Evolution of signatures of quasifission in reactions forming curium

E. Williams, D. J. Hinde, M. Dasgupta, R. du Rietz, I. P. Carter, M. Evers, D. H. Luong, S. D. McNeil, D. C. Rafferty, K. Ramachandran, and A. Wakhle

Phys. Rev. C 88, 034611 – Published 25 September 2013

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.

Received 29 July 2013

DOI:https://doi.org/10.1103/PhysRevC.88.034611

Authors & Affiliations

E. Williams^*, D. J. Hinde, M. Dasgupta, R. du Rietz^†, I. P. Carter, M. Evers, D. H. Luong, S. D. McNeil, D. C. Rafferty, K. Ramachandran^‡, and A. Wakhle

Department of Nuclear Physics, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 0200, Australia

^*Corresponding author: elizabeth.williams@anu.edu.au
^†Current address: Malmö University, Malmö 205 06, Sweden.
^‡Permanent address: Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India.

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Vol. 88, Iss. 3 — September 2013

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Figure 1
The ANU CUBE spectrometer's Multiwire Proportional Counters can be configured in two ways: with the back detector at (a) $θ = 135^{\circ}$ , which provides better angular coverage for anisotropy measurements and reactions using light projectiles, and (b) $θ = 90^{\circ}$ , which is preferred for mass-angle distribution measurements and reactions with heavier projectiles. The angular coverages (mirrored across both $θ_{c.m.} = 90^{\circ}$ and $M_{R} = 0.5$ ) for the $θ = 135^{\circ}$ and $θ = 90^{\circ}$ configurations, respectively, are illustrated using mass-angle distributions in panels (c) and (d).Reuse & Permissions
Figure 2
(a) Experimental fission cross sections are compared to calculated fusion cross sections for $^{30} Si +^{206} Pb$ . Calculations were performed using the code ccfull following the procedure described and are remarkably close to observed values considering that no optimization of the Woods-Saxon potential parameters has been attempted. The evaporation-residue cross sections are expected to be negligible for this system; thus, $σ_{cap} \sim σ_{fis}$ . (b) Average angular momentum squared from ccfull corresponding to the cross-section calculations shown in panel (a). As one can see, the diffuseness parameter $a$ has a small effect on the average angular momentum squared.Reuse & Permissions
Figure 3
(Top row) Mass-angle distributions for each reaction are displayed for a single representative beam energy for the reactions indicated. The ratio of center-of-mass energy to average capture barrier ( $E / V_{B}$ ) and the compound-nucleus excitation energy $E^{*}$ (in MeV) are also given. (Bottom row) Mass ratios are compared to single- or double-Gaussian fits as described in the text.Reuse & Permissions
Figure 4
(a) $σ_{M R}$ values from the single-Gaussian fit plotted versus excitation energy $E^{*}$ for all reactions where such a fit was possible. In all but the reaction with the lightest projectile ( $^{12} C +^{232} Th$ ), the fit range was narrowed to $0.27 \leq m_{R} \leq 0.73$ in order to exclude the elastics regions from the fit. In this portion of the figure, the lines connecting the data points are included to guide the eye. (b) $〈 ℓ^{2} 〉$ values calculated using ccfull and ccdegen for a diffuseness parameter of $a = 0.65$ . The color scheme is consistent with that used for the data in the top figure.Reuse & Permissions
Figure 5
Observed and calculated anisotropies are shown for reactions using the lightest three projectiles. The angular anisotropy data presented for $^{12} C +^{232} Th$ are from Ref. [37]; data presented for $^{28} Si +^{208} Pb$ have been taken from Refs. [53, 57]. All other data are from this work. The TSM calculation results (presented as solid lines) are shown for calculations using three different diffuseness parameters, as discussed in Sec. 4. For the middle panel, TSM calculations are for $^{30} Si +^{206} Pb$ , using the same color scheme for the diffuseness parameters as shown in the legend for the panel to the right. In all three cases, the TSM is inconsistent with experiment.Reuse & Permissions

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