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Karl-Heinz Schmidt
Fission of relativistic secondary projectiles - a new experimental approach The discovery of nuclear fission by Otto Hahn and Fritz Straßmann in 1939 represents a fundamental progress in our understanding of the dynamical properties of cold and slightly heated nuclear matter. In numerous experiments, fission proved to be a unique source of information on shell effects at extreme deformation and on nuclear viscosity. However, many questions still remain open, although nuclear fission is the most intensively studied nuclear reaction. The reason is that only few nuclei have been investigated up to very recently. Experiments were restricted to spontaneously fissioning isotopes and primordial (nuclei which can be found in nature) or long-lived target nuclei (radioactive nuclei which are produced in nuclear reactors or in other nuclear reactions with sufficient life-time to be used as target). In addition, many experiments were aimed to provide technical data and did not address the fundamental understanding of nuclear dynamics. The experimental installations of GSI Darmstadt offer unique possibilities to extend the research on nuclear fission appreciably. Microscopic amounts of short-lived nuclei are produced by fragmentation of stable 238U at relativistic energies, and the fission of individual nuclei is investigated. Within in the limits given by the production cross sections, the nuclei to be investigated can be freely chosen. In this way, important characteristics of the fission process have been measured for 70 fissioning nuclei. By this experiment, the number of nuclei for which the fission properties have been studied has almost doubled. The experimental method, adapted to the low secondary-beam intensities and the inverse-kinematic conditions has been developed in collaboration with Corinne Böckstiegel, Prof. Hans-Georg Clerc, Axel Grewe, Andreas Heinz, Manuel de Jong, Jochen Müller, Arnd R. Junghans, Steffen Steinhäuser and Bernd Voss from the "Institut für Kernphysik" of the "Technische Universität Darmstadt". Dedicated detectors (a large twin ionisation chamber and a TOF wall) have been constructed by Bernd Voss, K.-H. Behr and K. Burkard. Adapted procedures to analyse the element yields, the total kinetic energies, and the fission cross sections under these specific experimental conditions have been developed by Steffen Steinhäuser, Corinne Böckstiegel, and Andreas Heinz, respectively, in their PhD theses.
Fig. 1: Isotopes investigated in low-energy fission. Blue circles: Mass distributions measured in previous experiments at excitation energies lower than 10 MeV above the fission barrier. Green crosses: Systems investigated in the GSI experiment. Additionally, examples of previously measured fission-fragment mass distributions are shown. For orientation, the primordial isotopes are indicated by black squares.
Fig. 2: Schematic drawings of the fragment separator to prepare the secondary beams (upper part) and of the set-up for the fission experiment in inverse kinematics.
Fig. 3: Fission-fragment yields (upper part) and mean total kinetic energies (lower part) in electromagnetic-induced fission, measured in the GSI secondary-beam experiment, are shown as a function of the atomic number (small frames) on a chart of the nuclides. The systems shown in the figure cover the transition from symmetric (single-humped) to asymmetric (double-humped) fission which has been systematically covered for the first time. These experiments revealed a new understanding of even-odd effects in fission-fragment distributions. Previously deduced values on the viscosity of cold nuclear matter have to be re-evaluated. Due to the systematic variation of neutron and proton number of the fissioning system, the new experiment also revealed a new insight into the influence of shell effects at extreme deformation on the fission process.
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