Giant dipole resonance studies in Ba isotopes at E / A ≈ 5 MeV

Exclusive measurements of high-energy $\ensuremath{\gamma}$ rays have been performed in $^{124}\mathrm{Ba}$ and $^{136}\mathrm{Ba}$ at the same excitation energy $(\ensuremath{\sim}49\phantom{\rule{4pt}{0ex}}\mathrm{MeV})$ to study the properties of the giant dipole resonance (GDR) over a wide $N/Z$ range. The high-energy $\ensuremath{\gamma}$ rays are measured in coincidence with the multiplicity of low-energy $\ensuremath{\gamma}$ rays to disentangle the effect of temperature $(T)$ and angular momentum $(J)$. The GDR parameters are extracted employing a simulated Monte Carlo statistical model analysis. The observed $\ensuremath{\gamma}$-ray spectra of $^{124}\mathrm{Ba}$ can be explained with prolate deformation, whereas a single-component Lorentzian function which corresponds to a spherical shape could explain the $\ensuremath{\gamma}$-ray spectra of $^{136}\mathrm{Ba}$. The observed GDR width in $^{136}\mathrm{Ba}$ is narrower compared to that of $^{124}\mathrm{Ba}$. The statistical model best-fit GDR cross sections are found to be in reasonable agreement with the thermal shape fluctuation model (TSFM) calculations. Further, it is shown that the variation of GDR width with $T$ is well reproduced by the TSFM calculations over the temperature range of 1.1--1.7 MeV.