Shape coexistence in ${}^{178}\mathrm{Hg}$

Lifetime measurements of excited states in ${}^{178}\mathrm{Hg}$ have been performed using the ${}^{103}\mathrm{Rh}({}^{78}\mathrm{Kr},p2n)$ reaction at a beam energy of 354 MeV. The recoil-decay tagging (RDT) technique was applied to select the ${}^{178}\mathrm{Hg}$ nuclei and associate the prompt $\gamma$ rays with the correlated characteristic ground-state $\alpha$ decay. Lifetimes of the four lowest yrast states of ${}^{178}\mathrm{Hg}$ have been determined using the recoil distance Doppler-shift (RDDS) method. The experimental data are compared to theoretical predictions with focus on shape coexistence. The results confirm the shift of the deformed prolate structures to higher lying states but also indicate their increasing deformation with decreasing neutron number.

Figure 1

Level energy systematics of the even-mass mercury isotopes. Red filled squares refer to the assumed intruder states while blue open squares refer to the assumed oblate states. The figure is adopted from Ref. [6].

Figure 2

Spectrum of observed $\alpha$-particle energies occurring within 2000 ms of an ion being implanted into the same DSSSD pixel. Assignments for the peaks are indicated. See text for details.

Figure 3

Background subtracted $\gamma$-ray energy spectra measured with the JUROGAM II detectors in the ring at $157.6^{\circ }$ (five detectors) for the shortest and longest distances. Assignments for the lowest four yrast transitions are indicated. See text for details.

Figure 4

Partial level scheme of ${}^{178}\mathrm{Hg}$ up to the ${11}_1^{-}$ state, which is the highest lying state taken into account for feeding. $\gamma$ energies, spins, and parity are taken from Ref. [27]. See text for details.

Figure 5

Decay curves for the ground-state band up to the $8_1^{+}$ state in ${}^{178}\mathrm{Hg}$ constructed from the RDT singles $\gamma$-ray spectra recorded with five JUROGAM II detectors at $157.6^{\circ }$. The smooth lines correspond to the best fits.

Figure 6

Absolute values of transition quadrupole moments for the neutron-deficient Hg isotopes near midshell. See text for details.

Figure 7

Kinetic moments of inertia ${\Im }^{\left(1\right)}\left(J\right)$ for the Hg isotopes as a function of frequency $\omega (J\to J-2)$ for transitions along the yrast line up to the ${10}^{+}$ state. The figure is adopted from Ref. [31]. See text for details.