Measurement of the fusion probability,PCN, for hot fusion reactions

Abstract

Background: The cross section for forming a heavy evaporation residue in fusion reactions depends on the capture cross section, the fusion probability, P CN , i.e., the probability that the projectile-target system will evolve inside the fission saddle point to form a completely fused system rather than re-separating (quasifission), and the survival of the completely fused system against fission. P CN is the least known of these quantities. Methods: We measured the fission fragment angular distributions for these reactions and used the formalism of Back to deduce the fusion-fission and quasifission cross sections. From these quantities we deduced P CN for each reaction. Conclusions:The new measured values of P CN agree roughly with the semi-empirical systematic dependence of P CN upon fissility for excited nuclei.

Shape evolution and collective dynamics of quasifission in the time-dependent Hartree-Fock approach

“…In principle, it has to be treated dynamically, while equation 3 is based on a statistical approximation. In addition, the usual choice for the nuclear moment of inertia for the quasifission component, 0 / ef f = 1.5 [21,26,34], is somewhat arbitrary. Here, 0 is the moment of inertia of an equivalent spherical nucleus.…”

“…The detailed definition of various mathematical functions can be found in Refs. [21,99]. The quantum number K is known to play an important role in fission [102].…”

“…In particular, they can be used to disentangle fast quasi-fission processes (few zeptoseconds) to longer reaction mechanisms associated with contact times between the fragments exceeding 10 − 20 zs (i.e., long-time quasifission and fusion-fission). However, the analysis of such fragment mass-angle distributions [30,31], as well as statistical descriptions of fragment angular distributions [26], require external parameters such as moment of inertia and temperature which are chosen somewhat arbitrarily [21,26,34]. It is therefore important to provide realistic evaluation of these parameters, in particular for the quasifission mechanism which requires a description of the complex nuclear dynamics.…”

“…(1), the determination of P CN contains the most uncertainty which can be as much as 1−2 orders of magnitude [20,21]. Experimentally, P CN can be extracted from the measurement of fusion-evaporation residue crosssections [22,23].…”

“…For instance, assuming that the quasifission process, as fusion-fission, is statistical, the critical angular momentum J CN between fusionfission and quasifission can be adjusted to reproduce the experimental angular distribution of symmetric frag-ments [21,26,27]. Although this approach has been widely used, it could have limitations due to the fact that quasifission is not a statistical decay but a dynamical process, and that fusion-fission could be asymmetric due to, e.g., late-chance fusion-fission at low excitation energies [28] as well as shell structure of pre-scission configurations [29].…”