$\beta$-delayed neutron spectroscopy of ${}^{133}\mathrm{In}$

The decay properties of ${}^{133}\mathrm{In}$ were studied in detail at the ISOLDE Decay Station. The implementation of the Resonance Ionization Laser Ion Source allowed separate measurements of its $9/2^{+}$ ground state (${}^{133g}\mathrm{In}$) and $1/2^{-}$ isomer (${}^{133m}\mathrm{In}$). With the use of $\beta$-delayed neutron and $\gamma$ spectroscopy, the decay strengths above the neutron separation energy were quantified in this neutron-rich nucleus for the first time. The allowed Gamow-Teller transition $9/2^{+}\to 7/2^{+}$ was located at 5.93 MeV in the ${}^{133g}\mathrm{In}$ decay with a $\log ft=4.7\left(1\right)$. In addition, several neutron-unbound states were populated at lower excitation energies by the first-forbidden decays of ${}^{133g,m}\mathrm{In}$. We assigned spins and parities to those neutron-unbound states based on the $\beta$-decay selection rules, the $\log ft$ values, and systematics.

Figure 1

Schematic drawing of experimental setup at IDS. The radioactive beam from ISOLDE was implanted on the tape at the center of the setup. The $\beta$-delayed $\gamma$ rays from ${}^{133}\mathrm{In}$ were detected by four HPGe clover detectors at backward angles relative to the beam direction. The INDiE array was placed on the other side to measure neutron spectroscopy following the ${}^{133}\mathrm{In}$ decay.

Figure 2

The neutron data taken in coincidence with the ${}^{133}\mathrm{In}\beta$ decay, with figures on the left corresponding to the pure ground-state decay and those on the right to an admixture of ground-state (40%) and isomeric decays (60%). Panels (a) and (e) plot the neutron TOF against their energy loss in INDiE, with the projections along TOF in panels (b) and (f), respectively. Panel (c) shows the ground-state neutron spectrum in coincidence with the 4041-keV $\gamma$ decay in ${}^{132}\mathrm{Sn}$. Panels (d) and (g) focus on the TOF of high-energy neutrons with $E_n>3$ MeV. They were made by projecting panels (a) and (e), respectively, along TOF with energy loss greater than $1{\mathrm{MeV}}_{ee}$. All the neutron TOF spectra were fit by the neutron response functions (magenta), with the neutron peaks attributed to the ground-state and isomeric decays drawn in blue and red, respectively. The dashed line is the $\beta$- and $\gamma$-ray background in the neutron TOF spectra.

Figure 3

Constructed decay schemes of ${}^{133g}\mathrm{In}$ ($9/2^{+}$) and ${}^{133m}\mathrm{In}$ ($1/2^{-}$) between $S_{\mathrm{n}}=2.399$ MeV [28] and $E_{ex}=6.0$ MeV, where isolated resonances were observed in ${}^{133}\mathrm{Sn}$. Decays associated with the ground state and isomer are differentiated in blue and red, respectively. The states below the neutron separation energy are included for completeness [13, 14]. Half-lives and neutron-emission probabilities ($P_n$) in ${}^{133}\mathrm{In}$ are taken from Refs. [14, 15]. The excitation energy of the $1/2^{-}$ isomer is from Ref. [29]. The states with a solid spin-parity assignment are highlighted by “$*$” on their excitation energies.

Figure 4

Portion of $\gamma$-ray spectra measured in coincidence with the ground-state decay of ${}^{133}\mathrm{In}$. The candidate $\gamma$ deexcitations from the ${}^{133}\mathrm{Sn}$ neutron-unbound states are marked by “$•$,” to be distinguished from the $\gamma$ decays in ${}^{132}\mathrm{Sn}$. SE stands for “single-escape” peak. See text for details.

Figure 5

The experimental $\beta$-decay strength distribution (in $S_{\beta }=1/ft$ [31]) of the ${}^{133}\mathrm{In}$ ground state (blue) and isomer (red) between $E_{ex}=2.5$ and 11 MeV.

Figure 6

Reduced level schemes of ${}^{131}\mathrm{Sn}$ and ${}^{133}\mathrm{Sn}$. Each group of states is labeled by the dominant neutron p-h configuration in the wave functions. Information in ${}^{131}\mathrm{Sn}$ is taken from Refs. [32, 33]. In ${}^{133}\mathrm{Sn}$, the spins, parities, and excitation energies of the $\nu 1\mathrm{p}$-0h states are taken from Refs. [13, 14], while those of $\nu 2\mathrm{p}$-1h are from this work. The dashed lines connect groups of states with the same odd-even combination of neutron p-h configuration.