Extremely asymmetric shears band in ${}^{143}\mathrm{Sm}$

A dipole sequence has been observed and investigated in the ${}^{143}\mathrm{Sm}$ nucleus populated through the heavy-ion-induced fusion-evaporation reaction and studied using the Indian National Gamma Array as the detection system. The sequence has been established as a magnetic rotation (MR) band primarily from lifetime measurements of the band members using the Doppler shift attenuation method. A configuration based on nine quasiparticles with highly asymmetric angular momentum blades has been assigned to the shears band in light of the theoretical calculations within the framework of the shears mechanism with the principal axis cranking (SPAC) model. This is hitherto the maximum number of quasiparticles along with the highest asymmetricity associated with a MR band. Furthermore, as it has followed from the SPAC calculations, the contribution of the core rotation to the angular momentum of this shears band is substantial and greater than in any other similar sequence, at least, in the neighboring nuclei. This band can thus be perceived as a unique phenomenon of shears mechanism in operation at the limits of quasiparticle excitations as manifested in MR bandlike phenomena, evolving into collectivity.

Figure 1

(a) Proposed negative-parity level structure above the 7027-keV $35/2^{-}$ state in ${}^{143}\mathrm{Sm}$. The newly observed $\gamma$ rays are marked by asterisks. (b) Summed gated spectrum of the 352.7- and 632.4-keV transitions shows $\gamma$ rays marked by “$\times$” (DB III), “$\oplus$” (DB I), and “$\mathrm{\Delta }$” (below 8614 keV) in ${}^{143}\mathrm{Sm}$. The contamination peaks from ${}^{144}\mathrm{Sm}$ populated in the same reaction have been marked with “$\#$.” The aligned angular momentum ($i_x$) of the bands is shown in (c). The Harris parameters used in the calculation of $i_x$ are $J_0=12{\hslash }^2{\mathrm{MeV}}^{-1}$ and $J_1=25{\hslash }^4{\mathrm{MeV}}^{-3}$ [25]. The band-head spin ($K$) is determined from the fitting of excited energy $\left[E\right(I\left)\right]$ against spin ($I$).

Figure 2

Panels (a), (c), and (e) and (b), (d), and (f) exhibit the experimental spectra along with the fitted line shapes for the 352.7- and the 449.4-keV $\gamma$ transitions, respectively, of band DB III in ${}^{143}\mathrm{Sm}$. The top, middle, and bottom rows correspond to the shapes in the ${148}^{\circ },{123}^{\circ }$, and ${90}^{\circ }$ detectors, respectively. The obtained line shape of $\gamma$ transition, contaminant peaks, and total line shapes are represented by the blue, green, and red curves, respectively.

Figure 3

Comparison of the experimental results for dipole bands DB III (represented by the black filled squares) in ${}^{143}\mathrm{Sm}$ with the modified SPAC model calculations for different values of $\chi$ represented by the solid ($\chi =1.0$), dashed ($\chi =1.2$), and dotted ($\chi =1.4$) black lines. The $B\left(M1\right)$ and $B\left(E2\right)$ transition strengths against spin ($I$) have been depicted in (a) and (b), respectively. The nature of spin ($I$) against the rotational frequency ($\omega$) is shown in the inset of (a). The inset of (b) shows the variation of the shears and core angular momentum with the asymmetry of the shears blades of the MR bands in the Eu and Sm nuclei.