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Two large area multistep position sensitive (two dimensional) multiwire proportional counters have been developed for experiments involving study of fission dynamics using general purpose scattering chamber facility at IUAC. Both detectors have an active area of 20×10 cm2 and provide position signals in horizontal (X) and vertical (Y) planes, timing signal for time of flight measurements and energy signal giving the differential energy loss in the active volume. The design features are optimized for the detection of low energy heavy ions at very low gas pressures. Special care was taken in setting up the readout electronics, constant fraction discriminators for position signals in particular, to get optimum position and timing resolutions along with high count rate handling capability of low energy heavy ions. A custom made charge sensitive preamplifier, having lower gain and shorter decay time, has been developed for extracting the differential energy loss signal. The position and time resolutions of the detectors were determined to be 1.1 mm full width at half maximum (FWHM) and 1.7 ns FWHM, respectively. The detector could handle heavy ion count rates exceeding 20 kHz without any breakdown. Time of flight signal in combination with differential energy loss signal gives a clean separation of fission fragments from projectile and target like particles. The timing and position signals of the detectors are used for fission coincidence measurements and subsequent extraction of their mass, angular, and total kinetic energy distributions. This article describes systematic study of these fission counters in terms of efficiency, time resolution, count rate handling capability, position resolution, and the readout electronics. The detector has been operated with both five electrode geometry and four electrode geometry, and a comparison has been made in their performances. [ABSTRACT FROM AUTHOR]

Evaporation residue (ER) cross sections for the 16,18O + l98Pt reactions are measured in order to investigate fission hindrance. Compound nuclei (2l4,216Rn) are formed in the above fusion reactions at excitation energies in the range of 40-68 MeV. The experimental ER cross sections are compared with predictions from the statistical model calculations of compound nuclear decay where Kramers' fission width is used. The strength of nuclear dissipation is treated as a free parameter in the statistical model calculations in order to fit the experimental data. [ABSTRACT FROM AUTHOR]

Pre- and post-scission neutron multiplicities are measured for the three isotopes of Fr (217Fr, 215Fr, and 213Fr) in the excitation energy range of 48-91.8 MeV. Out of these three isotopes, 213Fr has shell closure (Nc = 126) while the other two are non-closed-shell nuclei. Statistical model calculations using Kramers' fission width are performed to investigate shell effects on the dissipation strength which fit the experimental data. It is observed that shell correction to the binding energies of the evaporated particles strongly affects the fitted values of the dissipation strength. However, the best-fit dissipation strength is only weakly influenced by the inclusion of shell correction in fission barrier [ABSTRACT FROM AUTHOR]

The prescission neutron multiplicity excitation function is measured for the first time for three isotopes across a major closed shell in order to investigate the shell effects on fission hindrance. Three isotopes of Fr (213,215,217Fr) are populated by fusion of 19F+194,196,198Pt in the excitation energy range of 46.6-91.8 MeV. While 213Fr has a major neutron shell closure at N=126, 215Fr and 217Fr are away from the closed shell. It is found from the statistical model analysis of the experimental data that the strengths of nuclear dissipation for nuclei away from shell closure are very similar. On the other hand, the dissipation is relatively weaker for a shell-closed nucleus in comparison to adjacent nuclei away from shell closure. [ABSTRACT FROM AUTHOR]