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Freiburg 1999 – scientific programme

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HK: Physik der Hadronen und Kerne

HK 50: Kernphysik VI / Spaltung

HK 50.2: Talk

Thursday, March 25, 1999, 14:15–14:30, B

The 238U(n,f) reaction: final results — •F.-J. Hambsch1, F. Vivès1, S. Oberstedt2, H. Bax1, and G. Barreau31JRC-IRMM, Retieseweg, B-2440 Geel, Belgium — 2ILL, F-38042 Grenoble, France — 3CENBG, F-2440 Bordeaux, France

The fission fragment properties of the reaction 238U(n,f) have been studied at different incident neutron energies ranging from En=1.2 to 5.8 MeV. The pre-neutron emission fission fragment mass, mean total kinetic energy (TKE) and ang ular distributions have been investigated with a double Frisch-gridded ionization chamber. The influence of the subthreshold vibrational resonances and of the proton pairing effect on the fission fragment properties is clearly visible. The TKE averaged over all fission fragment masses shows an increasing trend up to En=3.5 MeV with a sudden drop at roughly En=3.8 MeV which has been attributed to the onset of pair breaking. Above En=3.8 MeV, TKE is again continuously incr easing. The changes in the mass yield and TKE(A) distributions have been studied as a function of the compound nucleus excitation energy, and their individual contribution to the observed variations in the TKE have been also determined. The t wo-dimensional mass-TKE distributions have been convoluted in terms of fission modes and compared with theoretical calculations performed recently [1] in the frame of the multi-modal random neck-rupture model[2]. Although two possible solutions assuming either two or three asymmetric fission modes exist, only one solution leads to meaningful physical interpretation in terms of shell structure effects. However, in any case the super-long symmetric mode had to be included, in order to explain the dip in overlineTKE close to (mass) symmetry.

[1] S. Oberstedt, F.-J. Hambsch and F. Vivès; accepted by Nucl. Phys. A

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

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