Fine Structure of the Giant $M1$ Resonance in ${}^{90}\mathrm{Zr}$

The $M1$ excitations in the nuclide ${}^{90}\mathrm{Zr}$ have been studied in a photon-scattering experiment with monoenergetic and linearly polarized beams from 7 to 11 MeV. More than 40 $J^{\pi }=1^{+}$ states have been identified from observed ground-state transitions, revealing the fine structure of the giant $M1$ resonance with a centroid energy of 9 MeV and a sum strength of 4.17(56) ${\mu }_N^2$. The result for the total $M1$ strength and its fragmentation are discussed in the framework of the three-phonon quasiparticle-phonon model.

Figure 1

(a) Energy distribution of the incident photon beam (the vertical error bars represent statistical uncertainties) normalized to the flux deduced from strong $E1$ transitions of ${}^{90}\mathrm{Zr}$, shown as data points. Measured spectra at ($\theta$, $\varphi$) of (90°, 90°) (b) containing $E1$ transitions, of (135°, 0°) (c) and of (90°, 0°) (d) containing mostly $M1$ transitions. The vertical blue lines show some of the strong $M1$ transitions.

Figure 2

Results for (a) the measured $B(M1)$ strength of discrete levels in ${}^{90}\mathrm{Zr}$ compared with the detection limits (red solid line) and (b) predictions from the quasiparticle-phonon model. A comparison of the measured and calculated QPM cumulative $M1$ strength is shown in panel (c). The dashed line continuing the solid line above 10 MeV represents the $M1$ strength obtained from the continuum. The shaded area gives the uncertainty of the experimental values. QRPA results of the total cumulative $M1$ strength up to 20 MeV are shown in the insert.