Near- and sub-barrier fission fragment anisotropies and the failure of the statistical theory of fission decay rates

Lestone, J. P.; Sonzogni, A. A.; Kelly, Michael; Vandenbosch, R.

doi:10.1088/0954-3899/23/10/024

Cited by 33 publications

(45 citation statements)

References 24 publications

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“…Similar observations were made by Refs. [25,26] using light projectiles ranging from 7 Li to 16 O on a range of actinide targets, confirming the effect of deformation on QF. However, measurements of ER cross sections for the 16 O + 238 U reaction showed that CN formation is still present at sub-barrier energies for this reaction [27].…”

Section: Introductionmentioning

confidence: 60%

Strong evidence for quasifission in asymmetric reactions formingPo202

et al. 2008

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Fission fragment mass-angle correlations and mass ratio distributions have been measured for the reactions 16 O + 186 Os, 24 Mg + 178 Hf, 34 S + 168 Er, and 48 Ti + 154 Sm, forming the 202 Po composite nucleus, at near barrier energies. Systematic analysis based on the expected dependence of the variance of the mass distribution on the angular momentum and temperature of the compound nucleus indicate that the two lighter systems evolve through true compound nucleus fission. Evidence of quasifission was observed for the two most mass-symmetric reactions, through strong mass-angle correlations for the 48 Ti + 154 Sm reaction and a broadened mass ratio distribution for the 34 S + 168 Er reaction. Furthermore, the increase in mass width at near barrier energies shows the influence of the alignment of statically deformed target nuclei.

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Section: Introductionmentioning

confidence: 60%

Strong evidence for quasifission in asymmetric reactions formingPo202

et al. 2008

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“…[19,54] is reasonable and allows the reproduction of the anisotropy of the angular distribution for heavy fissioning nuclei. The obtained relaxation time of the K coordinate τ K ∼ 10 −20 s is in agreement with previous studies.…”

Section: Discussionmentioning

confidence: 99%

“…The above-mentioned value of the friction parameter γ K used in Refs. [17,19] was obtained by Lestone and coauthors in their early study [54] from an analysis of fission fragment angular distribution measured for a number of fusion-fission reactions. This value was further used in Refs.…”

Section: Modelmentioning

confidence: 99%

Four-dimensional Langevin dynamics of heavy-ion-induced fission

et al. 2012

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A four-dimensional dynamical model based on Langevin equations was developed and applied to calculate a wide set of experimental observables for the reactions 16 O + 208 Pb → 224 Th and 16 O + 232 Th → 248 Cf over a wide range of excitation energy. The fusion-fission and evaporation residue cross sections, fission fragment mass-energy distribution parameters, prescission neutron multiplicities, and anisotropy of angular distribution of fission fragments could be reasonably reproduced using a modified one-body mechanism for nuclear friction with a reduction coefficient of the contribution from a wall formula k s 0.25 and a dissipation coefficient for the orientation degree of freedom (K coordinate) γ K 0.077 (MeV zs) −1/2 . Inclusion of the K coordinate into calculation of potential energy changes the stiffness of the nucleus with respect to mass asymmetry coordinate for the values of K = 0 and results in a shift of the Businaro-Gallone point towards larger Z 2 /A values. The experimental data on the fission fragment mass-energy distribution parameters together with mean prescission neutron multiplicity for heavy fissioning nuclei are reproduced through the four-dimensional Langevin calculations more accurately than through three-dimensional calculations.

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“…Experimental measurements of A have been selected from the literature [16,36,51,[62][63][64][65][66] for reactions of projectiles from B to S, bombarding targets of 232 Th and 238 U. Selection was based on consistency between measurements by different authors, the effectiveness of the method of separation of transfer- induced fission (and its likely influence on the anisotropies), and the consistent variation of A as a function of beam energy within a given experiment.…”

Section: Experimental Anisotropies For Reactions With Thu Targetsmentioning

confidence: 99%

Sub-barrier quasifission in heavy element formation reactions with deformed actinide target nuclei

et al. 2018

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Background: The formation of superheavy elements (SHEs) by fusion of two massive nuclei is severely inhibited by the competing quasifission process. Low excitation energies favor SHE survival against fusion-fission competition. In "cold" fusion with spherical target nuclei near 208 Pb, 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 Z P = 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 Z P ∼ 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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Near- and sub-barrier fission fragment anisotropies and the failure of the statistical theory of fission decay rates

Cited by 33 publications

References 24 publications

Strong evidence for quasifission in asymmetric reactions formingPo202

Strong evidence for quasifission in asymmetric reactions formingPo202

Four-dimensional Langevin dynamics of heavy-ion-induced fission

Sub-barrier quasifission in heavy element formation reactions with deformed actinide target nuclei

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