Shape coexistence and isomeric states in neutron-rich ${}^{112}\mathrm{Tc}$ and ${}^{113}\mathrm{Tc}$

Isomeric states in ${}^{112}\mathrm{Tc}$ and ${}^{113}\mathrm{Tc}$, with half-lives of 150(17) ns and 500(100) ns, respectively, have been observed following the relativistic fission of ${}^{238}\mathrm{U}$. The fission fragments have been separated in a fragment separator and identified by means of energy-loss and time-of-flight techniques. In both nuclei, the ground-state configuration is calculated to have an oblate shape and the isomerism is proposed to arise due to transitions from a triaxial excited state to a low-lying oblate state.

Figure 1

Particle identification plot with the location of the ${}^{113}\mathrm{Tc}$ (1) and ${}^{106}\mathrm{Nb}$ (2) ions indicated.

Figure 2

$\gamma$-ray energy spectra obtained in coincidence with ${}^{112}\mathrm{Tc}$ ions (a) and ${}^{113}\mathrm{Tc}$ ions (b) and with a time gate of width 1.25 $\mu \mathrm{s}$ starting 300 ns after implantation (to reduce the bremsstrahlung background).

Figure 3

$\gamma$-ray energy spectra obtained in coincidence with ${}^{112}\mathrm{Tc}$ ions and (a) the 92-keV transition, (b) the 258-keV transition.

Figure 4

Background-subtracted time distributions of (a) the 258-keV transition in ${}^{112}\mathrm{Tc}$ and (b) the 114-keV transition in ${}^{113}\mathrm{Tc}$, along with associated fits.

Figure 5

Potential-Energy Surface (PES) calculations for ${}^{112}\mathrm{Tc}$ for a negative-parity neutron and a positive-parity proton $(-\nu ,+\pi )$ (a), $+\nu$, $+\pi$ (b), and $-\nu ,-\pi$ (c). Isolines are placed at intervals of 200 keV and the energies of the minima in (b) and (c) are 411 and 474 keV relative to the $-\nu ,+\pi$ ground state. The $+\nu ,-\pi$ configuration is also calculated to be triaxial but at an even higher excitation energy.

Figure 6

The calculated energy spectrum for ${}^{112}\mathrm{Tc}$. The in-band oblate states assume a moment of inertia one-half of that of a rigid body. The triaxial states correspond to the configurations in Figs. 5b and 5(c).

Figure 7

PES calculations for the lowest-energy positive-parity (a) and negative-parity (b) states in ${}^{113}\mathrm{Tc}$. Iso-lines are placed at intervals of 200 keV.