Exploring quasifission characteristics forS34+Th232formingSg266

Abstract

Background: Fission fragments from heavy ion collisions with actinide nuclei show mass-asymmetric and mass-symmetric components. The relative probabilities of these two components vary rapidly with beam energy with respect to the capture barrier, indicating a strong dependence on the alignment of the deformed nucleus with the partner in the collisions.Purpose: To study the characteristics of the mass-asymmetric quasifission component by reproducing the experimental mass-angle distributions to investigate mass evolution and sticking times.Methods: Fission fragment mass-angle distributions were measured for the $^{34}\text{S}+^{232}\text{Th}$ reaction. Simulations to match the measurements were made by using a classical phenomenological approach. Mass ratio distributions and angular distributions of the mass-asymmetric quasifission component were simultaneously fit to constrain the free parameters used in the simulation.Results: The mass-asymmetric quasifission component---predominantly originating from tip (axial) collisions with the prolate deformed $^{232}\text{Th}$---is found to be peaked near $A=200$ at all energies and center-of-mass angles. A Monte Carlo model using the standard mass equilibration time constant of $5.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}$ s predicts more symmetric mass splits. Three different hypotheses assuming (i) a mass halt at $A=200$, (ii) a slower mass equilibration time, or (iii) a Fermi-type mass drift function reproduced the main experimental features.Conclusions: In tip collisions for the $^{34}\text{S}+^{232}\text{Th}$ reaction, mass-asymmetric fission with $A\ensuremath{\sim}200$ is the dominant outcome. The average sticking time is found to be $\ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}$ s, independent of the scenario used for mass evolution.