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Title: Variation of variance of fission fragment mass distribution: a probe to study the dynamics of fusion-fission reactions

Abstract

Fragment mass distributions in fusion-fission reactions of light projectiles (12C, 16O and 19F) on thorium and bismuth targets in near and below Coulomb barrier energies are investigated. Precise and systematic measurements of mass distribution shows a sudden anomalous increase in variances of mass distributions ({sigma}{sub m}{sup 2}) near Coulomb barrier energies for all three projectiles with deformed thorium target, in contrast to a smooth variation of {sigma}{sub m}{sup 2} with energy for spherical bismuth target. Microscopic effects due to change in entrance channel shape compactness for projectiles hitting the polar region of prolate thorium target is postulated to reach a almost symmetric saddle without complete fusion for events of anomalous fragment widths.

Authors:
 [1]
  1. Variable Energy Cyclotron Centre, 1/AF, Bidhannagar, Kolkata 700064 (India)
Publication Date:
OSTI Identifier:
21056772
Resource Type:
Journal Article
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 891; Journal Issue: 1; Conference: 6. Symposium on nuclear physics, Tours (France), 5-8 Sep 2006; Other Information: DOI: 10.1063/1.2713547; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
73 NUCLEAR PHYSICS AND RADIATION PHYSICS; BISMUTH 209 TARGET; CARBON 12 REACTIONS; COULOMB FIELD; FISSION; FISSION FRAGMENTS; FLUORINE 19 REACTIONS; HEAVY ION FUSION REACTIONS; MASS DISTRIBUTION; OXYGEN 16 REACTIONS; THERMONUCLEAR REACTIONS; THORIUM 238 TARGET; VARIATIONS

Citation Formats

Ghosh, Tilak Kumar. Variation of variance of fission fragment mass distribution: a probe to study the dynamics of fusion-fission reactions. United States: N. p., 2007. Web. doi:10.1063/1.2713547.
Ghosh, Tilak Kumar. Variation of variance of fission fragment mass distribution: a probe to study the dynamics of fusion-fission reactions. United States. doi:10.1063/1.2713547.
Ghosh, Tilak Kumar. Mon . "Variation of variance of fission fragment mass distribution: a probe to study the dynamics of fusion-fission reactions". United States. doi:10.1063/1.2713547.
@article{osti_21056772,
title = {Variation of variance of fission fragment mass distribution: a probe to study the dynamics of fusion-fission reactions},
author = {Ghosh, Tilak Kumar},
abstractNote = {Fragment mass distributions in fusion-fission reactions of light projectiles (12C, 16O and 19F) on thorium and bismuth targets in near and below Coulomb barrier energies are investigated. Precise and systematic measurements of mass distribution shows a sudden anomalous increase in variances of mass distributions ({sigma}{sub m}{sup 2}) near Coulomb barrier energies for all three projectiles with deformed thorium target, in contrast to a smooth variation of {sigma}{sub m}{sup 2} with energy for spherical bismuth target. Microscopic effects due to change in entrance channel shape compactness for projectiles hitting the polar region of prolate thorium target is postulated to reach a almost symmetric saddle without complete fusion for events of anomalous fragment widths.},
doi = {10.1063/1.2713547},
journal = {AIP Conference Proceedings},
number = 1,
volume = 891,
place = {United States},
year = {Mon Feb 26 00:00:00 EST 2007},
month = {Mon Feb 26 00:00:00 EST 2007}
}
  • Fission-fragment mass distribution has been studied for the {sup 16}O+{sup 232}Th,{sup 209}Bi systems over an energy range of 102.8-78.6 MeV and 81.6-72.6 MeV, respectively, in a laboratory frame. The variance of the mass distribution ({sigma}{sub m}{sup 2}) for the {sup 16}O+{sup 209}Bi system varies linearly with center of mass energy, while a significant anomalous behavior is found for the system {sup 16}O+{sup 232}Th. Coupled with our earlier observation for the system {sup 19}F+{sup 232}Th [T. K. Ghosh et al., Phys. Rev. C 69, 031603(R) (2004)], we propose that the accurate measurement of mass distribution is a powerful tool to lookmore » for the onset of a nonstatistical reaction mechanism in heavy-ion-induced fission of deformed heavy nuclei.« less
  • The energy dependence of the variance of the mass distribution of fission fragments was investigated, and it was shown that for nuclei with Zless than or equal to90 this distribution was formed at the saddle point.
  • Mass distributions of fission fragments from the compound nuclei 180Hg and 190 Hg formed in fusion reactions 36Ar + 144 Smand 36Ar + 154Sm, respectively, were measured at initial excitation energies of E*( 180Hg) = 33-66 MeV and E*( 190Hg) = 48-71 MeV. In the fission of 180Hg, the mass spectra were well reproduced by assuming only an asymmetric-mass division, with most probable light and heavy fragment masses more » $$\overline{A}_L$$/ $$\overline{A}_H$$ = 79/101. The mass asymmetry for 180Hg agrees well with that obtained in the low-energy ╬▓ +/EC-delayed fission of 180Tl, from our earlier ISOLDE(CERN) experiment. Fission of 190Hg is found to proceed in a similar way, delivering the mass asymmetry of $$\overline{A}_L$$/ $$\overline{A}_H$$ = 83/107, throughout the measured excitation energy range. The persistence as a function of excitation energy of the mass-asymmetric fission for both proton-rich Hg isotopes gives strong evidence for the survival of microscopic effects up to effective excitation energies of compound nuclei as high as 40 MeV. In conclusion, this behavior is different from fission of actinide nuclei and heavier mercury isotope 198Hg.« less
  • The analyses and results of the application of quantum molecular dynamics plus fission model to intermediate-energy-proton-induced reactions on {sup 208}Pb are described. The mass distribution of residual nuclei and spallation fragments induced by protons incident on {sup 208}Pb with energies of 322, 660, 759, and 1600 MeV are investigated, and the results are found to be in good agreement with experimental data.
  • The fragment angular distributions and excitation functions of the fission following complete fusion (FFCF) have been measured after separating them from targetlike-fragment fission (TLFF) for the {sup 19}F+{sup 232}Th system in the bombarding energy range of 84.5 to 106.5 MeV. The fraction of the targetlike-fragment fission was observed to increase with decreasing bombarding energy below the Coulomb barrier. The excitation function for fission following complete fusion reaction agrees well with coupled channel calculations. However, the {l_angle}{ital l}{sup 2}{r_angle} values derived from the fragment anisotropy data of the FFCF events are found to be much larger than those calculated using themore » coupled channel transmission coefficient values. The discrepancy between the experimental and calculated {l_angle}{ital l}{sup 2}{r_angle} values increases as the bombarding energy is decreased below the barrier.« less