Evolution of signatures of quasifission in reactions forming curium

Abstract

Background: Quasifission, a fission-like reaction outcome in which no compound nucleus forms, is an important competitor to fusion in reactions leading to superheavy elements. The precise mechanisms driving the competition between quasifission and fusion are not well understood. Purpose: To understand the influence reaction parameters have on quasifission probabilities, an investigation into the evolution of quasifission signatures as a function of entrance channel parameters is required. Methods: Using the Australian National University's (ANU) CUBE detector for two-body fission studies, measurements were made for a wide range of reactions forming isotopes of curium. Important quasifission signatures-namely, mass-ratio spectra, mass-angle distributions, and angular anisotropies-were extracted. Results: Evidence of quasifission was observed in all reactions, even for those using the lightest projectile (12 C + 232 Th). But the observables showing evidence of quasifission were not the same for all reactions. In all cases, mass distributions provided some evidence of the possible presence of quasifission but were not sufficient in most cases to clearly identify reactions for which quasifission was important. For reactions using light projectiles (12 C, 28,30 Si, 32 S), experimental angular anisotropies provided the clearest signature of quasifission. For reactions using heavier projectiles (48 Ti, 64 Ni), the presence of mass-angle correlations in the mass-angle distributions provided strong evidence of quasifission and also provided information about quasifission timescales. Conclusions: The observable offering the clearest signature of quasifission differs depending on the reaction timescale.

Systematic study of quasifission characteristics and timescales in heavy element formation reactions

“…Quasifission mass-angle distributions (MAD) first measured at GSI in the 1980s [2,5] showed that quasifission timescales could often be shorter than the rotation time of ∼10 −20 s. However, subsequently only a few measurements [6,7] were made until recent years, when an extensive series of experiments (using the Australian National University Heavy Ion Accelerator Facility and CUBE spectrometer) were carried out [3,[8][9][10][11][12][13][14][15][16]. The kinematic coincidence technique used in the measurements [2,3,17] provides direct information on the mass-ratio of the fragments at scission; thus, the data are represented in terms of mass ratio M R , rather than pre-or According to the characteristics of the MAD (minimum mass yield at symmetry, mass-angle correlation with peak yield at symmetry, and no significant mass-angle correlation), they are assigned as type MAD1, MAD2 and MAD3 respectively [3].…”

How signatures of quasifission evolve in reactions forming Curium

“…Pulsed beams ranging from (f-j) Mass ratios for each studied reaction, compared with a single (red) or, where applicable, a dual (blue) Gaussian fit, as described in Ref. [18]. For all results in this figure, only one energy for each reaction is shown for brevity; similar results were obtained for other excitation energies.…”

“…A summary of the reactions chosen and any relevant reaction parameters is provided in Table I of Ref. [18].…”

“…Because of this competition between fusion and quasifission, the latter has long been the subject of intense study (see, e.g., Refs. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]). …”

“…[18], we will focus on three signatures of quasifission: mass distributions, angular anisotropies, and mass-angle distributions. Mass distributions have been shown to exhibit enhanced widths in reactions dominated by quasifission in comparison to equivalent reactions dominated by fusion-fission [13,22].…”