Publications (4)0 Total impact
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ABSTRACT: Fission fragment angular distributions following transfer have been measured for 232Th + 16O at energies of 86 and 90 MeV. The distributions for a given scattering angle are strongly peaked forward and backward of the recoil direction. However, when all recoil angles are considered, the integrated angular distribution, relative to the beam direction, is less pronounced than that observed when both fusion-fission and transfer-fission are included in the measurement. Thus the anomalously large fusion spin distribution extracted from such measurements persists, even when the effects of transfer are taken into account. However, our measurements suggest that transferfission can have large anisotropies when appropriate reaction Q-values are involved. Then, in a singles measurement, a decrease in anisotropy, with increasing energy, may be expected near the fusion barrier. Such behaviour has been observed for 232Th +19F. The study of particle-fragment correlations may prove to be a useful technique for evaluating the optimal angular momentum transfer in heavy-ion transfer reactions on deformed nuclei.
Nuclear Physics A.
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ABSTRACT: Proton and α-particle multiplicities associated with fission have been measured following the reactions 164,167,170Er + 28Si at energies between 140 and 185 MeV. The multiplicities at the lowest energies place limits on the level-density parameters. They also increase with energy much more rapidly than those calculated with a statistical model and delays to fission are essential to reproduce the observed behaviour. It is not possible to obtain fits to neutron, proton and alpha-multiplicities using only a pre-saddle delay to fission. However, a consistent picture emerges if a saddle-to-scission delay is also used. The resultant pre-saddle delay is ⩽ 10 × 10−21 s and implies the fissioning nucleus spends most of the time (∼ 50 × 10−21 s) at deformation considerably larger than the equilibrium value.
Nuclear Physics A.
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ABSTRACT: Neutrons have been measured in coincidence with fission fragments of the reaction 208Pb(16O,f) at bombarding energies ranging between 80 and 130 MeV. Pre-scission and post-scission multiplicities were deduced from the neutron spectra by application of a moving source fit procedure. Particles evaporated before the saddle point configuration of the fissioning nucleus will reduce the nuclear temperature of the transition nucleus. This effect of higher chance fission increases the anisotropies of fission fragment angular distributions compared to the assumption of first chance fission. Compound nuclei with nuclear temperatures considerably smaller than the fission barrier have statistical fission decay times that are much longer than saddle-to-scission times. With these conditions it is plausible that most of the particles evaporated from the fissioning nucleus are emitted before the saddle. Fission fragment angular distributions for 208Pb(16O,f) analyzed with the transition state model including the nuclear temperature reduction caused by pre-fission particle emission show ‘‘normal’’ anisotropies at low and near barrier energies.
Phys. Rev. C. 45(2).
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J. R. Leigh,
M. Dasgupta,
D. J. Hinde,
J. C. Mein,
C. R. Morton,
R. C. Lemmon,
J. P. Lestone,
J. O. Newton,
H. Timmers, J. X. Wei,
N. Rowley
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ABSTRACT: Fusion excitation functions for the reactions 144,148,154Sm and 186W + 16O and 144Sm + 17O have been measured with high precision, both in the cross sections and the small energy intervals, thus allowing meaningful fusion barrier distributions to be extracted. In this representation it is clearly seen that the excitation functions are not smooth and featureless; each is unique and is shown to depend on the details of the structure of the interacting nuclei. The effects of excitation of the collective single phonon states in 144Sm are evident. For the 17O projectile, the role of additional coupling to neutron stripping channels with positive Q values can be seen. As expected, the barrier distributions associated with 154Sm and 186W are dominated by deformation effects. However, the data appear to display sensitivity to additional couplings, even though they involve relatively weak inelastic and transfer channels.
Phys. Rev. C. 52(6).
