Identification of Excited States in the $T_z=1$ Nucleus ${}^{110}\mathrm{Xe}$: Evidence for Enhanced Collectivity near the $N=Z=50$ Double Shell Closure

Gamma-ray transitions have been identified for the first time in the extremely neutron-deficient ($N=Z+2$) nucleus ${}^{110}\mathrm{Xe}$, and the energies of the three lowest excited states in the ground-state band have been deduced. The results establish a breaking of the normal trend of increasing first excited $2^{+}$ and $4^{+}$ level energies as a function of the decreasing neutron number as the $N=50$ major shell gap is approached for the neutron-deficient Xe isotopes. This unusual feature is suggested to be an effect of enhanced collectivity, possibly arising from isoscalar $n\mathrm{\text{−}}p$ interactions becoming increasingly important close to the $N=Z$ line.

Figure 1

(a) Correlated decay energy spectrum obtained by requiring the implantation of a recoiling fusion product and a subsequent decay event in the same pixel of the DSSD, within a correlation time of 330 ms (corresponding to approximately three half-lives of ${}^{110}\mathrm{Xe}$). The largest peak is due to decays of the most strongly populated $\alpha$ emitter ${}^{109}\mathrm{Te}$ [28]. (b) As in (a) with the additional requirement of proper spatial and temporal correlations ($220\mu \mathrm{s}$) with the subsequent $\alpha$ decay of the daughter nucleus ${}^{106}\mathrm{Te}$. (c) As in (b), showing the energy spectrum corresponding to the daughter (${}^{106}\mathrm{Te}$) $\alpha$ decays.

Figure 2

(a) Recoil-correlated prompt $\gamma$-ray energy spectrum, dominated by transitions in the strongest fusion 3-proton evaporation channel, ${}^{109}\mathrm{Sb}$ [29]. (b) As in (a), with the additional selection of events tagged by the subsequent $\alpha$ decays of ${}^{110}\mathrm{Xe}$ and ${}^{106}\mathrm{Te}$. The energies (given in keV) and relative intensities (%) of the $\gamma$-ray transitions assigned to ${}^{110}\mathrm{Xe}$ are as follows: 469.7(2),100(9); 531.5(5),13(3); 586.3(4),39(5); 643.4(3),81(7); 671.7(5),36(5); 691.1(5),38(5); 735.4(5),13(3); 776.6(6),66(7); 816.8(3),19(3). (c) Recoil-decay-tagged prompt $\gamma \mathrm{\text{−}}\gamma$-coincidence spectrum extracted from a sum of gates on the 470, 643, and 777 keV transitions. (d) Level scheme showing the lowest levels assigned to ${}^{110}\mathrm{Xe}$.

Figure 3

Energies of $2_1^{+}$ (squares) and $4_1^{+}$ (circles) states plotted versus neutron number $N$ for even-even Xe isotopes in the mass region $110\le A\le 136$. Data are from the present work and Refs. 23, 30.

Figure 4

Systematics of theoretical $B(E2;2_1^{+}\to 0_1^{+})$ values compared with empirical data derived from experimental $2_1^{+}$ energies according to the formula proposed by Raman, Sheikh, and Bhatt [22]. Data points with error bars are directly measured $B(E2)$ values for ${}^{114,116}\mathrm{Xe}$ [25].