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Mesures de temps de fission par la technique d'ombre dans les monocristaux: un accès à la dissipation nucléaire

Abstract : The aim of this work is to measure fission times by single crystal blocking technique and to interpret them in terms of nuclear dissipation effects. We have studied fission of lead-like nuclei in the $^{208}$Pb\, (29 MeV/u)+Si reaction at GANIL. Fission selection has been performed by Z-identification of the two fission fragments F_(1) and F_(2). The F_(1) angular distribution around the <110>Si single crystal axis direction presents a dip in the <110>axis direction due to blocking effects. The filling of the dip and its shape are directly related to the time needed by the nucleus to undergo fission. An event-by-event excitation energy selection has been performed using the prompt response of ORION, a 4\pi neutron detector. The evolution of the blocking dips with excitation energy has thus been followed. The blocking dips present different evolutions with excitation energy, depending on the Z_(1)+Z_(2) value. For Z_(1)+Z_(2)\leq 82 and E^(*)<350 MeV, long lifetime components (\tau >3\times 10^(-19)s) are observed. Simulations of the blocking effects, including post-scission emission, have been used to perform fits to the angular distributions. An estimation of the fission time distribution has been inferred from these fits for large statistics. In the case of lower statistics, only the average fission times could be inferred. The fission times obtained for lead-like nuclei have been compared to the ones measured for uranium nuclei in a similar experiment. Both times have been interpreted in the framework of statistical models of fission including dissipative effects. For uranium nuclei with E^(*)\leq 250 MeV and for lead-like with 130\leq E^(*)\leq 270 MeV, the measured fission times have been found only compatible with rather high dissipation coefficients, \beta \geq 2\times 10^(21)s^(-1).
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https://tel.archives-ouvertes.fr/tel-00002018
Contributor : Stephane Basnary <>
Submitted on : Monday, March 31, 2003 - 4:28:46 PM
Last modification on : Thursday, February 1, 2018 - 1:35:55 AM
Long-term archiving on: : Monday, September 20, 2010 - 12:09:03 PM

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  • HAL Id : tel-00002018, version 2

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S. Basnary. Mesures de temps de fission par la technique d'ombre dans les monocristaux: un accès à la dissipation nucléaire. Physique Nucléaire Expérimentale [nucl-ex]. Université de Caen, 2002. Français. ⟨tel-00002018v2⟩

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