Effect of angular momentum on giant dipole resonance observables in the 28Si+116Cd reaction

Abstract

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 γ-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 γ 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 of 1.1 to 1.5 MeV. The high-energy γ rays were measured using the large NaI(Tl) detector in coincidence with the sum-spin multiplicity filter consisting of 32 NaI(Tl) detectors covering nearly 4π sr of solid angle. Results: The average angular momentum spanned the range of 25 to 60. The GDR centroid energies, widths, and deformation parameter (β) were extracted as a function of at three different 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. © 2013 American Physical Society.