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

Abstract

We report on a novel experimental approach for studying the dissipative spreading of collective motion in a meta-stable nuclear system, using, for the first time, highly fissile nuclei with spherical shape. This was achieved by fragmentation of 45 radioactive heavy-ion beams at GSI, Darmstadt. The use of inverse kinematics and a dedicated experimental set-up allowed for the identification in atomic number of both fission fragments. From the width of their charge distributions, a transient time of (3.3±0.7). 10-21 s is deduced for initially spherical nuclei.The problem of escape from a meta-stable state appears in many fields as various as fluid mechanics, chemistry or nuclear physics [1]. An excellent test case for such a process is nuclear fission. Once an excited system has been produced, the collective coordinates start adjusting to the potential-energy landscape, and the distribution of shapes inside the fission saddle develops towards thermo-dynamical quasi-equilibrium. After some time delay, all states in the quasi-bound region are populated according to the available phase space, including those above the fission saddle. Once the system leaves the quasi-bound region, it is driven by the dominating Coulomb force to more elongated shapes and finally separates into two fragments.The ideal scenario for studying the dynamics of the fission process is described in the pioneering theoretical work of Grangé, Weidenmüller and collaborators [2, 3]: They have chosen an initial excited system characterized by a spherical shape. In this case, the probability distribution starts from a configuration where the level density has a local maximum. Under this specific initial condition, the maximum of the probability distribution does not move. The distribution only spreads out under the influence of dissipation due to the fluctuating forces. In contrast, the evolution from saddle to scission is characterized by a strong driving force, and it is the friction force which dominates the dissipation mechanism by slowing down the directed motion towards scission and increasing the saddle-to-scission time relative to its non-viscous limit. Thus, in this ideal scenario starting from the bottom of a spherical potential well, the two stages of the fission process probe selectively the two dissipative phenomena, diffusion and friction, in nuclei [4]. The present work is motivated by the idea to study the time scale of nuclear diffusion. † Present address : Université Lyon I, CNRS/IN2P3, IPNL, Rue Enrico Fermi, 69622 Villeurbanne, France.

Dissipative effects in spallation-induced fission ofPb208at high excitation energies

“…The results presented in this work are in good agreement with other experiments based on fragmentation [6,7,31] and spallation reactions [19][20][21] where highly excited fissioning nuclei were produced with small deformation. Concerning the reaction 238 U + CH 2 at 1A GeV investigated by Jurado et al [6] we were also able to reproduce those data using a value for the dissipation strength of β = 4.5 × 10 21 s −1 but including the initial deformation of the prefragments in the calculation of the fission width according to Ref.…”

“…[7,31] it was shown that the quantitative values of the dissipation strength and transient time obtained by Jurado and collaborators were affected by the initial deformation of the fissioning nuclei that were investigated. Therefore a new experiment was proposed by Schmitt and collaborators based on the same experimental setup but investigating the fission of nearly spherical radioactive fissioning systems produced by fragmentation.…”

Disentangling effects of potential shape in the fission rate of heated nuclei

“…Soon afterward, Kramers derived several formulas for the fission rate, which included the strength of dissipation corresponding to the nuclear collective motion [2]. One of the formulas and its modifications, sometimes incorporating the Bohr-Wheeler formula, are widely used in modern dissipative statistical models of fission accompanied by light particle emission [3][4][5][6][7][8]. An alternative way of modeling the process relies on solving numerically the stochastic differential equations [9,10].…”

Significant role of deformation in probing postsaddle nuclear dissipation with light particle emission

“…An essential conclusion deduced from a comprehensive investigation for this phenomenon is that nuclear dissipation is deformation dependent, namely that presaddle friction is weak and postsaddle friction is strong [6]. Some recent works have reported how to make an accurate determination for the presaddle dissipation strength by analyzing various observables, such as evaporation residue cross sections [17][18][19] and its spin distributions [20,21], fission probabilities [22,23], and the width of the fission-fragment charge distributions [24,25]. However, the magnitude of presaddle friction is still debated [26][27][28][29].…”

“…Since these observables [17][18][19][20][21][22][23][24][25] mentioned above are uniquely connected to presaddle dynamics, to explore postsaddle friction, it is necessary to employ those observables that are sensitive to postsaddle dynamics. As light particles can be evaporated along the whole fission path when a compound nucleus proceeds toward scission, they are considered to be main indicators for the postsaddle dissipation effects.…”