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 electronic absorption spectrum of N,N-dimethylformamide (DMF) is studied in the 45 000-80 000 cm (5.6-9.9 eV) region using synchrotron radiation. The vacuum ultraviolet (VUV) spectrum comprises mostly of Rydberg series of ns, np, and nd types converging to the first two ionization potentials (IPs). Quantum defect values obtained are consistent with excitation of an electron from the highest occupied molecular orbitals localized on nitrogen (4a″) and oxygen (16a'); in addition, the 3s Rydberg transition converging to the third IP (3a″) is observed at 8.95 eV. A reinvestigation of the infrared spectrum of DMF in the 500-4000 cm region with the help of density functional theory (DFT) calculations establishes the planarity of the ground state and leads to revision of several vibrational assignments. Vertical excited state energies and their valence/Rydberg character are predicted using time dependent DFT calculations; excellent correlation is achieved between theoretical results and experimentally observed spectral features. Potential energy curves of the first few excited states give additional insights into the nature of the excited states and their role in photodissociation dynamics. The absorption spectrum of DMF in the region >63 400 cm (7.85 eV) as well as a complete set of spectral assignments in the VUV region (45 000-80 000 cm) is reported for the first time. This work represents a comprehensive study of the absorption spectra of DMF in the VUV and infrared regions.
The multiplicities of prescission protons and n particles in the reaction ' F+ Th at 104, 110, 116, and 118 MeV have been measured. The observed multiplicities are much larger than the prediction of the statistical model without the introduction of fission delay. An analysis of the prescission proton and u data along with that for neutrons (measured earlier) has been made using deformed optical model and deformation-dependent particle binding energies. Simultaneous fits to the charged particle and neutron data required a total fission time scale in the range of (25 -80)X 10 ' s. For this system, the mean kinetic energies of the prescission particles are insensitive to the division of the total delay time into presaddle and postsaddle components; however, the neutron and charged particle multiplicity data are sensitive to the deformation of the saddle-to-scission emitter.The analysis shows that postsaddle emission takes place close to the scission point. PACS number(s): 25.70.Jj, 25.85.6e
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