Experimental study of the isovector giant dipole resonance in Zr80 and Rb81

The isovector giant dipole resonance (IVGDR) $\ensuremath{\gamma}$ decay was measured in the compound nuclei $^{80}\mathrm{Zr}$ and $^{81}\mathrm{Rb}$ at an excitation energy of ${E}^{*}=54$ MeV. The fusion reaction $^{40}\mathrm{Ca}+^{40}\mathrm{Ca}$ at ${E}_{\mathrm{beam}}=136$ MeV was used to form the compound nucleus $^{80}\mathrm{Zr}$, while the reaction $^{37}\mathrm{Cl}+^{44}\mathrm{Ca}$ at ${E}_{\mathrm{beam}}=95$ MeV was used to form the compound nucleus $^{81}\mathrm{Rb}$ at the same excitation energy. The IVGDR parameters extracted from the analysis were compared with the ones found at higher excitation energy (${E}^{*}=83$ MeV). The comparison allows one to observe two different nuclear mechanisms: (i) the IVGDR intrinsic width remains constant with the excitation energy in the nucleus $^{81}\mathrm{Rb}$; (ii) the isospin-violating spreading width (i.e., Coulomb spreading width) remains constant with the excitation energy in the nucleus $^{80}\mathrm{Zr}$. The experimental setup used for the $\ensuremath{\gamma}$-ray detection was composed by the AGATA demonstrator array coupled to the large-volume ${\mathrm{LaBr}}_{3}$:Ce detectors of the ${\mathrm{HECTOR}}^{+}$ array.