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E-Verhandlungen 1998
Programm und Abstacts der Sitzung HK 51

•D. Pansegrau¹, H. Bauer¹, J. Eberth², T. Härtlein¹, F. Köck¹, P. Reiter³, D. Schwalm¹ und H.G. Thomas² für die German EUROBALL-Kollaboration
¹MPI für Kernphysik Heidelberg
²Institut für Kernphysik Köln
³Argonne National Laboratory

In order to investigate g-decays of excited states in the Second Minimum of the double-humped fission barrier of ²⁴⁰Pu, an experiment was performed during the CLUSTER-campaign of the German EUROBALL-collaboration at the MPI für Kernphysik in Heidelberg.

The superdeformed minimum was populated via an (a,2n)-reaction. After the prompt decay of excited states to the ground state of ^240mPu, the nucleus decays via fission with a half life of 3.8 ns which gives an unambiguous trigger for the population of the form isomer. The extremely rare g-decays in the Second Minimum were detected with six CLUSTER-detectors arranged in the highly efficient CUBE-setup.

The spectrum of prompt g-rays followed by a delayed fission is dominated by a transition at 786 keV. Other, weaker transitions in the region of 550 keV and 800 keV are observed, too. Together with the results of a previous experiment with the Heidelberg-Darmstadt crystal ball, an interpretation of the observed transitions will be given.

^* supported by BMBF

•L. Demattè¹, F.-J. Hambsch¹ und S. Oberstedt² für die German EUROBALL-Kollaboration
¹JRC-IRMM, Retieseweg, B-2440 Geel, Belgium
²ILL, F-38042 Grenoble, France

Fission mode calculations have been started for the lighter actinides in the frame of the multi-modal random neck-rupture model of Brosa et al. [1]. This is done in view of understanding the drastically changing charge distributions found in the recent GSI-experiments [2]. The calculations are in progress for the isotopes ^220,226,230Th and ²²⁶Ac. First results will be shown.

[1] U. Brosa, S. Groß mann and A. Müller; Physics Reports 197 (1990) 167

[2] S. Steinhäuser et al.; Proceedings Conference on Dynamical Aspects of Nuclear Fission, Casta-Papiernicka, Slovakia, 30.8-4.9.1996

•M. Wöstheinrich¹, R. Pfister¹, F. Gönnenwein¹, H. Faust² und S. Oberstedt² für die German EUROBALL-Kollaboration
¹Physikalisches Institut, Universität Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen
²Institut Laue-Langevin, F-38042 Grenoble

The emission of ternary particles from the fission reactions ²²⁹Th(n_th,f) and ²³⁹Pu(n_th,f) has been studied at the HFR of the Institut Laue-Langevin. Energy distributions were measured with the mass spectrometer Lohengrin for several ternary particles having not been observed in the past. These measurements form part of a systematic study of the yields of ternary particles originating in low energy fission of the actinides from Th up to Cf. Special emphasis is given to the search for the heaviest ternary particle still detectable with the present experimental technique. The mass/charge of these particles appears to depend on the mass/charge of the fissioning compound nucleus. A clear indication for this suggestion has been found in former experiments, e.g. in the reactions ²³³U(n_th,f) and ²⁴²Am(n_th,f), where it was observed that ternary particles with masses A heavier than A = 24 are emitted with sizable yields only in fission of the heavier Am compound [1], [2]. The results from the present experiment are in line with the above findings.

[1] M. Wöstheinrich et al., ILL report 1996

[2] M. Hesse et al., Proc. 3^rd DANF, Casta-Papiernicka, Slovakia, 1996 Supported by the BMBF, Bonn, under contract number 06Tü669

•Olivier Serot¹ und Cyriel Wagemans² für die German EUROBALL-Kollaboration
¹Institute for Reference Materials and Measurements, Retieseweg, B-2440 Geel
²Dept. of Subatomic and Radiation Physics, University of Gent, Proeftuinstraat 86, B-9000 Gent

Characteristics of Long Range Alpha particles emitted during spontaneous fission of 238, 240, 242 and 244Pu-isotopes have been measured under identical experimental conditions in order to get reliable results. Correlations between LRA emission probability, fission modes and available energy during fission are shown and discussed in terms of the cluster emission process.

