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

Abstract

38 pages, 15 figuresPeripheral 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

Recent highlights on fragmentation reactions

“…Although transient effects in fission are known since long ago [26], the novelty of the recent investigations performed at GSI is that they are based on the use of fragmentation reactions induced by heavy exotic nuclei. These reactions make possible to induce fission under well defined conditions such as low initial deformation and high excitation energy [27,28]. In this work the observables used to determine transient effects and most particularly the influence of the initial conditions in transient effects were the total fission cross section and the temperature of the fissioning system at saddle, determined from the width of the charge distribution of the final fission fragments.…”

Investigating the dynamics of fission with low-fissility highly-excited nuclei

“…However, the possible dependence of nuclear friction with temperature, deformation or fissility is still under debate [7,8]. On another hand, the manifestation of transient effects may depend on the particular conditions of the fissioning nuclei [9,10].…”

Dynamical effects in fission investigated at high excitation energy

“…An interesting alternative to fusion-fission is the use of spallation reactions induced by energetic protons [40,41] or anti-protons [42], and fragmentation reactions [30,43]. These reactions are expected to fulfill the conditions for the manifestation of dissipative effects in fission [6] because they can produced fissioning nuclei with large values of excitation energy but low angular momenta.…”

“…[43,44] where the charge distribution of the final fission fragments was sorted as a function of the sum of the atomic numbers of the two fragments. In this case, the sum of the atomic numbers of the two fragments represents the size of the fissioning nuclei.…”