Hardev Singh

Compact multiwire proportional counters for the detection of fission fragments

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 20x10 cm(2) 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.

Study of nuclear reaction dynamics through particle evaporation

The compound nucleus 76Kr* is formed in the heavy-ion fusion reactions by an asymmetric entrance channel 12C+64Zn and the symmetric entrance channel 31P+45Sc at the excitation energy of 75 MeV and angular momentum of 39 ℏ. Neutron energy spectra of the asymmetric system (12C+64Zn) at different angles are well described by the statistical model predictions using the normal value of the level density parameter a = A/8 MeV −1. However, in the case of the symmetric system (31P+45Sc), the statistical model interpretation of the data requires the change in the value of a = A/10 MeV−1. The delayed evolution of the compound system in case of the symmetric 31P+45Sc system may lead to the formation of a temperature equilibrated dinuclear complex, which may be responsible for the neutron emission at higher temperature, while the protons and alpha particles are evaporated after neutron emission when the system is sufficiently cooled down and the higher l-values do not contribute in the formation of the compound nucle...

Level Density Parameter: A Tool to Study the Particle Spectra

The compound nucleus 76Kr* is formed in the heavy‐ion fusion reactions by an asymmetric entrance channel 12C+64Zn and the symmetric entrance channel 31P+45Sc at the excitation energy of 75 MeV and angular momentum of 39 η. Neutron energy spectra of the asymmetric system (12C+64Zn) at different angles are well described by the statistical model predictions using the normal value of the level density parameter au2009=u2009A/8u2009MeV−1. However, in the case of the symmetric system (31P+45Sc), the statistical model interpretation of the data requires the change in the value of au2009=u2009A/10u2009MeV−1. The delayed evolution of the compound system in case of the symmetric 31P+45Sc system may lead to the formation of a temperature equilibrated dinuclear complex, which may be responsible for the neutron emission at higher temperature, while the protons and alpha particles are evap orated after neutron emission when the system is sufficiently cooled down and the higher λ‐values do not contribute in the formation of the compound nucle...

Pre‐fission neutron emission in 19F+209Bi reaction

The pre- and post-scission neutron multiplicities are measured for {sup 19}F+{sup 209}Bi reaction at E{sub lab} = 100, 104, 108, 112 and 116 MeV. The measured value of pre-scission neutron multiplicity was found to be increasing with the excitation energy. The comparison of experimental values with the statistical model calculations shows that the measured values are much larger than the model predictions. This difference in excess yield over the model predictions amounts to the survival time of 80{+-}5x10{sup -21} s for the {sup 228}U compound nucleus before it undergoes fission.

Entrance channel effects in fission of {sup 197}Tl

The pre- and post-scission neutron multiplicities are measured for {sup 16}O+{sup 181}Ta and {sup 19}F+{sup 178}Hf systems where the same compound nucleus {sup 197}Tl is formed at the same excitation energies (E*=72, 76, and 81 MeV). The measured pre-scission neutron multiplicities are found to be different for the two reactions and this difference in neutron yield increases with the excitation energy of the compound nucleus. The experimental pre-scission neutron yield is compared with predictions from the statistical model of compound nuclear decay containing the strength of nuclear viscosity as a free parameter. The magnitude of nuclear viscosity required to fit the experimental yield is found to be different for the two reactions. Because the two systems under consideration lie on the two sides of the Businaro-Gallone point, this observation indicates that the entrance channel mass asymmetry plays an important role in determining the number of neutrons emitted prior to scission in fusion-fission reactions.

Neutron evaporation as a probe for dynamical effects in heavy ion fusion reactions

The compound nucleus 76Kr* was populated at the excitation energy of 75 MeV and angular momentum of 39 ℏ in fusion reactions with two complementary, mass‐symmetric (31P+45Sc) and mass asymmetric (12C+64Zn) entrance channels. The neutron evaporation spectra were measured and compared with the predictions of the statistical model calculations. The results for the mass‐asymmetric reaction are found to be consistent with the predictions of the statistical model calculations. However, for the mass‐symmetric reaction (31P+45Sc), the experimental spectra are found to be harder than the theoretical neutron spectra. The dynamical model calculations of Feldmeier show that the formation time for the compound nucleus for the symmetric system is relatively larger as compared to the asymmetric system.