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HK: Hadronen und Kerne

HK 59: Postersitzung

HK 59.7: Poster

Donnerstag, 27. März 1997, 14:00–16:30, P

γ-decay above the shape-isomeric ground state of ²³⁹U — •S. Oberstedt — Institut Laue-Langevin, F-38042 Grenoble
Continuing the search for γ-decay towards the shape-isomeric ground state of ²³⁹U [1] γ-ray spectra of s-wave neutron subthreshold fission resonances of the intermediate structure around E_n 720 eV were investigated at γ-energies below 1.0 MeV. Since in this energy region possible γ-transitions towards the isomeric ground state are masked by the large quantity of ordinary γ -decay towards the normal ground state, direct observation is hardly possible. Therefore, in the first step all known γ-decay lines of a pure s-wave neutron capture resonance were fitted. In a second step, these fitted positions and widths are used to fit the γ-spectrum of fission resonances within the intermediate structure. Apparent transitions from the decay of a class-II state towards the shape-isomeric ground state should then appear as an overshoot in the residual spectrum.
In this way three hitherto unknown transitions at E_γ = 596.4 keV, 604.5 keV and 631.1 keV have been found. Both, the fact that they are not observed in other s-wave fission resonances and that the relative intensity pattern follows the class-II fraction of each resonance of the intermediate structure, indicate strongly the observation of γ-decay towards the shape-isomeric ground state.

[1] S. Oberstedt, F. Gunsing, Nucl. Phys. A589, 435 (1995)