Effect of angular momentum on giant dipole resonance observables in the28Si+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 $\ensuremath{\gamma}$-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 ${}^{144}$Sm.Methods: Exclusive measurements of GDR $\ensuremath{\gamma}$ rays were carried out in the ${}^{28}$Si+${}^{116}$Cd reaction populated at two different excitation energies. Beam energies of 125 and 140 MeV pumped the nuclei to average temperatures $\ensuremath{\langle}T\ensuremath{\rangle}$ of 1.1 to 1.5 MeV. The high-energy $\ensuremath{\gamma}$ 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 4$\ensuremath{\pi}$ sr of solid angle.Results: The average angular momentum $\ensuremath{\langle}J\ensuremath{\rangle}$ spanned the range of 25$\ensuremath{\hbar}$ to 60$\ensuremath{\hbar}$. The GDR centroid energies, widths, and deformation parameter ($\ensuremath{\beta}$) were extracted as a function of $\ensuremath{\langle}J\ensuremath{\rangle}$ at three different $\ensuremath{\langle}T\ensuremath{\rangle}$ 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.