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Present work aims to explicate the effect of entrance channel mass asymmetry on fusion dynamics for the Compound Nucleus 80Sr populated through two different channels, 16O+64Zn and 32S+48Ti, using cross-section and spin distribution measurements as probes. The evaporation spectra studies for these systems, reported earlier indicate the presence of dynamical effects for mass symmetric 32S+48Ti system.The CCDEF and TDHF calculations have been performed for both the systems and an attempt has been made to explain the reported deviations in the α-particle spectrum for the mass symmetric system.

Mass angle distribution measurements for 19F+238U reaction were carried out around the sub barrier energies. Mass angle correlation has not been observed at above and below the fusion barrier in present reaction. This infer the minimal presence of non compound like events at these bombarding energies range.

The spin distributions are measured for the compound nucleus 80Sr populated in the reactions 16O+64Zn and 32S+48Ti. The comparison of the experimental results for both the systems shows that the mean γ-ray multiplicity values for the system 32S+48Ti are lower than those for 16O+64Zn. The spin distribution of the compound nucleus populated through the symmetric channel is also found to be lower than the asymmetric channel. Present investigation directly shows the effect of entrance channel mass asymmetry on the reaction dynamics.

We present the evolution of giant dipole resonance (GDR) width as a function of angular momentum in the compound nucleus 144Sm in the temperature range of 1.5-2.0 MeV. The high energy γ rays emitted from the decay of excited 144Sm were measured using large NaI detector in coincidence with 4π sum spin spectrometer. GDR widths were found to comply with thermal shape fluctuation model in this temperature range over a wide range of spin. Experimental widths tend to increase rapidly at high angular momentum values.

Exclusive measurement of giant dipole resonance (GDR) γ rays has been performed in 144Sm nucleus which was populated at near barrier energy using the heavy ion reaction of 28Si beam on 116Cd target. GDR γ rays were detected in coincidence with low energy γ rays using 32 elements 4π sum-spin spectrometer. The 144Sm nucleus was populated at an excitation energy of 68 MeV in the temperature range of 1.1-1.3 MeV. The measured GDR widths in this temperature range are consistent with the Kusnezov’s parametrization.

The spin distribution of Evaporation Residues (ER) was measured for the reaction 30Si + 170Er. ER spin distributions were compared for the systems 16O + 184W, 19F + 181Ta and 30Si + 170Er forming the Compound Nucleus 200Pb. Mean gamma multiplicity vs. excitation energy curve shows saturation at a lower value in case of 30Si + 170Er. This was attributed to non compound nuclear processes.

Background: Complete fusion between two massive nuclei after capture inside the potential barrier is inhibited by competing fission-like processes. The target-projectile composite system may reseparate after capture without proceeding towards formation of the compound nucleus (CN), which is equilibrated in all degrees of freedom. The nature of these non-CN fission (NCNF) processes and factors that affect them are not completely known yet. Purpose: The nuclear mass regions from where NCNF processes begin to manifest themselves are not clearly demarcated. This work aims to study the onset of NCNF, if any, in the mass region ~200. Methods: Fission fragment (FF) mass and angular distribution (MAD) and pre-scission and post-scission neutron multiplicities were measured for the reaction 19F + 184W at a laboratory energy (Elab) range of 84-125 MeV. The measurements were carried out using two multiwire proportional counters (MWPC) to detect the FFs in coincidence and four neutron detectors to measure neutron time of flight (TOF). Statistical model (SM) calculation was performed. Results: No significant mass-angle correlation was observed in the MAD plots. Extracted mass ratio distributions were single-peaked and of Gaussian shape. Measured pre-scission neutron multiplicity values indicated dissipative nature of CN decay for this reaction. Conclusions: No clear signatures of NCNF were observed in the studied reaction, indicating that the target-projectile composite system predominantly proceeds towards formation of the CN after capture. [ABSTRACT FROM AUTHOR]

Background: Giant dipole resonance (GDR) has been used as an important tool for studying nuclear properties in hot rotating nuclei. Exclusive measurements using low-energy y-ray multiplicity filters provide more control over angular momentum selection in such measurements Purpose: Study the effect of angular momentum and temperature on nuclear deformations and GDR widths at high excitation energies in 144Sm. Methods: Exclusive measurements of GDR y rays were carried out in the 28Si + 116Cd reaction populated at two different excitation energies. Beam energies of 125 and 140 MeV pumped the nuclei to average temperatures (T) of 1.1 to 1.5 MeV. The high-energy y rays were measured using the large Nal(Tl) detector in coincidence with the sum-spin multiplicity filter consisting of 32 Nal(Tl) detectors covering nearly 4JT sr of solid angle. Results: The average angular momentum (J) spanned the range of 25h to 60h. The GDR centroid energies, widths, and deformation parameter (fi) were extracted as a function of (J) at three different (T) bins of 1.1, 1.3, and 1.5 MeV. The thermal shape fluctuation model (TSFM) calculations have been performed incorporating the fluctuations induced due to temperature and deformation in the nucleus using a numerically exact method. The calculations showed evidence of deformation throughout the experimental range. The GDR width data have been interpreted in terms of reduced width as a function of reduced angular momentum. Conclusions: The nucleus evolves to a deformed shape from spherical shape in ground state in the extracted temperature range as predicted by the theoretical calculations. Kusnezov's parametrization also holds good for the large experimental J range. [ABSTRACT FROM AUTHOR]

