Nuclear structure of Te isotopes beyond neutron magic number $N=82$

Newly observed decay schemes of the nuclei ${}^{137}\mathrm{Sb}$ and ${}^{138}\mathrm{Sb}$ are reported. The neutron-rich Sb isotopes were produced by the in-flight fragmentation of a ${}^{238}\mathrm{U}$ primary beam with an energy of 345 MeV/nucleon. Several new excited states of ${}^{137}\mathrm{Te}$ with tentatively assigned spin-parities of $(5/2^{-})$, $(9/2^{-})$, and $(7/2)$ have been established which play an important role in the evolution of neutron levels beyond $N=82$. The study of the $\beta$ decay of ${}^{138}\mathrm{Sb}$ led to a considerable extension of the level scheme of ${}^{138}\mathrm{Te}$ including the identification of several nonyrast states. The structure of ${}^{137}\mathrm{Te}$ and ${}^{138}\mathrm{Te}$ is discussed on the basis of large-scale shell-model calculations performed using two different effective interactions.

Figure 1

Background-subtracted $\beta$-delayed $\gamma$-ray singles spectrum of ${}^{137}\mathrm{Sb}$. Black numbers represent the energies of the $\gamma$-ray transitions following $\beta$ decay while red (blue) numbers marked by “&” (#) indicate transitions following the $\beta$-delayed emission of one (two) neutron(s). Several unlabelled peaks, such as in (b) and (c), could not be assigned due to their small statistics and absence of $\gamma \text{−}\gamma$ coincidence information. The inset in (c) shows the half-life measurement of ${}^{137}\mathrm{Sb}$ with gate on 606-, 608-, 633-, and 974-keV transitions.

Figure 2

$\gamma \text{−}\gamma$ coincidence spectra for (a) the 62-, (b) the 633-, and (c) the 974-keV transition in ${}^{137}\mathrm{Te}$. Coincident transitions are indicated with their energies and contaminants are labeled with asterisks (*).

Figure 3

$\beta$-decay scheme of ${}^{137}\mathrm{Sb}$. The half-life of the ground state of ${}^{137}\mathrm{Sb}$ is based on the present work. The $Q_{{\beta }^{-}}$ and neutron separation energies ($S_n$, $S_{2n}$) are taken from Ref. [23]. Level information such as excitation energies, spin-parities, $\beta$-branching ratios, and log $ft$ values are represented. Excited states newly established in the present work are represented in red.

Figure 4

Background-subtracted $\beta$-delayed $\gamma$-ray singles spectrum of ${}^{138}\mathrm{Sb}$. Black numbers represent the energies of the $\gamma$-ray transitions following $\beta$ decay while red (blue) numbers marked by “&” (#) indicate transitions following the $\beta$-delayed emission of one (two) neutron(s).

Figure 5

$\gamma \text{−}\gamma$ coincidence spectra with gate on (a) the 443-keV, (b),(c) the 461-keV, and (d) the 536-keV transition. Coincident transitions are indicated with their energies and contaminants are labeled with asterisks (*).

Figure 6

Half-life measurements of the ground state of ${}^{138}\mathrm{Sb}$. Red solid lines indicate the fit results with a single-component exponential decay curve (red-dashed lines) and a constant background (red dashed-dotted lines).

Figure 7

$\beta$-decay scheme of ${}^{138}\mathrm{Sb}$. The half-life of the ground state of ${}^{138}\mathrm{Sb}$ is based on the present work. The $Q_{{\beta }^{-}}$ and neutron separation energies ($S_n$, $S_{2n}$) are taken from Ref. [23]. Level information such as excitation energies, proposed spin-parities, $\beta$-branching ratios, and log $ft$ values are represented. Newly assigned excited states from the present work are represented in red.

Figure 8

Excitation energy differences of excited states in the $N=85$ isotones relative to the $7/2_1^{-}$ levels. Data for ${}^{137}\mathrm{Te}$ are taken from this work, and all others are taken from Ref. [15]. For $9/2_2^{-}$ states of ${}^{143}\mathrm{Ce}$ ($Z=58$) and ${}^{149}\mathrm{Gd}$ ($Z=64$), the levels at 817 keV and 1085 keV without explicit spin-parity assignment are included, based on systematics.

Figure 9

Comparison between the experimentally established negative-parity states of ${}^{137}\mathrm{Te}$ (middle) and theoretical predictions. The Napoli (left) and N3LOP (right) interactions are employed in the large-scale shell-model calculations.

Figure 10

Comparison between the experimentally established levels of ${}^{138}\mathrm{Te}$ (middle) and theoretical predictions. The Napoli (left) and N3LOP (right) interactions are employed in the large-scale shell-model calculations. A modified version of the Napoli interaction is shown in red, see text for details.