•A. Elhardt¹, T. Faestermann¹, R. Henkelmann², K. Knie¹, G. Korschinek¹, C. Lierse², G. Rugel¹, A. Stippschild² und A. Stollenwerk³ für die German EUROBALL-Kollaboration
¹Technische Universität München, Physik Department
²Technische Universität München, Institut für Radiochemie
³Wiederaufarbeitungsanlage Karlsruhe

Up to now fission yields from the low energy neutron induced fission of ²³⁵U are only known down to A=69. The high sensitivity for the detection of ⁶⁰Fe by accelerator mass spectrometry with the gasfilled analysing magnet system (GAMS) at the Munich Tandem Laboratory opens up the possebility to measure for the first time the fission yield for A=60.

⁶⁰Fe (T_1/2 = 1.5 Myr) is unique because of the missing direct production channels from stable nuclei like (n,g) or (n,p). To obtain ²³⁵U samples irridiated with a large neutron fluence, we have chemically extracted iron from the HAWC (high active waste concentrate) of the pilot reprocessing plant in Karlsruhe.

To estimate the background caused by double neutron capture on ⁵⁸Fe we have also measured the (n,g) cross-section of ⁵⁹Fe. The results will be reported.

•A. Kötzle¹, F. Gönnenwein¹, P. Jesinger¹, G. Petrov², V. Petrova², A. Gagarski², O. Zimmer³ und V. Nesvizhevsky³ für die German EUROBALL-Kollaboration
²PNPI, Gatchina 188350, Russia
³Institut Laue-Langevin, F-38042 Grenoble
¹Physikalisches Institut, Universität Tübingen , Auf der Morgenstelle 14, D-72076 Tübingen

Parity-nonconservation (PNC) in nuclear fission induced by polarized neutrons can be observed as an asymmetry of the fragment angular distribution relative to the direction of the neutron spin: W(q) ~ 1 + a_[n\vec]f([^n]·[^p]) . The aim of this work is to determine the mass- and energy dependence of the PNC-effect. One possible reason for such a dependence may be found in the different "`valleys"' of the potential surface, through which the fissioning nucleus descents from the saddle to the scission point (Brosa-modes). The results of this work should be a test of theoretical predictions connecting the size of the PNC-effect with the quantum-number K, the projection of the nuclear spin on the symmetry axis of the fissioning nucleus at scission.

In a new fourfold back-to-back ionization chamber about 5·10⁹ fissions events have been observed in a first run on the cold beam of polarized neutrons at the high flux reactor of the ILL Grenoble. The experimental setup and first results are presented.

•L. Demattè, F.-J. Hambsch und H. Bax für die German EUROBALL-Kollaboration
JRC-IRMM, Retieseweg, B-2440 Geel, Belgium

An investigation of the fission fragment mass and total kinetic energy distributions of ²³⁹Pu for resolved resonances has been started. The white neutron source at GELINA of the IRMM is used together with a double Frisch-gridded ionization chamber for fission fragment detection. ²³⁹Pu is expected to exhibit a more pronounced dependence of the fragment properties as a function of quantum numbers than ²³⁵U, already studied some time ago [1]. A possible reason could be that for resonance neutrons incident on ²³⁹Pu, fission occurs via J = 0⁺ and 1⁺-levels with mainly the K = 0⁺ and 1⁺ bands involved at the saddle point. Thus effects related to the quantum numbers should be detectable, because the energy difference of the two K-bands is about 1.25 MeV. The experimental setup and first results will be presented.

