Experimental study of the isovector giant dipole resonance in ${}^{80}\mathrm{Zr}$ and ${}^{81}\mathrm{Rb}$

The isovector giant dipole resonance (IVGDR) $\gamma$ 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 $\gamma$-ray detection was composed by the AGATA demonstrator array coupled to the large-volume ${\mathrm{LaBr}}_3$:Ce detectors of the ${\mathrm{HECTOR}}^{+}$ array.

Figure 1

CN $\gamma$-decay spectrum calculated with the cascade code before (solid line) and after (dashed line) the application of the detector response function.

Figure 2

Probability distribution $P(F_{\gamma },M_{\gamma })$ of triggering $F_{\gamma }$ in the AGATA demonstrator by a cascade of $M_{\gamma }\gamma$ rays. Three $F_{\gamma }$ conditions are considered. A total efficiency ${\epsilon }_{\mathrm{riv}}=7\%$ was considered in the calculations.

Figure 3

The same as Fig. 2 but for the ${\mathrm{HECTOR}}^{+}$ array. A total efficiency ${\epsilon }_{\mathrm{riv}}=5\%$ was considered in the calculations.

Figure 4

The same as Fig. 2 but for different coincidence conditions between AGATA and ${\mathrm{HECTOR}}^{+}$.

Figure 5

Top panel: The angular-momentum distributions of the compound nuclei ${}^{80}\mathrm{Zr}$ (red line) and ${}^{81}\mathrm{Rb}$ (black line) calculated with the cascade code are shown. Bottom panel: Angular-momentum distributions folded with the response function for the coincidence $F_{\gamma }=1$ in AGATA and $F_{\gamma }=1$ in ${\mathrm{HECTOR}}^{+}$ are shown.

Figure 6

Comparison between the inclusive $\gamma$-ray spectra (solid dots) and the time-gated $\gamma$-ray spectra (empty dots). The ${}^{37}\mathrm{Cl}+{}^{44}\mathrm{Ca}$ spectra are plotted on the left, whereas the ${}^{40}\mathrm{Ca}+{}^{40}\mathrm{Ca}$ spectra are plotted on the right.

Figure 7

Low-energy spectrum of the AGATA demonstrator in coincidence with a high-energy $\gamma$ ray ($E_{\gamma }\ge 20$ MeV) detected in ${\mathrm{HECTOR}}^{+}$. No transitions are visible and only the 511 keV peak is present.

Figure 8

$E_{\gamma }-E_{\gamma }$ matrix for the reaction ${}^{40}\mathrm{Ca}+{}^{40}\mathrm{Ca}$. On the $x$ axis the energy detected in the AGATA demonstrator is plotted, whereas on the $y$ axis the energy detected in ${\mathrm{HECTOR}}^{+}$ is plotted. Two regions in the matrix are indicated: $E_{\gamma }\le 4$ MeV (region 1) and $E_{\gamma }\ge 10$ MeV (region 2).

Figure 9

Comparison between the $\gamma$-ray spectra obtained with two different conditions: $E_{\gamma }<4$ MeV $\gamma$ ray detected in AGATA (empty dots), $E_{\gamma }>10$ MeV $\gamma$ ray detected in AGATA (crosses). The ${}^{37}\mathrm{Cl}+{}^{44}\mathrm{Ca}$ spectra are plotted on the left, whereas the ${}^{40}\mathrm{Ca}+{}^{40}\mathrm{Ca}$ spectra are plotted on the right.

Figure 10

Comparison between the time-of-flight (TOF) gated $\gamma$-ray spectrum (solid dots) and the $\gamma$-ray spectrum after the Bremsstrahlung subtraction (empty dots). The red dashed line is the estimated background contribution.

Figure 11

The low-energy spectra of the AGATA demonstrator are shown. The top panel is related to the ${}^{80}\mathrm{Zr}$ decay, while the bottom panel is related to the ${}^{81}\mathrm{Rb}$ decay. The circles represent the transitions of the more populated residues.

Figure 12

Residual-nuclei population obtained from the analysis of the AGATA demonstrator spectrum as a function of the energy detected in the ${\mathrm{HECTOR}}^{+}$ array. The experimental data were corrected with the AGATA demonstrator efficiency. The statistical-model calculations were performed using a Monte Carlo cascade code.

Figure 13

Residual-nuclei population obtained from the analysis of the AGATA demonstrator spectrum as a function of the fold request. $F_{\gamma }=1$ corresponds to one $\gamma$-ray detected in the AGATA demonstrator while $F_{\gamma }=2,3,...,6$ corresponds to $1,2,...,5\gamma$ rays detected in the AGATA demonstrator and one in ${\mathrm{HECTOR}}^{+}$, respectively. The statistical-model calculations were performed using a Monte Carlo cascade code.

Figure 14

Energy spectra of the AGATA demonstrator in the reaction ${}^{40}\mathrm{Ca}+{}^{40}\mathrm{Ca}$ related to two different $F_{\gamma }$ conditions: (top panel) $F_{\gamma }=1$ spectrum (one $\gamma$ ray detected in the AGATA demonstrator and zero in ${\mathrm{HECTOR}}^{+}$, respectively); (bottom panel) $F_{\gamma }=6$ spectrum (five $\gamma$- detected in the AGATA demonstrator and one in ${\mathrm{HECTOR}}^{+}$, respectively).

Figure 15

Linearized $\gamma$-ray spectra compared with theoretical calculations performed using a CTFM calculation for the reaction ${}^{44}\mathrm{Ca}+{}^{37}\mathrm{Cl}$. The red band reflects the uncertainty on the value of the temperature. In the left panel, the experimental data at $E^{*}=83$ MeV obtained in Ref. [22] are shown. In the right panel, the data at $E^{*}=54$ MeV obtained in our experiment are shown.

Figure 16

GDR reduced-width evolution as a function of the parameter $\xi$ for different nuclei. The black curve represents the function $L\left(\xi \right)$ in the CTFM. The experimental data were taken from Ref. [12] and references therein.

Figure 17

Values of the Coulomb spreading width obtained in the IAS (black dots) data and data from the CN (red triangles) [26]. The arrow points to the value obtained in our work (blue circle), overlapping with the value obtained in Corsi 2011 [22] (empty green dot).