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Fission Time Measurements

The evolution with the temperature of an excited nucleus scission time is a powerful way to get information on the nuclear dissipation processes involved during the fission process [1].
An experimental program has been undertaken at GANIL in order to measure in a quite direct way, using the blocking technique in single crystals, the fission times as a function of the fissioning nucleus characteristics (excitation energy, mass, charge, angular momentum).
The first results [2,3] show that fission time scales inferred in previous experiments from pre- or post-scission particle multiplicities [1] are underestimated by orders of magnitude, due the lack of sensitivity of these experiments to long lifetime components.

The principle of fission time measurement [4] by the blocking technique can be briefly summarized as follows. When a single crystal is used as a target, blocking effects give rise to dips in the angular distributions of fission fragments detected around the directions of axes and planes of the crystal. The dips arise from the repulsive potential due to the atomic rows or planes that modifies the fission fragment trajectories within the crystal.
The distribution of the scission distances from the axis or plane of the crystal determines the blocking patterns. This technique has only a restricted range of time sensitivity. On the one hand, if the fissioning nuclei travel through the crystal on a distance on the average much larger than the spacing between two rows or planes, no dip is observed in the angular distribution. On the other hand, if the scissions occur on average within the thermal vibration domain of the atoms of the crystal, the potential experienced by the fission fragments is smeared and the dips in the angular distributions do not depend anymore on the fission times.The typical sensitivity range for the present experiments is between 10-19s and $10-16s.

 

Contact: M.Morjean

FIGURE 1: Average scission times of excited uranium nuclei inferred from the blocking patterns of fission fragments.

m.morjean_pict1

 

 

FIGURE 2: Blocking pattern associated with elasting scattering of uranium nuclei on silicon.

m.morjean_pict2

 

[1] D. Hilscher and H. Rossner, Ann. Phys. Fr. 17, 471 (1992)
[2] M. Morjean et al., Nucl. Phys. A630, 200c (1998)
[3] F. Goldenbaum et al., Phys. Rev. Lett. 82, 5012 (1999)
[4] W. M. Gibson, Ann. Rev. Nucl. Sci. 25, 465 (1975)

 

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