$\beta$ decay and $\beta$-delayed neutron decay of the $N=82$ nucleus ${}_{49}{}^{131}\mathrm{In}_{82}$

The half-lives of three $\beta$-decaying states of ${}_{49}{}^{131}\mathrm{In}_{82}$ have been measured with the GRIFFIN $\gamma$-ray spectrometer at the TRIUMF-ISAC facility to be $T_{1/2}(1/2^{-})=328\left(15\right)\mathrm{ms}$, $T_{1/2}(9/2^{+})=265\left(8\right)\mathrm{ms}$, and $T_{1/2}(21/2^{+})=323\left(55\right)\mathrm{ms}$, respectively. The first observation of $\gamma$ rays following the $\beta n$ decay of ${}^{131}\mathrm{In}$ into ${}^{130}\mathrm{Sn}$ is reported. The $\beta$-delayed neutron emission probability is determined to be $P_{1n}=12\left(7\right)\%$ for the $21/2^{+}$ state and $2.3\left(3\right)\%$ from the combined $1/2^{-}$ and $9/2^{+}$ states of ${}_{49}{}^{131}\mathrm{In}_{82}$ observed in this experiment. A significant expansion of the decay scheme of ${}^{131}\mathrm{In}$, including 17 new excited states and 35 new $\gamma$-ray transitions in ${}_{50}{}^{131}\mathrm{Sn}_{81}$ is also reported. This leads to large changes in the deduced $\beta$-branching ratios to some of the low-lying states of ${}^{131}\mathrm{Sn}$ compared to previous work with implications for the strength of the first-forbidden $\beta$ transitions in the vicinity of doubly magic ${}_{50}{}^{132}\mathrm{Sn}_{82}$.

Figure 1

$\beta$-coincident $\gamma \text{−}\gamma$ spectra gated on (a) the 391-keV and (b) the 1221-keV $\gamma$ rays. These spectra show the presence of excited states of ${}^{130}\mathrm{Sn}$ following the $\beta n$ decay of ${}^{131}\mathrm{In}$.

Figure 2

The decay scheme of ${}^{130}\mathrm{Sn}$ observed following the $\beta n$ decay of ${}^{131}\mathrm{In}$. The half-lives of the $\beta$-decaying $\left(7^{-}\right)$ and $0^{+}$ states are from Refs. [40, 41] and were used in determining the total number of $\beta n$ decays. The half-life of the $\left({10}^{+}\right)$ isomeric state was measured in this work. See text for details.

Figure 3

Half-life measurement for the 2435-keV, $I^{\pi }=\left({10}^{+}\right)$ isomer in ${}^{130}\mathrm{Sn}$ obtained from the $\beta \text{−}\gamma$ time difference gated on the 391-keV photopeak in ${}^{130}\mathrm{Sn}$. The half-life was measured from these data to be $1.52\left(9\right)\mu \mathrm{s}$, in good agreement with the previous measurement of $1.61\left(15\right)\mu \mathrm{s}$ [42]. This state was populated following the $\beta n$ decay of ${}^{131}\mathrm{In}$. See text for details.

Figure 4

A partial decay scheme of ${}^{130}\mathrm{Sb}$ following the $\beta$ decay of the $0^{+}$ and $\left(7^{-}\right)$ states of ${}^{130}\mathrm{Sn}$. The intensities of these $\gamma$ rays were used to determine the total number of $\beta$ decays of ${}^{130}\mathrm{Sn}$ observed in this work, which is related to the total number of $\beta n$ decays of ${}^{131}\mathrm{In}$.

Figure 5

The decay scheme of ${}^{131}\mathrm{Sn}$ observed following the $\beta$ decay of the $21/2^{+}$ isomeric state in ${}^{131}\mathrm{In}$. The widths of the arrows represents the relative intensity of each transition. The low-lying isomeric state is denoted with the energy $X$ although evidence is presented in the current work that this state has an excitation energy of $X=65.1\left(5\right)\mathrm{keV}$. Previously unobserved transitions and excited states are highlighted in red, while levels and $\gamma$-ray transitions which were previously identified in fission decays [11] are highlighted in blue.

