Cross-section and neutron-emission data from heavy-ion fusion-fission reactions are consistent with the fission of fully equilibrated systems with fission lifetime estimates obtained via a Kramers-modified statistical model which takes into account the collective motion of the system about the ground state, the temperature dependence of the location and height of fission transition points, and the orientation degree of freedom. If the "standard" techniques for calculating fission lifetimes are used, then the calculated excitation-energy dependence of fission lifetimes is incorrect. We see no evidence to suggest that the nuclear viscosity has a temperature dependence. The strong increase in the nuclear viscosity above a temperature of ~1.3 MeV deduced by others is an artifact generated by an inadequate fission model. DOI: PACS number(s) : 24.75.+i, 24.60.Ky, 25.70. ER fus ER fus f J J J J J ⎛ − + = J J J J T
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.
Frescission charged-particle multiplicities, following fusion of ^^^'i6'''''^0Er+^^Si, have been measured. The multipHcities at the lowest bombarding energies limit the statistical model level density parameters. More importantly, the a data restrict the time spent near equilibrium and suggest evaporation occurs predominantly from larger deformations.
The energy spectra of 344, 779, 964, 1086/1112 and 1408 keV y rays scattered through 5, 7, 10 and 15" by targets of AI, Cu, MO, Sn, Ta and Pb have been measured using a Ge(Li) detector. The Compton peaks are closely fitted by a convolution of an instrumental lineshape and a theoretical lineshape computed using the relativistic impulse model of bound Compton scattering. Integrated energy spectra yield incoherent scattering functions in very close agreement with theoretical values taken from existing tabulations based upon the Waller-Hartree form factor model and with values obtained by direct numerical integration of the relativistic impulse model lineshapes.
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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