First Measurement of the Giant Monopole and Quadrupole Resonances in a Short-Lived Nucleus: ${}^{56}\mathrm{Ni}$

The isoscalar giant monopole resonance (GMR) and giant quadrupole resonance (GQR) have been measured in the ${}^{56}\mathrm{Ni}$ unstable nucleus by inducing the ${}^{56}\mathrm{Ni}(d,d^{\prime })$ reaction at $50A\mathrm{MeV}$ in the Maya active target at the GANIL facility. The GMR and GQR centroids are measured at $19.3\pm 0.5\mathrm{MeV}$ and $16.2\pm 0.5\mathrm{MeV}$, respectively. The corresponding angular distributions are extracted from 3° to 7°. A multipole decomposition analysis using distorted wave Born approximation with random phase approximation transition densities shows that both the GMR and the GQR exhaust a large fraction of the energy-weighted sum rule. The demonstration of this new method opens a broad range of giant resonance studies at intermediate-energy radioactive beam facilities.

Figure 1

Top: Scatter plot of recoiling deuteron energy versus scattering angle in the laboratory frame for the ${}^{56}\mathrm{Ni}$ beam. The lines correspond to elastic scattering and the 14–22 MeV excitation energy of ${}^{56}\mathrm{Ni}$. The c.m. angles ares denoted by squares along these lines, with 1° step, starting from 0° for the lowest deuteron energy. Bottom left: geometrical efficiency of the detection setup in the c.m. frame. Bottom right: ${}^{56}\mathrm{Ni}$ excitation energy spectrum deduced from the deuteron kinematics and corrected for geometrical efficiency. The background that was subtracted is shown by the solid line. The inset shows the background subtracted inelastic data fitted with Gaussian distributions located at 16.5 and 19.5 MeV for the GQR and the GMR, respectively, (see text).

Figure 2

GMR and GQR inelastic angular distributions of ${}^{56}\mathrm{Ni}$ at $50A\mathrm{MeV}$ and DWBA calculations using microscopic double folded RPA form factor (see text). The data (squares) are also displayed. Bottom right: analysis of the data obtained by using two Gaussian distributions located at 16.5 and 19.5 MeV for the GQR and the GMR, respectively, with $\mathrm{FWHM}=5.2\mathrm{MeV}$ (see text).