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Fragmentation of spherical radioactive heavy nuclei as a novel probe of transient effects in fission

Abstract : Peripheral collisions with radioactive heavy-ion beams at relativistic energies are discussed as an innovative approach for probing the transient regime experienced by fissile systems evolving towards quasi-equilibrium. A dedicated experiment using the advanced technical installations of GSI, Darmstadt, permitted to realize ideal conditions for the investigation of relaxation effects in the meta-stable well. Combined with a highly sensitive experimental signature, it provides a measure of the transient effects with respect to the flux over the fission barrier. Within a two-step reaction process, 45 proton-rich unstable spherical isotopes produced by projectile-fragmentation of a stable 238U beam have been used as secondary projectiles. The fragmentation of the radioactive projectiles on lead results in nearly spherical compound nuclei which span a wide range in excitation energy and fissility. The decay of these excited systems by fission is studied with a dedicated set-up which permits the detection of both fission products in coincidence and the determination of their atomic numbers with high resolution. The width of the fission-fragment nuclear charge distribution is shown to be specifically sensitive to pre-saddle transient effects and is used to establish a clock for the passage of the saddle point. The comparison of the experimental results with model calculations points to a fission delay of (3.3+/-0.7).10-21s for initially spherical compound nuclei, independent of excitation energy and fissility. This value suggests a nuclear dissipation strength at small deformation of (4.5+/-0.5).1021s-1. The very specific combination of the physics and technical equipment exploited in this work sheds light on previous controversial conclusions.
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http://hal.in2p3.fr/in2p3-00471325
Contributor : Michel Lion <>
Submitted on : Thursday, April 8, 2010 - 9:00:49 AM
Last modification on : Saturday, April 14, 2018 - 11:46:03 AM

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C. Schmitt, K.-H. Schmidt, A. Kelić, A. Heinz, B. Jurado, et al.. Fragmentation of spherical radioactive heavy nuclei as a novel probe of transient effects in fission. Physical Review C, American Physical Society, 2010, 81, pp.064602. ⟨10.1103/PhysRevC.81.064602⟩. ⟨in2p3-00471325⟩

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