Low-lying dipole strength in ${}^{52}\mathrm{Cr}$

The low-lying dipole strength in ${}^{52}\mathrm{Cr}$ was measured in nuclear resonance fluorescence experiments using a quasimonochromatic, linearly polarized photon beam. The parities of the excited dipole states were determined by the intensity asymmetry of resonantly scattered $\gamma$ rays with respect to the polarization plane of the incident photon beam. We observed 62 resonances at excitation energies between 7.5 and 12.1 MeV. The observed $M1$ and $E1$ strengths were compared via random-phase-approximation calculations using the Skyrme interaction. The effects of two-particle–two-hole configuration mixing and tensor force on dipole strength distributions were investigated.

Figure 1

Typical energy distribution of the incident photon beam at $E_{\gamma }^{\max }=9.4$ MeV measured using a ${\mathrm{LaBr}}_3$(Ce) scintillation detector (solid line). The original LCS photon spectrum, shown as the dashed line, was obtained by unfolding the simulated energy distribution (dotted line).

Figure 2

Photon scattering spectra measured at polar and azimuthal angles of $(\theta ,\varphi )=({90}^{\circ },0^{\circ }$) and $({90}^{\circ },{90}^{\circ })$ using the photon beam with $E_{\gamma }^{\max }$ of (a) 8.2 MeV, (b) 8.8 MeV, and (c) 9.4 MeV, respectively. The $J^{\pi }$ assignments are indicated for the ground-state transitions in ${}^{52}\mathrm{Cr}$. Peaks labeled c and s are attributable to contamination from the opposite-parity transitions and single escapes, respectively.

Figure 3

Same as Fig. 2, but with $E_{\gamma }^{\max }$ of (a) 10.1, (b) 11.0, and (c) 12.1 MeV.

Figure 4

Azimuthal intensity asymmetry obtained for $E1$ (open circles) and $M1$ (filled circles) transitions in ${}^{52}\mathrm{Cr}$. The experimental sensitivity $q(=0.8)$ deduced from the numerical calculation is indicated by dashed lines.

Figure 5

SPLs of protons (top) and neutrons (bottom) for ${}^{52}\mathrm{Cr}$ calculated with the SGII and SGII+Te1 interactions. The dotted line in the right panel indicates the Fermi energy of the neutrons.

Figure 6

Distribution of (a) the measured $E1$ strength in units of ${\mu }_N^2$ in ${}^{52}\mathrm{Cr}$ compared with the values obtained from (b) RPA and (c) SRPA calculations with the SGII and SGII+Te1 interactions. Experimental data listed in Table 1 (Table 2) are shown with black (white) bars. In addition, experimental data below 7.5 MeV are taken from Refs. [29, 30].

Figure 7

Calculated $M1$ strength in units of ${\mu }_N^2$ obtained for the unperturbed states.

Figure 8

Comparison of the measured and calculated RPA and SRPA cumulative $M1$ strengths. The experimental values from Table 1 (Tables 1 and 2) are shown with solid (dotted) lines. Experimental $B\left(M1\right)$ data below 7.5 MeV are taken from Refs. [29, 30].

Figure 9

Distribution of (a) the measured $E1$ strength in units of ${\text{e}}^2{\mathrm{fm}}^2$ in ${}^{52}\mathrm{Cr}$ compared with the values obtained from (b) RPA and (c) SRPA calculations with the SGII and SGII+Te1 interactions. Experimental data listed in Table 1 (Table 2) are shown with black (white) bars. In addition, experimental data below 7.5 MeV are taken from Refs. [29, 30].

Figure 10

Comparison of the measured and calculated RPA and SRPA cumulative $E1$ strength. The experimental values from Table 1 (Tables 1 and 2) are shown with solid (dotted) lines. Experimental $B\left(E1\right)$ data below 7.5 MeV are taken from Refs. [29, 30].