The fission fragment anisotropies for 12 C 1 198 Pt populating 210 Po are found to be systematically higher than that for 12 C 1 194 Pt, the difference decreasing over the energy range 1.0 # E͞V B # 1.4. The parameters required for a statistical model analysis of the data were constrained by fission cross section data and by measuring evaporation residue cross sections. While the measured anisotropies for 12 C 1 194 Pt are in agreement with the saddle point model calculations, those for 12 C 1 198 Pt are considerably larger. The data are suggestive of a significant shell effect in the anisotropies of fission fragments emitted from 210 Po with neutron magic number, N 126. [S0031-9007(98)08289-1] PACS numbers: 25.70.JjShell effects play a central role in determining the stability of nuclei and the nuclear structure. It is generally believed that shell effects are washed out at high excitation energies and angular momenta produced in medium energy heavy ion reactions. It is of interest to investigate an energy region where shell effects are still visible and observe the gradual decrease of the effect with increasing excitation energy. This information is important in the context of production and stability of super heavy elements.In recent years, the unexpected deviation of some fission fragment angular anisotropies from the transition state model has been taken to imply noncompound reaction mechanisms [1]. Anomalous anisotropies have also been interpreted in terms of target deformation [2] and target spin [3]. It is interesting to consider fission fragment anisotropies as a probe for shell effects. We have made anisotropy measurements for 12 C 1 194,198 Pt. This allows a comparison of the fragment anisotropies for 210 Po (with N 126) to be made with that for 206 Po (with N 122). In the limited number of such measurements in the literature for this mass region, no significant isotopic dependence was observed for 12 C 1 182,183,184,186 W [4] and 6 Li 1 194,198 Pt [5].Statistical model calculations which take into account differences in angular momenta, fusion cross sections, fissility values, prefission neutrons and shell corrections to the level densities between the two systems have been made. The statistical model parameters have been constrained by making measurements of partial evaporation residue (ER) cross sections in addition to fission cross sections and also comparing the calculations with the earlier measurements [6-9] for 210,206 Po compound nuclei. Since shell corrections in the level densities are included in the calculation, comparison with the measured anisotropies for 206,210 Po could reveal the effects of the underlying shell structure on other nuclear properties such as the deformation at the fission saddle point.The measurements were carried out using 12 C beams from the BARC-TIFR 14UD Pelletron accelerator at Bombay in the energy range from 59 to 85 MeV. The targets were self supporting rolled foils of 194 Pt (97.4% enriched, 1 mg͞cm 2 thick) and 198 Pt (95.7% enriched, 1.3 mg͞cm 2 thick). The e...

The transfer-induced fission channel has been studied in the collision of 340 MeV 28 Si on 232 Th as a function of the atomic number of the projectilelike fragments ͑PLF's͒ by using a 4 detector array. It is found that the energy loss of the transfer reaction increases as a function of the net charge transfer ⌬Z from the projectile to the target nucleus, going from quasielastic to deep-inelastic regimes. The average excitation energy of the targetlike fragment ͑TLF͒ is derived from the measured energy loss, whereas its angular momentum has been obtained from the angular distribution of fission fragments. It is found that the populated TLF nuclei with Z TLF ϭ90-96 (⌬Zϭ0 -6) have average excitation energies up to about 100 MeV and angular momenta up to about 40ប. The measured ratio of transfer-fission yield to PLF singles, Y f , first increases with increasing net charge transfer up to ⌬Zϭ4 and then shows a plateau around the values Y f ϭ0.4-0.6 followed by a decrease for higher ⌬Z transfers. This ratio can be identified as the cumulative fission probability of the populated nuclei for net charge transfers up to ⌬Zр6, where a two-body mechanism for the first reaction step is supported by the experimental data. This result suggests a significant survival probability against fission of these TLF nuclei, in marked disagreement with the standard statistical model predictions. The observed survival probability implies that there is a strong hindrance to fission in the early stages of deexcitation, as also indicated by the large fission times (t f ϭ10-100 zs) derived from earlier neutron measurements in fusion-fission reactions. The importance of such effects in the population of nuclei in the heavy and superheavy mass regions by transfer reactions is discussed.

In order to determine the relative importance of the role played by inelastic excitations and transfer channels of the colliding nuclei, in near-barrier fusion enhancement, the fusion cross sections have been measured for 12 Cϩ 194,198 Pt in the energy range of 0.9рE/V B р1.2. Additional data of quasielastic and nucleon͑s͒-exchange cross sections have also been measured at an energy of 1.2V B . The strength of transfer form factors required for the simplified coupled-channels calculations has been obtained from the transfer reaction measurements using a semiclassical approach and calculations based on complex WKB approximations. Coupled-reactionchannels calculations have been performed to explain the complete data set that included fusion, quasielastic, and transfer cross sections. The dominant contributions to the enhancement of fusion cross section compared to a one-dimensional barrier-penetration model arise from coupling to inelastic channels. It has been shown for the first time that the lighter isotope ( 194 Pt) of a given nuclide that has a relatively larger collectivity (␤ ) and a larger neutron separation energy compared to the heavier isotope ( 198 Pt), exhibits larger enhancement of fusion cross section. The experimental fusion-barrier distributions were obtained from fusion and quasielastic scattering data.

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