Reexamination of the nuclear orientation model of quasifission reactions to explain anomalous fragment anisotropies at sub-barrier energies

Majumdar, N.; Bhattacharya, Purba; Biswas, D. C.; Choudhury, R. K.; Nadkarni, D.M.; Saxena, A.

doi:10.1103/physrevc.53.r544

Cited by 17 publications

(21 citation statements)

References 18 publications

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“…For the 16 O + 232 Th reaction, our folding angle distributions show clear evidence of fission following transfer reactions at near-and sub-barrier energies in agreement with the results of [16]. We have corrected for the fission events following transfer and obtain 16 O + 232 Th fusion-fission anisotropies in agreement with the results of [16].…”

Section: Resultssupporting

confidence: 80%

“…We have studied the cross sections, angular distributions and folding-angle distributions for fission fragments at near-and sub-barrier energies, for the reactions 7 Li, 11 B, 12 C and 16 O + 232 Th; 7 Li and 11 B + 238 U; and 12 C + 235,236,238 U and 237 Np, using pulsed heavy-ion beams from the University of Washington tandem plus superconducting linear accelerator [14]. For the long-lived isotopes 232 Th and 238 U, the targets were ∼250 µg cm −2 thick layers of ThF 4 and UF 4 evaporated onto ∼80 µg cm −2 Ni and ∼20 µg cm −2 C foils, respectively.…”

Section: Methodsmentioning

confidence: 99%

“…We estimated the prescission neutron multiplicities as a function of initial excitation energy using the data of [17]. Figure 1 shows (A exp − 1)/(A TSM − 1) for 7 Li, 11 B, 12 C, 16 O and 19 F + 232 Th reactions as a function of centre-of-mass energy relative to the fusion barrier, E c.m. /V B .…”

Section: Fission Anisotropies Involving 232 Th Targetsmentioning

confidence: 99%

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

Lestone¹,

Sonzogni²,

Kelly³

et al. 1997

J. Phys. G: Nucl. Part. Phys.

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The angular distributions of fission fragments have been measured over a range of near-and sub-barrier energies for reactions involving 7 Li, 11 B, 12 C and 16 O projectiles on 232 Th, 235,236,238 U and 237 Np targets. The discrepancies between our experimental fission anisotropies and the transition state model increase dramatically as the beam energy decreases through the region of the fusion barriers; decrease smoothly with projectile size with a fixed target; show no evidence of a discontinuous behaviour across the Businaro-Gallone ridge in the mass asymmetry degree of freedom; and, at sub-barrier energies, are strongly influenced by the ground-state spin of the targets. A good fit to measured fission anisotropies can be obtained if, immediately following fusion, the system has the K-state distribution of the entrance channel, and this initial distribution is broadened with time due to a coupling between the intrinsic and collective rotational degrees of freedom. If, at well above barrier energies, the entrance channel uniformly populates the K states for each J then the strength of the coupling between the intrinsic and the rotational degrees of freedom required to reproduce observed fission anisotropies leads to a limiting fission timescale of several 10 −20 s. This limiting time is not due to the slowing of nuclear shape changes caused by the viscosity of heated nuclear matter, but is due to the finite time required to change the angle of the symmetry axis relative to the direction of the total spin.

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Section: Resultssupporting

confidence: 80%

Section: Methodsmentioning

confidence: 99%

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

Lestone¹,

Sonzogni²,

Kelly³

et al. 1997

J. Phys. G: Nucl. Part. Phys.

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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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“…

Th Reactions"Majumdar et al [1,2] have recently published measurements of the anisotropies of fission fragments following fusion reactions with 11 B, 12 C, 16 O, and 19 F projectiles on 232 Th. These measurements show unexpected peaklike structures in the anisotropies as a function of center-of-mass energy in the near-and sub-barrier energy regions.

