and the kinematics of fission fragments from a variety of primordial and radioactive projectiles reveal new insight into the influence of shell effects and dissipation on the fission process. The present contribution gives an overview on the experimental methods, the experimental results and the prospects for future progress.

Fission Dynamics of Atomic Clusters and Nuclei, pp. 302-310 (2001) No AccessPUZZLING RESULTS ON NUCLEAR SHELLS FROM THE PROPERTIES OF FISSION CHANNELSK.-H. SCHMIDT, A. R. JUNGHANS, J. BENLLIURE, C. BÖCKSTIEGEL, H.-G. CLERC, A. GREWE, A. HEINZ, M. DE JONG, and S. STEINHÄUSERK.-H. SCHMIDTGesellschaft für Schwerionenforschung, Planckstraße 1, 64291 Darmstadt, Germany, A. R. JUNGHANSGesellschaft für Schwerionenforschung, Planckstraße 1, 64291 Darmstadt, Germany, J. BENLLIUREUniversidad de Santiago de Compostela, 15706 Santiago de Compostela, Spain, C. BÖCKSTIEGELInstitut für Kernphysik, TU Darmstadt, Schloßgartenstr. 9, 64289 Darmstadt, Germany, H.-G. CLERCInstitut für Kernphysik, TU Darmstadt, Schloßgartenstr. 9, 64289 Darmstadt, Germany, A. GREWEInstitut für Kernphysik, TU Darmstadt, Schloßgartenstr. 9, 64289 Darmstadt, Germany, A. HEINZInstitut für Kernphysik, TU Darmstadt, Schloßgartenstr. 9, 64289 Darmstadt, Germany, M. DE JONGInstitut für Kernphysik, TU Darmstadt, Schloßgartenstr. 9, 64289 Darmstadt, Germany, and S. STEINHÄUSERInstitut für Kernphysik, TU Darmstadt, Schloßgartenstr. 9, 64289 Darmstadt, Germanyhttps://doi.org/10.1142/9789812811127_0027Cited by:0 (Source: Crossref) PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail Abstract: At the secondary-beam facility of GSI, the fission properties of short-lived neutron-deficient nuclei have been investigated in inverse kinematics. Detailed features of the measured element distributions and total kinetic energies seem to contradict the present understanding according to which the neutron shells at N = 82 and N ≈ 90 are decisive for the asymmetric fission channels. FiguresReferencesRelatedDetails Recommended Fission Dynamics of Atomic Clusters and NucleiMetrics History PDF download

The secondary-beam facility of GSI provided the technical equipment for a new kind of fission experiment. Fission properties of short-lived neutron-deficient nuclei have been investigated in inverse kinematics. The measured element distributions reveal new kinds of systematics on shell structure and even-odd effects and lead to an improved understanding of structure effects in nuclear fission.

Nuclear-charge yields of fragments produced by fission of neutron-deficient isotopes of uranium, protactinium, actinium, and radium have been measured. These radioactive isotopes were produced as secondary beams, and electromagnetic fission was induced in a lead target with an average excitation energy around 11 MeV. The local even-odd effect in symmetric and in asymmetric fission of thorium isotopes is found to be independent of Z(2)/A. The charge yields of the fission fragments of the odd-Z fissioning protactinium and actinium show a pronounced even-odd effect. In asymmetric fission the unpaired proton predominantly sticks to the heavy fragment. A statistical model based on the single-particle level density at the Fermi energy is able to reproduce the overall trend of the local even-odd effects both in even-Z and odd-Z fissioning systems.

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.

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 correlatios. 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.

At GSI Darmstadt, a new technique to investigate low-energy fission was developed. Relativistic secondary projectiles are produced by the fragmentation of 1.A GeV U-238 primary beam, and identified in nuclear mass and charge number by using the fragment separator FRS. In a dedicated experimental setup, the Coulomb fission of these secondary beams in a lead target was investigated. The nuclear charge yields as well as the total kinetic energies TKE of the two fission fragments have been determined for the Coulomb fission of a large number of different secondary beams of neutron-deficient isotopes of the elements U, Pa, Th, Ac, and Ra. This allows systematic studies of low-energy fission properties, which had not been possible by conventional methods. The transition between asymmetric and symmetric fission is covered, and the TKE as well as the proton even-odd effect have been studied for asymmetric and symmetric fission. The observed strong local proton even-odd effects in the yields for asymmetric fission of both even-Z and odd-Z fissioning nuclei are described by a statistical model. As a result, the relation between the proton even-odd effect and pairbreaking processes in fission has to be reconsidered.

The unknown β-decay half-lives of 22 isotopes far off stability (5 < T Z < 10) in the region from Ti to Ni were measured at GSI, Darmstadt. The nuclei were produced in a fragmentation reaction of 500 A.MeV 86Kr-projectile impinging on a thick Be target. The isotopes of interest were separated and identified with the fragment separator, FRS, by a combination of Bρ,Z, and ToF techniques. An additional range separation was performed by a selective implantation into granular detectors. The spatial and time correlations of the implant with the consecutively detected β-particles were used to determine the unknown half-lives. For nuclei far off stability, β-decay chains were measured and analyzed as well, leading to an even more reliable evaluation of the lifetimes. The large discrepancies found between the measured and the theoretical values emphasize that most recent theoretical work is not an improvement over calculations made almost a decade ago.

A new experimental technique for nuclear-fission studies has been developed at GSI, Darmstadt. Relativistic secondary projectiles are produced by fragmentation of a 1 A GeV U-238 primary beam and identified in nuclear charge and mass number. The giant resonances are excited by electromagnetic interactions in a secondary lead target, and fission from excitation energies around 11 MeV is induced. The fission fragments are identified in nuclear charge, and their velocity vectors are determined. Nuclear-charge distributions and total kinetic energies have been determined for a number of neutron-deficient actinides and preactinides which were not accessible viith conventional techniques. The characteristics of multimodal fission of nuclei around Th-226 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.