Systematic evidence for quasifission in Be9−, C12−, and O16 -induced reactions forming No258,260

Abstract

Background: Cross sections for the formation of superheavy elements (SHE) by heavy ion fusion are suppressed by the competing quasifission process. This results in a fissionlike decay after capture but before formation of a compact compound nucleus. Fast quasifission is evident from very mass-asymmetric fission, focused in angle. In contrast, slow quasifission shows no significant mass-angle correlation, and a mass distribution peaked at symmetry. However, it shows angular distributions more anisotropic than those calculated for fission following fusion. Following fusion, low excitation energies should increase SHE survival through reduced competition from fission. However, in reactions with deformed actinide target nuclei, subbarrier fusion is highly suppressed by both fast and slow quasifission.Purpose: To investigate the threshold for quasifission by investigating signatures of slow quasifission in both fission angular and mass distributions, as a function of beam energy with respect to the capture barrier, for the projectiles $^{9}\mathrm{Be}$, $^{12}\mathrm{C},$ and $^{16}\mathrm{O}$ that form the neighboring compound nuclei $^{258,260}\mathrm{No}$.Methods: Fission mass and angular distributions have been measured from below to above-barrier energies using the kinematic coincidence method for the reactions $^{9}\mathrm{Be}+^{249}\mathrm{Cf}$, $^{12}\mathrm{C}+^{248}\mathrm{Cm}$, and $^{16}\mathrm{O}+^{244}\mathrm{Pu}$. Fission following transfer reactions can significantly contaminate fission events that follow capture, and must be rejected. Existing methods to reject transfer-induced fission have been refined to allow quantitative subtraction of the transfer fission component.Results: The capture-fission mass-angle distributions show no evidence for fast quasifission, as might be expected. However, measured fission fragment angular anisotropies are larger than transition state model (TSM) calculations for fusion fission. The deviations increase with larger projectile charge and for bombarding energies below the mean capture barrier energy. Even for the $^{9}\mathrm{Be}+^{249}\mathrm{Cf}$ reaction, the subbarrier angular anisotropy significantly exceeds the TSM calculation. Fission mass distributions measured at the same excitation energies also show a consistent dependence on the projectile charge.Conclusions: New refined analysis techniques have been developed to enable reliable separation of fission following capture from sequential fission following transfer reactions. For fission following capture at above-barrier energies, the $^{9}\mathrm{Be}$ angular anisotropies are close to the TSM predictions, supporting the validity of TSM calculations of fusion-fission for such heavy elements. At subbarrier energies the angular anisotropy data indicate a component of slow quasifission even for $^{9}\mathrm{Be}$, and a probability that increases rapidly with projectile charge. It is concluded that the probability of slow quasifission changes smoothly with projectile charge, having no sharp threshold.