Shears mechanism and development of collectivity in ${}^{141}\mathrm{Sm}$

High-spin states in the dipole structure of ${}^{141}\mathrm{Sm}$ have been investigated using the fusion-evaporation reaction ${}^{116}\mathrm{Cd}\left({}^{31}\mathrm{P},p5n\right)$ at beam energy 148 MeV using the Indian National Gamma Array. The spin parity of the observed dipole bands I and II has been established firmly from the spectroscopic measurements. Level lifetimes of several levels in the dipole bands I and II have been measured using the Doppler shift attenuation method. The smooth decrease of the $B\left(M1\right)$ values with spin exhibits a clear signature of the magnetic rotational character of the dipole band I. Comparisons between the experimental characteristics and the semiclassical shears mechanism with the principal axis cranking model calculation show that the dipole band I may be interpreted as a magnetic rotational band.

Figure 1

Theoretical $R_{\text{DCO}}$ values for different $\sigma /j$ value of the reaction (represented by red, orange, olive, and black lines) for the present experimental setup, calculated using angcorprogram. Experimental $R_{\text{DCO}}$ ratios for the 1151.6 ($17/2^{+}\to 15/2^{-}$)-keV $E1$ transition in ${}^{143}\mathrm{Eu}$ and 553.6 ($25/2^{-}\to 23/2^{+}$)-keV, 628.3 ($19/2^{+}\to 17/2^{-}$)-keV, and 823.1 ($21/2^{+}\to 19/2^{-}$)-keV $E1$ transitions in ${}^{141}\mathrm{Sm}$ (represented by the solid circles), reproduced assuming $\sigma /j=0.27$ and 0.26, 0.31, and 0.26, respectively.

Figure 2

In panel (a) total projection spectrum of the $E_{\gamma }\text{−}{E}_{\gamma }\text{−}{E}_{\gamma }$ cube shows the $\gamma$ rays of the nuclei populated in the present reaction. The peaks marked with the “$\mathrm{\Delta }$”, “$\oplus$”, and “$\times$” are the $\gamma$-ray transitions in ${}^{143}\mathrm{Eu}$, ${}^{142}\mathrm{Sm}$, and ${}^{141}\mathrm{Sm}$, respectively. The summed double-gated $\gamma$-ray spectrum of the 634.8-, 1089.0-, and 1418.0-keV $\gamma$ transitions showing the $\gamma$ rays in ${}^{141}\mathrm{Sm}$ is displayed in panel (b).

Figure 3

The partial level scheme of ${}^{141}\mathrm{Sm}$ obtained in the present work. The $\gamma$-ray energies are given in keV and the width of the arrows are proportional to the intensities of the transitions. The new $\gamma$ transitions have been marked by asterisks.

Figure 4

The variation of $R_{\text{DCO}}$ for with the mixing ratio ($\delta$) of different multipoles ($L$) for the 132.6-keV transition in ${}^{141}\mathrm{Sm}$.

Figure 5

Angular distribution of the 132.6-keV ($27/2^{-}\to 25/2^{-}$) $\gamma$-ray transition in ${}^{141}\mathrm{Sm}$. The solid red line represents the fit to the experimental data points. The solid black line represents the angular distribution of a pure dipole transition.

Figure 6

Evolution of the ratio of the angular distribution coefficients ($a_2/a_4$) with mixing ratio ($\delta$) for the 132.6-keV ($27/2^{-}\to 25/2^{-}$) $\gamma$-ray transition in ${}^{141}\mathrm{Sm}$.

Figure 7

Summed double-gated $\gamma$-ray spectrum of the 132.6-, 309.4-, 446.4-, and 527.8-keV $\gamma$ transitions shows the $\gamma$ rays in ${}^{141}\mathrm{Sm}$. The peaks marked with the asterisks are the newly observed member of dipole bandlike structures in ${}^{141}\mathrm{Sm}$.

Figure 8

The experimental observed spectra along with the fitted line shapes for the dipole transitions (a) 132.6, (b) 309.4, (c) 446.4, (d) 527.8, and (e) 253.7 keV of the dipole bands I and II in ${}^{141}\mathrm{Sm}$. The top, middle, and bottom rows correspond to the shapes in the ${140}^{\circ }$, ${90}^{\circ }$, and ${65}^{\circ }$ detectors, respectively. The obtained line shape of the $\gamma$ transition, contaminant peaks, and total line shapes are represented by the blue, olive, and red curves, respectively. The vertical dotted lines represent the stopped peak position for each transition whose shape is observed.

Figure 9

Comparison of the experimental results (represented by solid squares) for the dipole bands I and II in ${}^{141}\mathrm{Sm}$ in the framework of the SPAC-model (solid lines) calculations. In panel (a) $B\left(M1\right)$ transition strengths against spin ($I$); (b) angular momentum ($I$) vs $\hslash \omega$; (c) energy of the excited states as a function of ${\theta }_1$; and (d) evolution of the angle ${\theta }_1$ (solid blue line), ${\theta }_I$ (solid red line), and collective core rotational angular momentum ($R$) (solid black line) against spin for the dipole bands are shown. Here, ${\theta }_I$ is the angle between the total angular momentum vector ($I$) and the collective core rotational angular momentum ($R$). The parameters used for the calculations are given in the figures. The dotted lines represent the SPAC-model calculations for dipole band II assuming $j_1=10.5\hslash$, $j_2=9\hslash$, and $J=4.1{\hslash }^2/\mathrm{MeV}$ (see text for details).

Figure 10

Contour plots of the TRS calculations for the (a) $\pi h_{11/2}^2\otimes \nu h_{11/2}^{-1}$ and (b) $\pi h_{11/2}^2\otimes \nu h_{11/2}^{-3}$ configurations of the dipole bands I and II, respectively, in ${}^{141}\mathrm{Sm}$. The rotational frequency ($\hslash \omega$) for the calculations are (a) 0.170 and (b) 0.210 MeV. The energy difference between two contours is 0.400 MeV.

Figure 11

The experimental angular momentum as a function of the rotational frequency for the MR bands of Sm, Eu, and Gd [13, 14, 20, 32] nuclei in the $A\sim 140$ mass region.