New $\beta$-decay spectroscopy of the ${}^{137}\mathrm{Te}$ nucleus

Background: Nuclear spectroscopy of neutron-rich isotopes provides important information on their nuclear structure and has a valuable impact on the modeling of the $r$-process path. Particularly interesting are nuclei close to doubly-magic species, e.g., ${}^{132}\mathrm{Sn}$, with only several valence particles. Such is the barely explored ${}^{137}\mathrm{I}$ nucleus, investigated here in detail.

Purpose: To establish excited states in ${}^{137}\mathrm{I}$, $\beta$ decay of the ${}^{137}\mathrm{Te}$ ground state is studied. In addition, the unknown $\beta$-delayed neutron-emission channel of ${}^{137}\mathrm{Te}$ to ${}^{136}\mathrm{I}$ is inspected. Search for levels and for candidates for Gamow-Teller and first-forbidden transitions between the mother nucleus and excited states in the daughter nucleus is conducted within the experimental observations.

Methods: $\beta$-delayed $\gamma$-ray spectroscopy is employed to study excited states in ${}^{137}\mathrm{I}$. The nucleus is populated in the decay of a mass-separated beam of ${}^{137}\mathrm{Te}$, produced in neutron-induced fission of ${}^{235}\mathrm{U}$.

Results: The new level scheme of ${}^{137}\mathrm{I}$ populated in $\beta$ decay is established. The half-life $T_{1/2}$ of ${}^{137}\mathrm{Te}$ is determined to be 2.46(5) s. The $\beta$-delayed neutron-emission probability $P_n$ value of ${}^{137}\mathrm{Te}$ is deduced as a lower limit to be 2.63(85)%.

Conclusions: The experimental results are an important input to the theoretical description of nuclei in the region, being well interpreted within large-scale shell model calculations, and provide essential information on the first-forbidden transitions beyond $N=82$ and $Z=50$.

Figure 1

The setup of this experiment.

Figure 2

Time distribution of ${}^{137}\mathrm{Te}\beta$-decay events fitted with the resulting half-life.

Figure 3

The $\beta$-gated $\gamma$-ray singles spectrum obtained following the $\beta$ decay of ${}^{137}\mathrm{Te}$ for both $Q$ settings. Peaks belonging to ${}^{137}\mathrm{I}$ are marked by their energies. The main background comes from ${}^{137}\mathrm{Xe}$ granddaughter nuclei and a ${}^{142}\mathrm{Ba}$ contamination lines for $Q=25$ coming from the decay of ${}^{142}\mathrm{Cs}$.

Figure 4

The $\beta$-gated $\gamma$-ray singles spectrum obtained following the $\beta$ decay of ${}^{137}\mathrm{Te}$, after background subtraction for $Q=21$. The leftover from the background is from ${}^{137}\mathrm{Xe}$.

Figure 5

Coincidence $\gamma$-ray spectra gated on different transitions. All marked peaks (t for tentative) are in mutual coincidence.

Figure 6

Coincidence $\gamma$-ray spectra gated on the 247 and 569 keV transitions. All marked peaks are in mutual coincidence.

Figure 7

Energy spectra corresponding to the decay of characteristic $\gamma$ lines in one-second slices. Blue, red and green are for the first, second, and third seconds after closure of the beam chopper, respectively.

Figure 8

The proposed level scheme of ${}^{137}\mathrm{I}$ obtained from $\beta$ decay of ${}^{137}\mathrm{Te}$. The new levels and transitions are in red, tentative transitions are shown with dotted lines. The $Q({\beta }^{-}$) of ${}^{137}\mathrm{Te}$ is taken from Ref. [26]. For the levels marked with (*), some assumption for uniqueness may be possible (see text, Table 2).

Figure 9

Experimental levels in ${}^{137}\mathrm{I}$ compared to the theoretical shell-model (SM) calculations. The color code indicates the main theoretical configurations and some of the possible experimental correspondences.