Low-lying electric and magnetic dipole strengths in ${}^{207}\mathrm{Pb}$

Low-lying dipole transitions in ${}^{207}\mathrm{Pb}$ were measured via nuclear photon scattering using a quasimonochromatic, linearly polarized photon beam. The spins and/or parities of the states observed in ${}^{207}\mathrm{Pb}$ were determined from the intensity asymmetry of resonantly scattered $\gamma$ rays with respect to the polarization plane of the incident photon beam. The electric ($E1$) and magnetic ($M1$) dipole strengths were obtained for excitation energies from 5.49 to 6.75 MeV. The present experimental results, combined with $(\gamma ,n)$ data from the literature, were used to investigate the $E1$ and $M1$ photoabsorption cross sections near the neutron separation energy by comparison with predictions from the particle-vibration coupling with the quasiparticle random-phase approximation model.

Figure 1

Typical energy distribution of the incident photon beam with $E_{\gamma }^{\max }=5.8$ MeV measured using the HPGe detector (black line). The original LCS photon spectrum (blue dashed line) was obtained by unfolding the simulated energy distribution (red line).

Figure 2

Photon-scattering spectra observed at a polar angle of $\theta ={90}^{\circ }$ relative to the incident photon beam and azimuthal angles of $\varphi =0^{\circ }$ (top) and ${90}^{\circ }$ (bottom) relative to the polarization plane of the LCS photon beam with (a) $E_{\gamma }^{\max }=5.8$, (b) 6.3, and (c) 6.8 MeV. The transitions to the ground state in ${}^{207}\mathrm{Pb}$ are labeled with open circles in the upper panels.

Figure 3

Measured azimuthal intensity ratios $R^{\prime }$ for the dipole transitions of ${}^{207}\mathrm{Pb}$. The horizontal lines $R^{\prime }=0.42$, 1, and 0.24 indicate the calculated ratios for $J^{\pi }=3/2^{-}, J^{\pi }=1/2^{\pm }$, and $J^{\pi }=3/2^{+}$, respectively.

Figure 4

Integrated scattering cross sections deduced from the present experiment. Data below 5 MeV are taken from Ref. [35].

Figure 5

$E1$ photoabsorption cross sections in the energy range (a) from 5 to 10 MeV and (b) from 5 to 20 MeV, obtained from the present ($\vec{\gamma },{\gamma }^{\prime }$) experiment (red squares) and from $(\vec{\gamma },n)$ [7] (blue open circles) and $(\gamma ,n)$ [7, 38] (blue filled circles and green triangles) experiments compared with the PVC $+$ QRPA calculations using the ${\mathrm{SkM}}^{*}$ (red line), SGII (green broken line), and SkP (blue dotted line) forces. Also shown are the Lorentzian curves with constant width (long broken line) and an energy-dependent width (green dotted line) from RIPL3 [42].

Figure 6

$E1$ strength predicted by the PVC $+$ QRPA calculation using the (a) ${\mathrm{SkM}}^{*}$, (b) SGII, and (c) SkP forces.

Figure 7

$M1$ photoabsorption cross sections obtained from the present ($\vec{\gamma },{\gamma }^{\prime }$) experiment (red squares) and from the $(\vec{\gamma },n)$ experiment (blue circles) [7] compared with the PVC $+$ QRPA calculation using the ${\mathrm{SkM}}^{*}$ (red line), SGII (green broken line), and SkP (blue dotted line) forces.