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- Hinde, D. J., et al.
(author)
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Experimental investigation of the role of shell structure in quasifission mass distributions
- 2022
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In: Physical Review C. - : American Physical Society. - 2469-9985 .- 2469-9993. ; 106:6
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Journal article (peer-reviewed)abstract
- Background: To understand superheavy element synthesis reactions, quantifying the role of quantum shells in quasifission dynamics is important. In reactions with actinide nuclides, a wide peak in the binary quasifission mass yield is seen, centered close to the 208Pb mass. It is generally attributed to the 208Pb spherical closed shells causing a valley in the potential-energy surface, attracting flux to these mass splits. However, an early experiment studying 48Ca, 50Ti+238U reactions showed strong evidence that sequential fission plays an important role in generating the observed peak. These conflicting interpretations have not been resolved up to now.Purpose: This work aims to measure quasifission mass spectra for reactions with nuclei lighter than 208Pb, having negligible sequential fission, to search for systematic features correlated with the proton shells known to affect low-energy fission mass distributions of the same actinide elements.Methods: Systematic measurements have been made at energies near and below the capture barriers (where quasifission is most prominent) of mass-angle distributions for fission following collisions of 48Ti projectiles with even-even nuclides from 154Sm to 200Hg. Mean excitation energies above the ground-states ranged from 51 to 33 MeV, respectively.Results: With increasing compound nucleus atomic number ZCN, a rapid transition occurs from fission having characteristics of fusion-fission to fast quasifission. The heaviest reactions form 240Cf, 244Fm, and 248No. Low -energy fission of neighboring isotopes is mass asymmetric, correlated with proton number Z = 56. However, peak quasifission yields are at mass-symmetry for all reactions. There appears to be a very small (P-3%) systematic excess of yield correlated with Z = 56, however this is at the limit of sensitivity of the experiment.Conclusions: No significant (>3%) systematic features are seen in the quasifission mass spectra that can be unambiguously identified as resulting from shells. This small influence may result from attenuation of shell effects due to the excitation energy introduced, even in these near-barrier reactions giving low excitation energies typical of superheavy element synthesis reactions.
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| 2. |
- Hinde, D. J., et al.
(author)
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Sub-barrier quasifission in heavy element formation reactions with deformed actinide target nuclei
- 2018
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In: Physical Review C. - : American Physical Society. - 2469-9985 .- 2469-9993. ; 97:2
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Journal article (peer-reviewed)abstract
- Background: The formation of superheavy elements (SHEs) by fusion of two massive nuclei is severely inhibited by the competing quasifission process. Lowexcitation energies favor SHE survival against fusion-fission competition. In "cold" fusion with spherical target nuclei near Pb-208, SHE yields are largest at beam energies significantly below the average capture barrier. In "hot" fusion with statically deformed actinide nuclei, this is not the case. Here the elongated deformation-aligned configurations in sub-barrier capture reactions inhibits fusion (formation of a compact compound nucleus), instead favoring rapid reseparation through quasifission. Purpose: To determine the probabilities of fast and slow quasifission in reactions with prolate statically deformed actinide nuclei, through measurement and quantitative analysis of the dependence of quasifission characteristics at beam energies spanning the average capture barrier energy. Methods: The Australian National University Heavy Ion Accelerator Facility and CUBE fission spectrometer have been used to measure fission and quasifission mass and angle distributions for reactions with projectiles from C to S, bombarding Th and U target nuclei. Results: Mass-asymmetric quasifission occurring on a fast time scale, associated with collisions with the tips of the prolate actinide nuclei, shows a rapid increase in probability with increasing projectile charge, the transition being centered around projectile atomic number ZP = 14. For mass-symmetric fission events, deviations of angular anisotropies from expectations for fusion fission, indicating a component of slower quasifission, suggest a similar transition, but centered around ZP similar to 8. Conclusions: Collisions with the tips of statically deformed prolate actinide nuclei show evidence for two distinct quasifission processes of different time scales. Their probabilities both increase rapidly with the projectile charge. The probability of fusion can be severely suppressed by these two quasifission processes, since the sub-barrier heavy element yield is likely to be determined by the product of the probabilities of surviving each quasifission process.
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| 3. |
- Hinde, D. J., et al.
(author)
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Systematic study of quasifission characteristics and timescales in heavy element formation reactions
- 2016
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In: 12th International Conference on Nucleus-Nucleus Collisions 2015. - : EDP Sciences.
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Conference paper (peer-reviewed)abstract
- Superheavy elements can only be created in the laboratory by the fusion of two massive nuclei. Mass-angle distributions give the most direct information on the characteristics and time scales of quasifission, the major competitor to fusion in these reactions. The systematics of 42 mass-angle distributions provide information on the global characteristics of quasifission. Deviations from the systematics reveal the major role played by the nuclear structure of the two colliding nuclei in determining the reaction outcome, and in hindering or favouring heavy element production.
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