Nuclear level densities and $\gamma$-ray strength functions in samarium isotopes

The $\gamma$-strength functions and level densities in the quasicontinuum of ${}^{147,149}\mathrm{Sm}$ isotopes have been extracted from particle-$\gamma$ coincidences using the Oslo method. The nuclei of interest were populated via $(p,d)$ reactions on pure ${}^{148,150}\mathrm{Sm}$ targets and the reaction products were recorded by the Hyperion array. An upbend in the low-energy region of the $\gamma \mathrm{SF}$ has been observed. The systematic analysis of the $\gamma \mathrm{SF}$ for a range of Sm isotopes highlights the interplay between scissors mode and the upbend. Shell-model calculations show reasonable agreement with the experimental $\gamma \mathrm{SFs}$ and confirm the correspondence between the upbend and scissors mode.

Figure 1

A comparison of experimental $\gamma$-ray spectrum of a clover detector with geant4 simulation for ${}^{60}\mathrm{Co}$ source.

Figure 2

First-generation (primary) $\gamma$-ray matrices for ${}^{147}\mathrm{Sm}$ (a) and ${}^{149}\mathrm{Sm}$ (b).

Figure 3

A comparison of experimental primary $\gamma$-ray spectra (crosses) for four excitation energies and the product of $\rho \left(E_f\right)$ and $\mathcal{T}\left(E_{\gamma }\right)$ obtained from the ${\chi }^2$ fit (solid lines) of $P(E,E_{\gamma })$ in ${}^{149}\mathrm{Sm}$. The fit is performed for the entire first-generation matrix and, as shown here, works well for different subsets of excitation energies.

Figure 4

Systematics of level density as a function of neutron-separation energy $S_n$ for odd-$A$ Sm isotopes (black circles). The solid line corresponds to the linear fit of the ${}^{151,149,145}\mathrm{Sm}$ data points giving an estimate of the level density in ${}^{147}\mathrm{Sm}$ (red square). A reduction factor of 0.89 is used for all these $\rho \left(S_n\right)$ calculated using the parameters given in Ref. [37].

Figure 5

Level-density functions for ${}^{147}\mathrm{Sm}$ (a) and ${}^{149}\mathrm{Sm}$ (b). Experimental data are shown as black squares. The dashed line corresponds to the constant-temperature approximation extrapolating to $\rho \left(S_n\right)$ (open squares). The solid line is the known level density in the low-energy discrete region. Experimentally deduced $\gamma$-ray strength functions are shown in (c) and (d). For comparison, the analytical approximations for the $E1$ strength (blue dashed lines) and $M1$ strength (red dashed lines) as well as their sums (red solid lines) are shown.

Figure 6

$\gamma$-ray strength functions for all four Sm isotopes with the GDR contribution subtracted. Red solid lines indicate the fit to the upbend region, while the red dashed lines show the fit uncertainty. The results are compared with shell model calculations (black curve).

Figure 7

Level density functions for all four Sm isotopes. Solid symbols: data extracted using the Oslo method. Solid line: level density from shell-model calculations. Dashed line: known levels.