Nuclear shape and structure in neutron-rich ${}^{110,111}\mathrm{Tc}$

The high-spin nuclear structure of Tc isotopes is extended to more neutron-rich regions based on the measurements of prompt γ rays from the spontaneous fission of ${}^{252}\mathrm{Cf}$ at the Gammasphere. The high-spin level scheme of $N=67$ neutron-rich ${}^{110}\mathrm{Tc}$ ($Z=43$) is established for the first time, and that of ${}^{111}\mathrm{Tc}$ is extended and expanded. The ground band of ${}^{111}\mathrm{Tc}$ reaches the band-crossing region, and the new observation of the weakly populated $\alpha =-1/2$ member of the band provides important information on signature splitting. The systematics of band crossings in the isotopic and isotonic chains and a CSM calculation suggest that the band crossing of the ground band of ${}^{111}\mathrm{Tc}$ is due to alignment of a pair of $h_{11/2}$ neutrons. The best fit to signature splitting, branching ratios, and excitations of the ground band of ${}^{111}\mathrm{Tc}$ by the rigid triaxial rotor plus particle model calculations result in a shape of ${\varepsilon }_2=0.32$ and $\gamma =-{26}^{°}$ for this nucleus. Its triaxiality is larger than that of ${}^{107,109}\mathrm{Tc}$, which indicates increasing triaxiality in Tc isotopes with increasing neutron number. The identification of the weakly populated $K+2$ satellite band provides strong evidence for the large triaxiality of ${}^{111}\mathrm{Tc}$. In ${}^{110}\mathrm{Tc}$, the four lowest-lying levels observed are very similar to those in ${}^{108}\mathrm{Tc}$. At an excitation of 478.9 keV above the lowest state observed, ten states of a $\Delta I=1$ band are observed. This band of ${}^{110}\mathrm{Tc}$ is very analogous to the $\Delta I=1$ bands in ${}^{106,108}\mathrm{Tc}$, but it has greater and reversal signature splitting at higher spins.

Figure 1

No caption

Figure 2

High-spin level scheme of ${}^{110}\mathrm{Tc}$ proposed for the first time in the present work, assuming bandhead spin $I$, as for ${}^{106,108}\mathrm{Tc}$ in [16]. The coincident 53.5–131.3 keV transitions are also reported by measurements of ${}^{110}\mathrm{Mo}$ ${\beta }^{-}$ decay [14].

Figure 3

High-spin level scheme of ${}^{111}\mathrm{Tc}$ proposed in the present work. The $\alpha =+1/2$ member of the ground band reaches the band-crossing region; the weakly populated $\alpha =-1/2$ member of the ground band is identified for the first time. A $K+2$ satellite band, band 6, similar to those in ${}^{105,107,109}\mathrm{Tc}$ [8] is also identified in ${}^{111}\mathrm{Tc}$.

Figure 4

Level systematics of the high-spin bands in even-$A$ ${}^{106,108,110}\mathrm{Tc}$. The $\Delta I=1$ yrast band observed in ${}^{110}\mathrm{Tc}$ is very analogous to those recently observed in ${}^{106,108}\mathrm{Tc}$ [16], but it has greater and reversal signature splitting at higher spins.

Figure 5

Systematics of level patterns of ground bands of odd-$A$ ${}^{103-111}\mathrm{Tc}$ isotopes. Data are from the present work and [8, 12]. A smooth trend can be seen.

Figure 6

No caption

Figure 7

Polar coordinate plots of total Routhian surface (TRS) calculated at (a) $\hslash \omega =0.2$ and (b) 0.4 MeV for ${}^{111}\mathrm{Tc}$.

Figure 8

Calculated Routhian in ${}^{111}\mathrm{Tc}$ for (a) quasiprotons and (b) quasineutrons plotted vs rotational frequency. The parity and signature ($\pi ,\alpha )$ of the state are solid lines (+,+); dotted lines (+,−); dot-dashed lines (−,+); dashed lines (−,−).

Figure 9

Level systematics of band 6 of ${}^{105-111}\mathrm{Tc}$ and of the Rh isotopes. A rapid lowering of the low-lying bandhead of band 6 with increasing neutron number is observed for both Tc and Rh isotopes. Data are from the present work and [7, 8, 12, 28].

Figure 10

Comparison of theoretical and experimental signature splitting of the ground band of ${}^{111}\mathrm{Tc}$. Calculations were performed using the RTRP model. Good agreement is obtained by using parameters ${\varepsilon }_2=0.32$, $\gamma =-{26}^{°}$, $E(2^{+})=0.25$ MeV, and $\xi =0.8$.

Figure 11

Experimental signature splittings of the ground bands of ${}^{105-111}\mathrm{Tc}$. ⋄ ${}^{105}\mathrm{Tc}$; □ ${}^{107}\mathrm{Tc}$; • ${}^{109}\mathrm{Tc}$; ▴ ${}^{111}\mathrm{Tc}$. Signature splitting of ${}^{111}\mathrm{Tc}$ is the largest among all the Tc isotopes, implying a larger triaxiality. Data are from the present work and [8].

Figure 12

Comparison of theoretical and experimental excitation energies of the ground band (band 1) and band 6 of ${}^{111}\mathrm{Tc}$. Calculations were performed using the RTRP. Good agreement is obtained by using the same parameters as in Fig. 10.