[1] F.-J. Hambsch, H.-H. Knitter, C. Budtz-Jø rgensen and J.P. Theobald; Nucl. Phys. A491 (1989) 56

F. Vivès¹, •F.-J. Hambsch¹, S. Oberstedt², H. Bax¹ und G. Barreau³ für die German EUROBALL-Kollaboration
¹JRC-IRMM, Retieseweg, B-2440 Geel, Belgium
²ILL, F-38042 Grenoble, France
³CENBG, F-2440 Bordeaux, France

The ²³⁸U(n,f) fission reaction has been studied at the 7 MV Van de Graaff accelerator of IRMM. Fission fragment detection was performed with a double Frisch-gridded ionization chamber at incident neutron energies ranging from 1.2 to 5.5 MeV. The experimental mass and TKE distribution have been compared to theoretical predictions [1] in the frame of the multi-modal random neck-rupture model of Brosa et al. [2]. A discontinuity in the mean TKE as a function of the incident neutron energy has been observed at about E_n = 3.5 MeV. This is about the energy above the barrier (E_n-E_B ~ 2 MeV) needed to additionally break a proton pair. It is in line with observed changes in the number of prompt neutrons as a function of E_n [3]. The experimental results and the comparison with theoretical predictions will be presented and discussed.

[1] S. Oberstedt, F.-J. Hambsch and F. Vivès; Proc. Int. Symp. Large-Scale Collective Motion of Atomic Nuclei, Brolo, Italy, 15-19.10.1996.

[2] U. Brosa, S. Groß mann and A. Müller; Physics Reports 197 (1990) 167

[3] J. Frehaut; NEANDC(E) 238/L (1986)

•Hans J. Krappe¹ und Serguei Fadeev² für die German EUROBALL-Kollaboration
¹Hahn-Meitner Institut Berlin, D-14091 Berlin
²Nuclear Physics Department, State University, RU-198904 St. Petersburg

Along the path of cold fission one would construct the wave function for the fissioning system with pairing correlations by projecting the BCS wave function onto the total numbers of protons and neutrons in the parent system, but beyond scission on the two nucleon numbers in each fragment. We propose to interpolate between these limiting shapes by a generator-coordinate ansatz for the wave function with the nucleon-number differences between the nascent fragments as the generator coordinates. The ansatz is motivated by the gauge-symmetry properties of the fissioning system. We also define the space of collective states in which the system evolves in time. We present first results of a model calculation in the static Hill-Wheeler box with a d-type diaphragm.

•F. Goldenbaum¹, M. Chevallier², C. Cohen³, D. Dauvergne², J. Dural⁴, J. Galin¹, D. Jacquet⁵, R. Kirsch², E. Lienard¹, B. Lott¹, M. Morjean¹, A. Peghaire¹, Y. Perier¹, J.C. Poizat², G. Prevot³, J. Remillieux², D. Schmaus³ und M. Toulemonde⁴ für die German EUROBALL-Kollaboration
³GPS, 75251 Paris Cedex 05, France
⁴CIRIL, BP 5133, 14040 Caen Cedex, France
⁵Institut de Physique Nucleaire d'Orsay, BP 1, F-91406 Orsay Cedex, France
¹GANIL (DSM/CEA,IN2P3/CNRS), BP 5027, 14076 Caen Cedex 5, France
²Institut de Physique Nucleaire de Lyon, F-69622 Villeurbanne Cedex, France

Die Zeitskala der Spaltung wurde für uran-artige Kerne in der Reaktion 24 A.MeV U+Si unter Anwendung der sog. blocking Technik an Ein-Kristallen studiert [1]. Aufgrund der inversen Kinematik und der großen Geschwindigkeit der fissilen projektil-artigen Fragmente sind Spaltzeiten zugänglich, die vorangehenden Experimenten bei dieser Methode bislang verschlossen blieben. Spaltzeiten größer als 10^-19 s wurden für Anregungsenergien bis 250 MeV bestimmt. Diese Zeitskalen sind sehr viel länger als diejenigen, die aus Messungen der Neutronen-, geladenen Teilchen oder GDR-g Multiplizitäten geschlußfolgert werden und deuten auf sehr große Dissipationskoeffizienten und eine hohe Viskosität heißer Materie hin.

[1] M. Morjean et al., Ganil Preprint GANIL P97 21, (1997)

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