New yrast and non-yrast states in ${}^{136}\mathrm{I}$ and medium-spin structure of ${}^{136}\mathrm{I}$ and ${}^{136}\mathrm{Xe}$: Roles of the proton $d_{5/2}$ and the neutron $h_{9/2}$ orbitals beyond ${}^{132}\mathrm{Sn}$

In this article we report on the observation of new yrast and non-yrast states in ${}^{136}\mathrm{I}$, populated in the prompt-$\gamma$ spectroscopy EXILL campaign at the ILL using both ${}^{235}\mathrm{U}$(n,f) and ${}^{241}\mathrm{Pu}$(n,f) reactions. We propose an extension of the level scheme and interpretation of the new spectroscopic results in comparison to state-of-the-art shell-model calculations. We discuss the role of the proton $d_{5/2}$ orbital in the structure of this nucleus with $Z=53$ and the energy split between the $\pi d_{5/2}\text{and}\pi g_{7/2}$ orbitals. We also provide a complete overview of all experimental information about this nucleus and include new data from ${}^{252}\mathrm{Cf},{}^{248}\mathrm{Cm}$ fission and $\beta$ decay of ${}^{136}\mathrm{Te}$ to ${}^{136}\mathrm{I}$. We give a new interpretation of ${}^{136}\mathrm{I}$ and its decay to ${}^{136}\mathrm{Xe}$ in terms of the influence of the neutron $h_{9/2}$ orbital and the Gamow-Teller strength for $N=83$.

Figure 1

Coincidence spectra for ${}^{136}\mathrm{I}$ from neutron-induced fission of ${}^{241}\mathrm{Pu}$ (red) and ${}^{235}\mathrm{U}$ (blue) with gates on 87–135 keV transitions with a zoom in the large inset and on 87–1784 keV in the small inset, as measured at EXILL. The new transitions are marked by asterisk (*) symbol.

Figure 2

Coincidence spectra for ${}^{136}\mathrm{I}$ from neutron-induced fission of ${}^{241}\mathrm{Pu}$ (red) and ${}^{235}\mathrm{U}$ (blue) with gates on (a) 87–1212 keV, (b) 87–2078 keV, and (c) 87–1305 keV transitions, as measured at EXILL. The new transitions are marked by asterisk (*).

Figure 3

Experimental level scheme of ${}^{136}\mathrm{I}$ populated in fission of actinides (yrast) and in $\beta$ decay of fission products (non-yrast). All new transitions obtained in this work are marked with asterisk (*). The new ones from neutron-induced fission on ${}^{235}\mathrm{U}$ and ${}^{241}\mathrm{Pu}$ are in red, the new ones from spontaneous fission of ${}^{252}\mathrm{Cf}$ fission are in dark blue. The dashed lines represent tentative transitions (levels).

Figure 4

Experimental spectra for ${}^{136}\mathrm{I}$ from neutron-induced fission of ${}^{241}\mathrm{Pu}$ and ${}^{235}\mathrm{U}$ with gates on (a) 42–1111 keV transitions, (b) 261–1111 keV, and (c) 261–1644 keV transitions, as measured at EXILL. The unidentified peaks belong to the complementary channels/other background. In the insets gates on (b) 261–243 keV and (c) 1111–1463 keV transitions are shown in addition.

Figure 5

$\gamma$-ray spectrum doubly gated on 1111–42 keV lines of $A=136$, following the spontaneous fission of ${}^{252}\mathrm{Cf}$, as measured at Gammasphere. Strongly nonlinear energy calibration is applied.

Figure 6

$\gamma$-ray spectrum doubly gated on 1111–61 keV lines of ${}^{136}\mathrm{I}$, following the spontaneous fission of ${}^{252}\mathrm{Cf}$, as measured at Gammasphere. Energy calibration as in the 1111–42 keV gated spectrum.

Figure 7

$\gamma$-ray spectrum doubly gated on the 1058-keV line (1st gate) and the sum of the 261- to 1111-keV lines (2nd gate) of ${}^{136}\mathrm{I}$, following the spontaneous fission of ${}^{252}\mathrm{Cf}$, as measured at Gammasphere. Energy calibration as in the 1111–42 keV gated spectrum.

Figure 8

$\gamma$ singles spectrum for $A=136$ nuclei with low (a) and high-energy (b) regions, following ${\beta }^{-}$ decay of ${}^{136}\mathrm{Te}$, as measured at Lohengrin. A strongly nonlinear energy calibration is applied.

Figure 9

$\gamma$-ray spectrum gated on the 578-keV line of ${}^{136}\mathrm{I}$, following ${\beta }^{-}$ decay of ${}^{136}\mathrm{Te}$, as measured at Lohengrin. Energy calibration as in the singles spectrum for $A=136$.

Figure 10

Level scheme of ${}^{136}\mathrm{I}$ populated in ${\beta }^{-}$ decay of ${}^{136}\mathrm{Te}$, as obtained in the present work. The new transitions are in red, the newly added by Aas et al. [25]—in dark blue, and the observed by Schussler et al.—in black [17]. The dashed lines represent tentative transitions (levels).

Figure 11

Angular correlations analysis for the 1321–1313 keV cascade in ${}^{136}\mathrm{Xe}$, populated in ${\beta }^{-}$ decay of ${}^{136}\mathrm{I}$, for (a) $I=2$ and (b) $I=1$ spin for the 2634-keV level.

Figure 12

Schematic drawing of the proposed decay scenario in $A=136$ isobars, involving the $\nu h_{9/2}\to \pi h_{11/2}$ GT decay.

Figure 13

Comparison of calculated and experimental levels in ${}^{136}\mathrm{I}$. Calculations are normalized to experiment at the g.s. level. Dominating configurations are given. See text for more details.

Figure 14

Comparison of experimental and calculated multiplets in ${}^{136}\mathrm{I}$, The “$-$” symbol in the Table signifies contribution below $1\%$.

Figure 15

Comparison of calculated and experimental levels in ${}^{136}\mathrm{Xe}$. Calculations are normalized to the experiment at the g.s. level.

Figure 16

Calculated Gamow-Teller strength for transitions from ground states of ${}^{136}\mathrm{Te}$ and ${}^{136}\mathrm{I}$ to $1^{+}$ and $2^{-}$ levels in ${}^{136}\mathrm{I}$ (a) and ${}^{136}\mathrm{Xe}$ (b), respectively.