Structure of low-lying states in ${}^{116}\mathrm{Te}$

Low-lying excited levels in ${}^{116}\mathrm{Te}$ were studied by measuring level lifetimes with the recoil-distance Doppler-shift method in the ${}^{112}\mathrm{Sn}({}^{12}\mathrm{C},{}^8\mathrm{Be}){}^{116}\mathrm{Te}\alpha$-transfer reaction and angular correlations following the $\beta$ decay of ${}^{116}\mathrm{I}$. Both experiments were performed at the Cologne FN Tandem accelerator. Several new levels were discovered below 3.2 MeV excitation energy, spins and multipole mixing ratios were determined via angular correlations. Lifetimes were measured for the $2_1^{+}$, $0_2^{+}$, $2_2^{+}$, and $4_1^{+}$ and upper limits for lifetimes were determined for higher lying levels. The experimental findings are compared to calculations in the U(5) limit of the interacting boson model and are also discussed in the framework of shape coexistence which is expected in midshell Te isotopes.

Figure 1

Particle spectrum observed by the solar cells. Protons from fusion-evaporation reactions like ${}^{112}\mathrm{Sn}({}^{12}\mathrm{C},np){}^{122}\mathrm{Cs}$, ${}^{112}\mathrm{Sn}({}^{12}\mathrm{C},2p){}^{122}\mathrm{Xe}$ were found at energies below channel 700. The gate was set for higher energies, to select only $\alpha$ particles. The corresponding coincident spectrum in the HPGe detectors is show in Fig. 2. The structure at around channel 600 is introduced by the electronics of the Plunger device.

Figure 2

Doppler-corrected $\gamma$-ray spectrum obtained in coincidence with solar cells in the $\alpha$-transfer experiment. All target-to-stopper distances are summed for the full statistics. An artificial vertical offset of 40 counts has been added to the spectrum of the detectors at forward angle to make the figure more illustrative. Note that the Doppler correction leads to a broadening of the stopped component and a narrowing of the flight component. The flight component is on the nominal energy and the stopped component is shifted. Transitions in ${}^{116}\mathrm{Te}$ are denoted by spin and parity of the initial and final state, states with unknown spin and parity are denoted with a question mark. Contaminating transitions—stemming from, e.g., target impurities—are denoted by the nucleus in which they take place.

Figure 3

Spectrum showing $\gamma$ rays in coincidence with the 679 keV transition in ${}^{116}\mathrm{Te}$ observed in the $\beta$-decay experiment. The statistics of all detectors were summed for this figure. Note that for illustrative purposes not all visible $\gamma$ rays were marked.

Figure 4

Angular correlations of $\gamma \text{−}\gamma$ cascades in ${}^{116}\mathrm{Te}$. Experimental intensities are shown in red. The parameters of the best fit, which is shown in black, can be found in Table 3.

Figure 5

Spin and multipole mixing ratio determination for the 959-679 keV cascade via angular correlations: For different spin hypotheses of $J_i\to 2\to 0$ cascades, the resulting $S^2$ minimization is shown depending on the multipole mixing ratio ${\delta }^2=\frac{T(L_{\min }+1)}{T\left(L_{\min }\right)}$. Here, $T\left(L\right)$ is the probability to have a transition with angular momentum $L$. All hypotheses above the 99% confidence limit are rejected and spin 2 is adopted for the 1637 keV state. The multipole mixing ratio is extracted from the minimum $S^2$ value and the uncertainty is taken at $S_{\min }^2+1$.

Figure 6

Doppler corrected spectra obtained from particle-$\gamma \text{−}\gamma$ coincidences in the $\alpha$-transfer experiment. The $\gamma$ gate is set on the transition shown in the upper left corner of each panel, the transition energy is rounded to 10 keV. The particle gate just requires a particle to be detected in the solar cells. The positions of the 680 keV doublet and the $2_2^{+}\to 2_1^{+}$ transition at 540 keV are marked with red dashed lines. The statistics of all HPGe detectors and all target-to-stopper distances are summed.

Figure 7

Level scheme of ${}^{116}\mathrm{Te}$ observed in the current study. New results are marked with blue color. Blue transitions indicate, that the transition was observed for the first time. Blue level energies and level lines correspond to levels that were placed for the first time. Blue spin and parity imply that the spin and parity were assigned for the first time or clarified. Indentation of lines and numbers is used to improve the readability of the figure.

Figure 8

Decay curve $R\left(x\right)$ of the $0_2^{+}$ state for detectors at forward and backward angles depending on absolute distances $x$. The uncertainty on the absolute distances is not represented in this picture. The uncertainty of the offset is about 5 µm and much larger than the uncertainty of the relative distances which is below 1 µm.

Figure 9

Spectra of the $2_1^{+}\to 0_1^{+}$ transition for different relative target-to-stopper distances from detectors at backward angles. A full gate on both components was set on the $2_2^{+}\to 2_1^{+}$ (540 keV) transition and a solar cell condition. Since the spectra are not Doppler-corrected, the shifted component is significantly broader than the unshifted component.

Figure 10

Comparison of the IBM-1 calculation with the experimental results. $B\left(E2\right)$ strengths are given in W.u. For simplicity, for the experimental data the $4_2^{+}\to 4_1^{+}$ transition is assumed to be a pure $E2$ transition as no multipole mixing ratio was determined.

Figure 11

Systematics of low-lying levels in midshell Te isotopes [11, 26, 27, 31, 32, 33]. States where the spin and parity assignment could be improved in this study are marked with a $⬠$ symbol. Several states in ${}^{114}\mathrm{Te}$ are marked in brackets since the spin assignment is tentative.