Experimental measurements of evaporation residue (ER) cross sections for the 19F + 194,196,198Pt reactions forming 213,215,217Fr corripound nuclei are reported. The cross sections are measured at beam energies in the range of 101-137.3 MeV. The survival probability of the 213Fr compound nucleus with neutron number N = 126 is found to be lower than the survival probabilities of 215Fr and 217Fr with neutron numbers N = 128 and 130 respectively. Statistical model analysis of the ER cross sections show that an excitation energy dependent scaling of the finite-range rotating liquid drop model fission barrier is necessary to fit the experimental data. The fitted scaling factors for 2l3Fr are found to be smaller than those of 215Fr and 217Fr for almost the entire range of excitation energies. [ABSTRACT FROM AUTHOR]

Background: Synthesis of a doubly magic spherical nucleus beyond Pbs is a key question in contemporary nuclear physics. Such nuclei can exist solely because of shell stabilization. As the formation cross section of super heavy elements is prohibitively low, attempts have been made to understand stabilizing effects of closed proton (Z) and neutron (N) shells in the vicinity of Pb. Purpose: The present work attempts to elucidate the stabilizing effect of shell closure, in general, and the same of Z = 82, in particular. Methods: The evaporation residue (ER) excitation function and ER-gated /-multiplicity distribution have been measured for the reaction 31P + 170Er at a laboratory energy range of 134--172 MeV. The measurements have been carried out using the HYbrid Recoil mass Analyzer (HYRA) in gas-filled mode and a 4π spin spectrometer consisting of 29 Nal(Tl) detectors. Results of the present reaction have been compared with those of the reaction 30Si + 170Er. Statistical model calculation has been performed for both the systems. Results: The two reactions, induced by 30Si and 31P projectiles, resulted in compound nuclei (CN) 200Pb (Z = 82) and 201Bi (Z = 83), respectively. To reproduce experimental ER cross sections, the liquid drop fission barrier (Bf) had to be scaled in the statistical model calculation. The scaling factor (Kf) varies from 0.75 to 1.05 and 0.90 to 1.05 for 30Si and 31P induced reactions, respectively. No significant differences have been found between γ-multiplicity distribution and the distribution moments of the two systems. [ABSTRACT FROM AUTHOR]

Mass-angle correlations and mass ratio distributions of fission fragments have been measured in 12C + 238U and 180 + 232Th reactions, both leading to the same compound nucleus 250Cf (fissility = 0.86), with matching excitation energies at near and sub-barrier energies. In both systems, there is no mass-angle correlation in the entire energy region studied. However, it is found that the variance of the mass ratio distribution in the 180 + 232Th system is consistently higher than in the 12C + 238U system at all excitation energies. This points to the fact that mass equilibration has not been fully achieved in the 180 + 232Th reaction in comparison to the 12O + 238U reaction, indicating the presence of quasifission in the 180 + 232Th system. [ABSTRACT FROM AUTHOR]

Background: Presence of closed proton and/or neutron shells causes deviation from macroscopic properties of nuclei, which are understood in terms of the liquid-drop model. Efforts to synthesize artificial elements are driven by the prediction of the existence of closed shells beyond the heaviest doubly magic nucleus found in nature. It is important to investigate experimentally the stabilizing effects of shell closure, if any, against fission. Purpose: This Rapid Communication aims to investigate probable effects of proton shell (Z=82) closure in the compound nucleus in enhancing survival probability of the evaporation residues formed in heavy ion-induced fusion-fission reactions. Method: Evaporation residue cross sections have been measured for the reactions 19F+180Hf,19F+181Ta, and 19F+182W from ≃9% below to ≃42% above the Coulomb barrier; leading to the formation of compound nuclei with the same number of neutrons (N=118) but different numbers of protons across Z=82 employing the Heavy Ion Reaction Analyzer. Measured excitation functions have been compared with a statistical model calculation in which the reduced dissipation coefficient is the only adjustable parameter. Results: Evaporation residue cross section, normalized by the capture cross section, is found to decrease gradually with increasing fissility of the compound nucleus. Measured evaporation residue cross sections require inclusion of nuclear viscosity in the model calculations. Reduced dissipation coefficient in the range of 1-3×1021s-1 reproduces the data quite well. Conclusions: Since entrance channel properties of the reactions and structural properties of the heavier reaction partners are very similar, the degree of presence of noncompound nuclear fission, if any, is not expected to be significantly different in the three cases. No abrupt enhancement of evaporation residue cross sections has been observed in the reaction forming a compound nucleus with Z=82. Thus, this Rapid Communication does not find enhanced stabilizing effects of the Z=82 shell closure against fission in the compound nucleus. One may attempt to measure cross sections of individual exit channels for further confirmation of our observation. [ABSTRACT FROM AUTHOR]

