Parity assignment for low-lying dipole states in ${}^{58}\mathrm{Ni}$

Low-lying dipole states in the singly closed-shell nucleus ${}^{58}\mathrm{Ni}$ were studied via nuclear resonance fluorescence experiments using a quasimonoenergetic, linearly polarized photon beam. The parity quantum numbers of the dipole states were determined by the intensity asymmetry of resonantly scattered $\gamma$ rays with respect to the polarization plane of the incident photon beam. The electric and magnetic dipole ($E1$ and $M1$) strengths at excitation energies between 5.9 and 9.8 MeV were obtained based on dipole strengths taken from literature. The $E1$ and $M1$ strength distributions were compared with large-scale shell-model predictions in the $fp$ shell using the SDPFSDG-MU interaction.

Figure 1

A typical photon scattering spectrum observed at polar and azimuthal angles of $(\theta ,\varphi )=({90}^{\circ },0^{\circ }$) and $({90}^{\circ },{90}^{\circ })$ from the measurement with the maximum beam energy of $E_{\gamma }^{\max }=8.8$ MeV. The $J^{\pi }$ assignments are indicated for the ground state transitions in ${}^{58}\mathrm{Ni}$. Peaks labeled s and d are due to single and double escapes, respectively. The dashed line shows the energy distribution of the incident photon beam.

Figure 2

Experimental $E1$ strength distribution below 10 MeV in ${}^{58}\mathrm{Ni}$ compared with shell-model calculations with $B\left(E1\right)\uparrow >0.2\times {10}^{-3}{e}^2{\mathrm{fm}}^2$.

Figure 3

Experimental $M1$ strength distribution below 10 MeV in ${}^{58}\mathrm{Ni}$ compared with shell-model calculations with $B\left(M1\right)\uparrow >0.01{\mu }_N^2$.

Figure 4

Spin ($M_S$) and orbital ($M_L$) components of the transition matrix elements corresponding to the transitions shown in Fig. 3 obtained using the SDPFSDG-MU effective interaction. The $M1$ strength is given by $B\left(M1\right)={[M_S+M_L]}^2$.

Figure 5

Spin components of the transition matrix elements for (a) protons ($M_{SP}$) and (b) neutrons ($M_{SN}$) are shown. The total spin component shown in Fig. 4 is given by $M_S=M_{SP}+M_{SN}$.