Physics – Nuclear Physics – Nuclear Experiment
Scientific paper
2010-04-07
Physics
Nuclear Physics
Nuclear Experiment
38 pages, 15 figures
Scientific paper
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.
Heinz Achim
Jurado Beatriz
Kelic Aleksandra
Nadtochy P. N.
Schmidt Karl-Heinz
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