…”

mentioning

confidence: 99%

Comment on “Anomalous Peaklike Structure in the Fission Fragment Anisotropies at Sub-barrier Energies in11B,12C,
Lestone¹,
Sonzogni²,
Kelly³
et al. 1998
Phys. Rev. Lett.
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Th Reactions"Majumdar et al. [1,2] have recently published measurements of the anisotropies of fission fragments following fusion reactions with 11 B, 12 C, 16 O, and 19 F projectiles on 232 Th. These measurements show unexpected peaklike structures in the anisotropies as a function of center-of-mass energy in the near-and sub-barrier energy regions. The 16 O and 19 F measurements are made difficult by the presence of a significant yield of fission following transfer reactions at near-and sub-barrier energies. However, for reactions involving 11 B and 12 C projectiles on Th, U, and Np targets, the transfer-fission yield is small enough that the anisotropy of fragments following full momentum transfer can be obtained by simply measuring the singles fission fragments at all but the lowest beam energies [3][4][5][6].Recent measurements of 12 C 1 232 Th fission fragment anisotropies [3,6] do not show the peaklike structure seen in the data of Ref.[2]. We take this opportunity to respond quickly to the 11 B 1 232 Th data of Majumdar et al. [1]. We have measured the angular distribution of fission fragments in the 11 B 1 232 Th reaction in the energy range E c.m. 46 to 66 MeV, using beams from the University of Washington tandem plus superconducting linear accelerator. The target consisted of a 225 mg͞cm 2 layer of 232 ThF 4 evaporated onto a 100 mg͞cm 2 Ni foil. Fission fragments were detected with Si surface barrier telescopes and identified using energy and time-of-flight information. Folding angle distributions were obtained FIG. 1. Folding angle distributions for u c.m. ϳ 90 ± fission fragments from 11 B 1 232 Th at E c.m. 50.6 MeV (solid circles) and 62.5 MeV (open squares). The E c.m. 62.5 MeV results have been shifted by 1.6 ± to facilitate a comparison of the shape of the folding angle distribution at these two different energies. FIG. 2. 11 B 1 232 Th fission segment anisotropies as a function of center-of-mass energy E c.m. . by observing fission fragments in Si telescopes and the complementary fragments in a large area position sensitive Si detector. Figure 1 shows our u c.m. ϳ 90 ± fission fragment folding angle distributions at E c.m. 50.6 MeV and 62.5 MeV; note the single peaklike structure over 3 orders of magnitude and the lack of any significant change in the shape of the folding angle distribution as a function of energy. Our folding angle distributions confirm that over the energy range of our study, the fission yield in 11 B 1 232 Th reactions is dominated by fission following complete fusion. Figure 2 compares our fission fragment anisotropies (solid circles) to those of Majumdar et al. (open squares). Our anisotropies vary smoothly with center-of-mass energy and give no hint of a peaklike structure. In view of our results and those in Refs. [3,6] we feel that the conclusions drawn in Ref.[1] should be viewed with caution.

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Reexamination of the nuclear orientation model of quasifission reactions to explain anomalous fragment anisotropies at sub-barrier energies

Cited by 17 publications

References 18 publications

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

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

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

Comment on “Anomalous Peaklike Structure in the Fission Fragment Anisotropies at Sub-barrier Energies in11B,12C,
Lestone¹,
Sonzogni²,
Kelly³
et al. 1998
Phys. Rev. Lett.
4
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2
0
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Reexamination of the nuclear orientation model of quasifission reactions to explain anomalous fragment anisotropies at sub-barrier energies

Cited by 17 publications

References 18 publications

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

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

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

Comment on “Anomalous Peaklike Structure in the Fission Fragment Anisotropies at Sub-barrier Energies in11B,12C,Lestone1, Sonzogni2, Kelly3 et al. 1998Phys. Rev. Lett.4020View full textAdd to dashboardCite

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Comment on “Anomalous Peaklike Structure in the Fission Fragment Anisotropies at Sub-barrier Energies in11B,12C,
Lestone¹,
Sonzogni²,
Kelly³
et al. 1998
Phys. Rev. Lett.
4
0
2
0
View full text Add to dashboard Cite