Background: It is a well established fact that nuclear deformation and vibration influence fusion dynamics around the Coulomb barrier. This effect was observed for several systems with the inclusion of inelastic excitations in coupled-channels calculations. Sub-barrier fusion cross sections were also observed to be affected by neutron transfer in systems carrying positive Q value for transfer channels. However, recent experimental analysis with a few systems showed that inelastic excitations are enough to explain the sub-barrier fusion behavior, and no effect was noticed due to positive Q-value transfer channels. Purpose: The motivation behind present investigation is to explore the effects of colliding nuclei structure and the transfer channel on enhancement of sub-barrier fusion cross sections. Method: An experiment was performed with Heavy Ion Reaction Analyzer (HIRA) at New Delhi to measure the fusion cross sections for 28Si+92,96Zr systems. These cross sections were later compared with quantum mechanical coupled-channels calculations. In order to explore the effects of nuclear deformation on fusion cross sections, the present data were compared with those of other researchers on fusion who have used various projectiles of different structural properties on a 96Zr target. Results: Experimental fusion cross sections have been extracted around the Coulomb barrier. In the coupled-channels framework, inclusion of inelastic excitations of both projectile (28Si) and targets (92,96Zr) could reproduce the experimental cross sections around the Coulomb barrier, but they deviated substantially in the sub-barrier region. This indicates that positive Q-value neutron transfer channels may need to be included in the calculations to reproduce the experimental cross sections at sub-barrier energies. Conclusions: The nuclear structure of interacting nuclei has a strong influence on sub-barrier fusion enhancement. The effect of multineutron transfer channels was observed to be significant for fusion, especially for those systems where interacting nuclei are less deformed or spherical. [ABSTRACT FROM AUTHOR]

Background: Couplings with various reaction channels are known to enhance sub-barrier fusion cross sections by several orders in magnitude. However, a few open questions still remain. For example, the influence of higher order static deformations on sub-barrier fusion cross sections is yet to be comprehensively understood. Purpose: We study the role of hexadecapole nuclear deformation effect on sub-barrier fusion cross sections. Also, this work aims to extract hexadecapole deformation (β4) in nuclei in the lanthanide region. Method: The evaporation residue (ER) excitation functions for 16O+174,176Yb were measured at laboratory beam energies (Elab) in the range of 64.6-103.6 MeV. Measurements were carried out by employing the recoil mass spectrometer Heavy Ion Reaction Analyzer (HIRA) at IUAC, New Delhi. Fusion barrier distributions (BDs) were extracted from data. Results from the experiment were subjected to coupled-channels analysis, in which β4 was varied as a free parameter. Results: Experimental fusion cross sections at energies below the barrier expectedly showed strong enhancement compared to the predictions from the one-dimensional barrier penetration model. Data were satisfactorily reproduced after inclusion of negative β4 for both the targets in the coupled-channels calculation. Conclusions: The significant role of hexadecapole deformation was observed in the sub-barrier fusion of 16O+174,176Yb. The proposed value of β4 reproduced the measured fusion excitation function reasonably well. The BDs from these data were also extracted but no definitive conclusions could be drawn from them. [ABSTRACT FROM AUTHOR]

High-accuracy mass measurements of neutron-deficient Yb isotopes have been performed at TRIUMF using TITAN's multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS). For the first time, an MR-TOF-MS was used on line simultaneously as an isobar separator and as a mass spectrometer, extending the measurements to two isotopes further away from stability than otherwise possible. The ground state masses of ^{150,153}Yb and the excitation energy of ^{151}Yb^{m} were measured for the first time. As a result, the persistence of the N=82 shell with almost unmodified shell gap energies is established up to the proton drip line. Furthermore, the puzzling systematics of the h_{11/2}-excited isomeric states of the N=81 isotones are unraveled using state-of-the-art mean field calculations.