Evidence of antimagnetic rotation in an odd-odd nucleus: The case of ${}^{\mathbf{142}}\mathbf{Eu}$

The present Rapid Communication reports conclusive evidence for an antimagnetic rotational band in the odd-odd ${}^{142}\mathrm{Eu}$ nucleus. The parity of the states of a quadrupole sequence in this nucleus is identified firmly from polarization measurements using the Indian National Gamma Array, and lifetimes of some of the states in the same structure are measured using the Doppler-shift attenuation method. The decreasing trends of the deduced quadrupole transition strength $B\left(E2\right)$ with spin in the sequence along with the increasing $J^{\left(2\right)}/B\left(E2\right)$ values conclusively establish the origin of these states as arising from antimagnetic rotation. The results are well reproduced by numerical calculations within the framework of a semiclassical geometric model.

Figure 1

(a) The partial level scheme of ${}^{142}\mathrm{Eu}$ obtained in the present Rapid Communication. The $\gamma$ energies are given in keV, and the widths of the arrows are proportional to the intensities of the transitions. The energies are rounded off to the nearest keV. Asterisk-marked transitions are newly observed. (b) The polarization value against DCO ratio $\left(R_{\mathrm{DCO}}\right)$ for different transitions of the observed structure in ${}^{142}\mathrm{Eu}$.

Figure 2

(a) The measured angular distribution (in arbitrary units) along with the fitted curve (solid red line) for the 601.5-keV transition. The theoretical distribution of this transition for the spin sequences $13\to 11$ (dashed violet line) and $11\to 11$ (dotted black line) are also shown. The ${\chi }^2$ of angular distribution coefficients for different mixing ratios $\left(\delta \right)$ plotted in the inset of this figure for the spin sequences $13\to 11$ (dotted violet line), $12\to 11$ (dashed black line), and $11\to 11$ (solid red line), respectively. (b) The experimental angular distributions along with the fitted curves for the 1021.1- and 1104.5-keV transitions. The ${\chi }^2$ analysis for the experimental distribution of the 1021.1-keV transition assuming the spin sequence $12\to 11$ is shown in the inset of this figure and gives the mixing ratio of $\delta =-0.39\left(0.01\right)$.

Figure 3

Angular distribution coefficients $a_2$ vs $a_4$ plot for different mixing ratios $\left(\delta \right)$ using multiple spin sequences [23, 24]. The black cross indicates the data point for the 601.5-keV transition.

Figure 4

The experimental observed spectra along with the fitted line shapes for the 820.1-, 1075.6-, and 1281.0-keV transitions of the quadrupole band of ${}^{142}\mathrm{Eu}$. (a)–(d) correspond to the shapes in the ${140}^{\circ },{90}^{\circ },{65}^{\circ }$, and ${40}^{\circ }$ detectors. The spectrum for the 1281.0-keV transition at ${40}^{\circ }$ is omitted since the peak shapes merge into the background. The obtained line shape of the $\gamma$ transition, contaminant peaks, and the total line shapes are represented by the blue, green, and red curves, respectively. The vertical dotted line represents the stopped peak position for each transition, whose shape is observed.

Figure 5

(a) $B\left(E2\right)$ values with spin $\left[I\right(\hslash \left)\right]$ and (b) $I(\hslash )$ against rotational frequency $\left(\omega \right)$ for the quadrupole band in ${}^{142}\mathrm{Eu}$. The variation of $R$ is shown in the inset plot. The solid and dashed lines represent the theoretical SPRM calculation for unstretched and stretched conditions, respectively. The parameters used for this calculation are $V_{\pi \pi }=0.2$ and $V_{\pi \nu }=1.7\mathrm{MeV}$ [13], $j_{\pi }^1=j_{\pi }^2=4.5$ (unstretched), $j_{\nu }=17,n=8,a=3.78$, and $e{Q}_{\mathrm{eff}}=1.26e\mathrm{b}$ (calculated as in Ref. [12]). The parameters have the same meaning as in Refs. [6, 7, 8, 9, 13]. The upward arrow in (a) represents the lower limit of the $B\left(E2\right)$ value for the ${25}^{-}$ state of the quadrupole structure. The $B\left(E2\right)$ values for the quadrupole band $I$ in ${}^{143}\mathrm{Eu}$ [13] are also shown (red squares) (see the text for details).