Statistical properties of the well deformed ${}^{153,155}\mathrm{Sm}$ nuclei and the scissors resonance

The nuclear level densities (NLDs) and the $\gamma$-ray strength functions ($\gamma \mathrm{SFs}$) of ${}^{153,155}\mathrm{Sm}$ have been extracted from ($d,p\gamma$) coincidences using the Oslo method. The experimental NLD of ${}^{153}\mathrm{Sm}$ is higher than the NLD of ${}^{155}\mathrm{Sm}$, in accordance with microscopic calculations. The $\gamma \mathrm{SFs}$ of ${}^{153,155}\mathrm{Sm}$ are in fair agreement with QRPA calculations based on the D1M Gogny interaction. An enhancement is observed in the $\gamma \mathrm{SF}$ for both ${}^{153,155}\mathrm{Sm}$ nuclei around 3 MeV in excitation energy and is attributed to the $M1$ scissors resonance (SR). Their integrated strengths were found to be in the range 1.3–2.1 and 4.4–6.4 ${\mu }_N^2$ for ${}^{153}\mathrm{Sm}$ and ${}^{155}\mathrm{Sm}$, respectively. The strength of the SR for ${}^{155}\mathrm{Sm}$ is comparable to those for deformed even-even Sm isotopes from nuclear resonance fluorescence measurements, while that of ${}^{153}\mathrm{Sm}$ is lower than expected.

Figure 1

The raw [panels (a), (d)], unfolded [panels (b), (e)], and first generation (primary) $\gamma$-ray matrix [panels (c), (f)] of ${}^{153,155}\mathrm{Sm}$. The dashed lines indicate the energy regions from which the NLDs and $\gamma \mathrm{SFs}$ were extracted.

Figure 2

Spin distributions at $B_n$ estimated from HFB+comb (blue solid line) and RMI (red dashed line) models for ${}^{153}\mathrm{Sm}$.

Figure 3

Experimental NLDs of (a) ${}^{153}\mathrm{Sm}$ and (b) ${}^{155}\mathrm{Sm}$ from the present ($d,p\gamma$) experiments, normalized using the RMI and HFB+comb models and extrapolated with the CT and Fermi gas models to match the estimated $\rho (B_n$). The error bars on the NLD data points represent only statistical uncertainties, whereas the error band represent both statistical uncertainties as well as systematic uncertainties (see text for details). The error bands beyond the last data point (grey shaded) indicate the range of the extrapolation models and the solid line represent the average of the limits. The NLDs of ${}^{153,155}\mathrm{Sm}$ are compared to the NLDs of ($p,d\gamma ){}^{151,153}\mathrm{Sm}$ from Ref. [26] in panel (c).

Figure 4

The experimental $\gamma \mathrm{SF}$ of ${}^{153}\mathrm{Sm}$ (a) and ${}^{155}\mathrm{Sm}$ (b) compared to the ${}^{152,154}\mathrm{Sm}$ photonuclear reaction GEDR measurements of [62], and $E1$ and $M1$ $(n,\gamma ){}^{155}\mathrm{Sm}$ data measured in ARC experiments [29]. The $\gamma \mathrm{SFs}$ of ${}^{153}\mathrm{Sm}$ and ${}^{155}\mathrm{Sm}$, which only include the systematic errors, are compared in panel (c). The $\gamma \mathrm{SFs}$ of ${}^{151,153}\mathrm{Sm}$ from Ref. [26] are included for comparison.

Figure 5

The extracted SR of ${}^{153}\mathrm{Sm}$ (a) and ${}^{155}\mathrm{Sm}$ (b), fitted with the exponential background and SLo (see text for details).

Figure 6

Comparison between ${}^{153}\mathrm{Sm}$ (solid lines) and ${}^{155}\mathrm{Sm}$ (dashed lines) NLDs obtained within the combinatorial (red curves) [53] and statistical, ${\rho }_{\mathrm{stat}}$, (dark blue curves) [68] models. Also shown are the energy-dependent level density parameters $a_{\mathrm{stat}}$ (light blue curves) obtained within the microscopic statistical approach [68].

Figure 7

The experimental low-lying $M1$ strength, $B_{\mathrm{SR}}\left(M1\right)$, plotted against mass number $A$. The present measurements (lines) extracted over a full energy range are compared to ${}^{151,153}\mathrm{Sm}$ data from Ref. [26] (blue squares) and ${}^{150,152,154}\mathrm{Sm}$ from NRF measurements of Ref. [67] (green stars).

Figure 8

Comparison between the D1M+QRPA+0lim [32, 74] $E1$ (blue solid lines), $M1$ (red solid lines), and $E1+M1$ (black solid lines) with experimental data, (a) for ${}^{153}\mathrm{Sm}$ and (b) for ${}^{155}\mathrm{Sm}$. The present Oslo data including systematic uncertainties correspond to gray squares and the ARC data to triangles (blue for $E1$ and red for $M1$). To give a fair approximation of the $\gamma \mathrm{SF}$ in the GDR region, the $\gamma \mathrm{SF}$ extracted from photoneutron cross section of the neighboring ${}^{154}\mathrm{Sm}$ is shown by solid [62] and open triangles [75].