Figure 6

The $\beta \text{−}\gamma \text{−}\gamma$ coincidence spectrum gated on the 4273-keV $\gamma$-ray transition following the $\beta$ decay of the $21/2^{+}$ isomer in ${}^{131}\mathrm{In}$. The most intense transitions in the spectrum from ${}^{131}\mathrm{Sn}$ are labeled with their measured energies.

Figure 7

The $\beta \text{−}\gamma$ add-back spectrum showing the candidate 2369-keV $M2$ transition in ${}^{131}\mathrm{Sn}$. This $\gamma$ ray is proposed to decay from the 2434 keV $\nu g_{7/2}^{-1}$ state to the 65 keV $\nu h_{11/2}^{-1}$ state. A future confirmation of the placement of this transition via $\gamma \text{−}\gamma$ coincidences would result in the firm placement of the $\nu h_{11/2}$ single-particle state at 65.1(5) keV.

Figure 8

Fit of the $\beta \text{−}\gamma$ time difference spectrum gated on the 158-keV $\gamma$ ray. The half-life of the $4606+X\mathrm{keV}$ high-spin isomer measured from this fit is 316(5) ns. The fit included a component for the $1.5\text{−}\mu \mathrm{s}$ isomer in ${}^{130}\mathrm{Sn}$ resulting from Compton-scattered $\gamma$ rays as well as a constant background.

Figure 9

The difference between the time that any $\gamma$ ray was detected relative to the (a) 270-keV, (b) 158-keV, and (c) 285-keV $\gamma$ rays. This method was used in conjunction with the $\gamma$-ray energy differences and coincidence relationships to assign the locations in the level scheme of the $\gamma$ transitions and excited states of ${}^{131}\mathrm{Sn}$ populated following the decay of the $21/2^{+}$ isomer of ${}^{131}\mathrm{In}$. See text for details.

Figure 10

Measurement of the half-life of the $21/2^{+}$ isomer in ${}^{131}\mathrm{In}$. The activity curve from the “beam-off” portion of the cycle was generated by placing a gate on the 171-, 173-, and 285-keV $\gamma$ rays, which are emitted following the $\beta$ decay of the $21/2^{+}$ state. The half-life from this fit was measured to be 323(55) ms.

Figure 11

Fit of the SCEPTAR $\beta$ detector activity summed over all 182 cycles. The intensity of the high-spin ${}^{131}\mathrm{In}$ decay was fixed to reproduce the total measured $\beta$ decay intensity into the high-spin states of ${}^{131}\mathrm{Sn}$ as described in Sec. 3b.

Figure 12

The decay scheme of ${}^{131}\mathrm{Sn}$ observed following the $\beta$ decay of the $9/2^{+}$ isomeric state in ${}^{131}\mathrm{In}$. The widths of the arrows represents the relative intensities of each of the transitions. Previously unobserved transitions and excited states are highlighted in red. The location of the 2369-keV $\gamma$-ray transition could not be confirmed.

Figure 13

Fit of the half-life of the $9/2^{+}$ ground state of ${}^{131}\mathrm{In}$. The activity curve shown was generated by placing a gate on the 2434-keV $\gamma$ ray, which is emitted primarily following the $\beta$ decay of the $9/2^{+}$ state. A narrow gate was chosen in order to exclude the contribution from the 2429-keV $\gamma$-ray transition from the $21/2^{+}$ isomeric decay. The half-life from this fit was measured to be 265(8) ms.

Figure 14

Decay scheme of ${}^{131}\mathrm{Sn}$ observed following the $\beta$ decay of the $1/2^{-}$ state in ${}^{131}\mathrm{In}$. The widths of the arrows represents the relative intensities of each of the transitions. Previously unobserved transitions and excited states are highlighted in red.

Figure 15

Fit of the half-life of the $1/2^{-}$ isomer in ${}^{131}\mathrm{In}$. The activity curve shown was generated by placing a gate on the 332-keV $\gamma$ ray which is emitted primarily following the $\beta$ decay of the $1/2^{-}$ state. The feeding from the $\beta$ decays of the other ${}^{131}\mathrm{In}$ states was negligible. The 332-keV $\gamma$ decay in ${}^{131}\mathrm{Sb}$ following the $\beta$ decay of the ${}^{131}\mathrm{Sn}$ daughter was also accounted for. The half-life from this fit was measured to be 328(15) ms.