Fission time scales from anisotropic in-plane distributions inMo100+

Abstract

The characteristics of the fission step following a binary deep-inelastic interaction have been reconstructed for three-body events detected in the reactions 100 Mo + 100 Mo at 18.7A MeV and 120 Sn -f-120 Sn at 18.AA MeV. The observed anisotropy of the in-plane angular distributions points to the fast decay of a rotating (and strongly deformed) nuclear object formed at the end of the deep-inelastic interaction. The derived time scale of the process indicates that asymmetric divisions are faster than symmetric ones. PACS numbers: 25.70.Lm, 25.70.Gh Interest in nuclear fission in general and particularly in the determination of its characteristic time scale has been revived after a series of recent measurements of prescission neutrons (see Ref.[1], and references therein).As compared to particle emission, nuclear fission is expected to be a slower process, due to the complex change of collective degrees of freedom involved in it [2]. Experimentally, this expectation was repeatedly confirmed by the observation of isotropic in-plane angular distributions in a large number of fusion-fission reactions, as well as in the sequential fission decay of heavy reaction products [3]. This fact points to the nucleus undergoing, on the average, at least one full rotation before scission and sets a lower limit of several 10~2 1 s for the fission time scale. Recent studies on neutron emission in fusion-fission reactions have argued that the motion towards scission is highly viscous and leads to fission times of the order of 10 -20 s to 10 -19 s [1,4-6]. This time seems to decrease with increasing mass asymmetry, as reported in recent works based on the detection of neutrons [1,7,8] or light charged particles [9] in coincidence with heavy fragments.Some indications of short time scales (« 10~2 1 s) were obtained also from fragment correlations in sequential fissionlike decays following collisions at intermediate bombarding energies [10,11] and from "proximity" modulations of the relative velocity of sequential-fission fragments in the 129 Xe+ 122 Sn collision at 12.5,4 MeV [12]. In this latter case, anisotropic in-plane angular distributions were also observed, pointing to three-body events characterized by a preferential collinearity of the three fragments, with the lighter fission fragment roughly located in between the other two.This Letter presents for the first time evidence that the in-plane angular distribution of a fissionlike decay evolves from a "well behaved" isotropic shape for symmetric splits towards a strong anisotropy for the most asymmetric splits. Moreover, a novel and independent method of estimating the time scale of the process is suggested, based on the analysis of the shape of the in-plane angular distribution.Two symmetric systems were investigated, namely, 100 Mo + 100 Mo at 18.7A MeV and 120 Sn + 120 Sn at 18.4A MeV. Heavy fragments (A > 20) were detected with twelve position-sensitive gas detectors covering about 75% of the forward hemisphere [13,14]. Prom the measured velocity vectors, primary (pre-e...

Origins of intermediate velocity particle production in heavy ion reactions

“…Recent results for the heavier system 116 Sn+ 93 Nb at 29.5 MeV/nucleon presenting the necessity of adding a surface emission component to the neck component at mid-velocity are compatible with this expectation [27]. It can also be noted that the phenomenon of delayed aligned asymmetric breakup is potentially related to previously observed processes of dynamical projectile splitting [29] and fast asymmetric fission [30] at lower bombarding energies. It will be interesting in future analysis and experiments to study the isospin dependance of the IV products according to their specific origin, in order to disentangle shape, thermal and chemical equilibration processes.…”

Dynamical effects in multifragmentation at intermediate energies

“…Up to now, these entrance channel effects have been studied by tracking deviations from the standard fission process in the quasi-projectile break-up [13,14,18,19,22]. Such deviations have been seen on the angular distributions, which show the focusing of the break-up axis along the quasi-projectile velocity direction.…”

“…Statistical decay models are widely used and dedicated to describe the decay of hot fully equilibrated systems, defined as systems having reached energy, shape and isospin equilibrium [1,2,3,4,5,6,7,8,9]. However other experimental works show strong effects of the entrance channel on the decay modes of the formed hot systems [10,11,12,13,14,15,16,17,18,19,20,21,22,23]. These effects which are not taken into account in the statistical decay models can provide complementary information on the nuclei such as the characteristics of the nucleon-nucleon interaction [24,25,26,27] and/or the viscosity of the nuclear matter.…”

Coulomb chronometry to probe the decay mechanism of hot nuclei

“…This information is of great importance in view of constraining reaction models with predictive power in this energy regime. Several methods have been proposed for time-scale measurement in peripheral heavy-ion collisions, which are dominated by deep-inelastic reactions, neck formation and decay, and so-called dynamical fission of projectileor target-like nuclei [6,16,17]. Such methods were recently used to probe the isospin equilibration between projectile and target nuclei [18][19][20][21].…”

“…Only by detecting all reaction products in coincidence and achieving a full kinematical reconstruction event by event can we hope to better understand the underlying reaction and decay mechanisms. Such exclusive experimental data are relatively scarce for multibody exit channels in this energy range [4][5][6][7] leaving room for ambiguities in the interpretation of the reaction mechanism. New theoretical efforts are made to cover this energy range including time dependent microscopic approaches [8,9], transport models [10] and molecular dynamics calculations [11], which require comparison with new exclusive measurements in order to advance.…”