  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.
