Fast Decision in Favor of the Slow Fission Process

Abstract

The fission probability P f of highly excited targetlike nuclei produced in reactions of 2.5 GeV protons on Au, Bi, and U was studied as a function of excitation energy E whereby E is deduced eventwise from the multiplicity of evaporated light particles. At the highest E of 1000 MeV P f amounts to 30% with all 3 target nuclei irrespective of the initial fissility. Statistical-model calculations satisfactorily reproduce the observed evolution of P f with E -provided that no extra transient delay is introduced. Fission thus is decided upon very fast and early in the long deexcitation chain towards scission which comprises as much as 80% of all evaporated alpha particles. DOI: 10.1103/PhysRevLett.95.162701 PACS numbers: 24.75.+i, 24.10.2i, 25.40.Sc, 24.60.Dr At low excitation energy nuclear fission is strongly influenced by nuclear shell effects, resulting, for instance, for U in the well known binary split into two mass asymmetric fission fragments. With increasing excitation these nuclear structure effects are washed out, and macroscopic properties such as nuclear dissipation dominate the collective flow of nuclear matter from the equilibrium deformation via the saddle towards scission into two fragments of about equal size. The overall time elapsed from the equilibrium deformation up to scission is found to be a few times 10 ÿ20 s. This time is quite long compared to other processes such as the emission of neutrons which is 10 to 100 times faster. This observation has led to the notion of fission being a slow process [1].Since the magnitude of nuclear dissipation is most likely deformation dependent [2], the characteristic time governing the flow over the saddle point, where the decision about fission is made, might be considerably shorter than the one close to the scission point. In particular, at high excitation energies, where the emission times of light charged particles are very short, a long transient delay for fission at the saddle point would strongly favor the emission of charged particles, thus reducing the fissility and, consequently, second-and higher-chance fission probabilities. Conversely, low dissipation at the saddle or a minimum transient delay tends to keep fission competitive with particle evaporation even at high excitation. In other words, although the entire fission process is slow, the decision to fission can be fast.We have chosen 2.5 GeV proton-induced reactions to excite three target nuclei, Au, Bi, and U with different fissilities Z 2 =A 31:7, 33.0, and 35.6, respectively, in order to study the evolution of the inelastic reaction, inel , and the fission cross section, f , as a function of excitation energy, E , and fissility. Their ratio, the fission probability P f E f E = inel E , provides the best possible evidence for the presence of dissipative or transient effects in fission. The selected reactions, similar to antiproton [3,4] or peripheral relativistic heavy-ion reactions [5,6], are thought to deposit high thermal excitation with minimum ballast from collective excitations ...

Fission time-scale from the measurement of pre-scission light particles and γ-ray multiplicities

“…For proton and α emission, the combination of binding energy and emission barrier decides whether τ ssc increases or decreases with Z ssc . The result also indicated that the pre-saddle time is less than 2 zs which is in agreement with the conclusion of recent fission time-scale measurements using the fission probability probe [19,20].…”

“…Fission lifetime can be measured using direct methods like crystal blocking technique [14], K-vacancy production [15] and indirect methods like neutron clock [1,12], light charged particle clock [1,16], GDR clock [2,10], evaporation residues (ER) corss-section [2,17,18] and fission probabilities [19,20]. Early measurements [10,12] using the neutron and GDR clocks showed that the fission time-scales were much larger than those predicted by the standard statistical model.…”

CHARACTERISTICS OF LIGHT CHARGED PARTICLE EMISSION IN THE TERNARY FISSION OF 250CF AND 252CF AT DIFFERENT EXCITATION ENERGIES

“…3). Our results nicely agree with the non-Gaussian low-energy tailing observed by Tischenko et al [5], confirmed also by Mutterer et al [6]. …”

First Experiment on Fission Transients in Highly Fissile Spherical Nuclei Produced by Fragmentation of Radioactive Beams

“…[27,28,29]. The present work goes a step further by utilising for the first time highly fissile spherical systems for which fission properties can be studied over a much wider range of excitation energy.…”