First observation of band structure in ${}^{76}\mathrm{As}$: Possible chirality and octupole correlations

High-spin states in ${}^{76}\mathrm{As}$ have been observed for the first time by using the ${}^4\mathrm{He}+{}^{74}\mathrm{Ge}$ reaction at beam energies of 58.6 and 62.6 MeV. Two positive-parity and three negative-parity bands have been found in ${}^{76}\mathrm{As}$. The two positive-parity bands with $\pi g_{9/2}\otimes \nu g_{9/2}$ configurations are tentatively interpreted as chiral doublet bands, which are supported by triaxial particle rotor model calculations. The electric dipole transitions linking the yrast positive- and negative-parity bands are also observed in this work, implying the possible presence of octupole correlations in ${}^{76}\mathrm{As}$.

Figure 1

Partial level scheme of ${}^{76}\mathrm{As}$. The energies of the $\gamma$ transitions and levels are given in keV. The thicknesses of the arrows correspond to the $\gamma$-ray intensities. New transitions and levels are shown in red.

Figure 2

(a) Typical $\gamma \text{−}\gamma$ coincidence spectrum gated on the 164.7-keV transition showing all new transitions, and $\gamma \text{−}\gamma \text{−}\gamma$ coincidence spectra (b) gated on 164.7 and 85.8 keV $+$ 164.7 and 114.6 keV showing the placement of bands 1 and 2, (c) gated on 164.7 and 45.8 keV $+$ 164.7 and 74.6 keV showing the placement of bands 3 and 4, and (d) gated on 164.7 and 223.0 keV $+$ 164.7 and 276.5 keV showing the placement of band 5. Peaks labeled with a “c” are contaminate transitions.

Figure 3

The systematics of the excitation energies for the positive yrast (black) and side (red) bands above $7^{+}$ in ${}^{70-78}\mathrm{As}$ [18, 35, 36, 37]. The states of ${}^{76}\mathrm{As}$ were deduced from the present work.

Figure 4

The experimental excitation energies $E\left(I\right)$, energy staggering parameters $S\left(I\right)=\left[E\right(I)-E(I-1\left)\right]/2I$, and reduced transition probability ratios $B\left(M1\right)/B\left(E2\right)$ as functions of spin for bands 1 and 2 in ${}^{76}\mathrm{As}$ in comparison with the TPRM calculations.

Figure 5

The root-mean-square components along the intermediate (i, circles), short (s, squares), and long (l, triangles) axes of the core, valence neutron, and valence proton angular momenta calculated as functions of spin by means of the particle rotor model for the doublet bands in ${}^{76}\mathrm{As}$. “R” represents the core, “$\pi g_{9/2}$” represents the unpaired proton in $g_{9/2}$ orbital, and “$\nu g_{9/2}$” represents the unpaired neutron in $g_{9/2}$ orbital.

Figure 6

The experimental $B\left(E1\right)/B\left(E2\right)$ ratios between the positive- and negative-parity bands as a function of spin in ${}^{76}\mathrm{As}$, together with those in ${}^{74}\mathrm{As}$ [18], ${}^{78}\mathrm{Br}$ [14], and ${}^{76}\mathrm{Br}$ [16].