Mass measurements of neutron-rich indium isotopes for $r$-process studies

A new series of neutron-rich indium mass measurements is reported from the TITAN multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS). These mass measurements cover ${}^{125-134}\mathrm{In}$ ($N=76–85$) and include ground states as well as isomeric states. The masses of nuclei in this region are known to be of great importance for accurately modeling $r$-process nucleosynthesis, and the significance of the reported neutron-rich indium masses is discussed in this context. Results are compared with earlier experimental data where available as well as theoretical mass models. The measurements reported here include the first mass measurements of ${}^{133,134}\mathrm{In}$, as well as the first direct mass measurement of ${}^{132}\mathrm{In}$. The masses of ${}^{125-131}\mathrm{In}$ ground states and several isomers were previously measured to higher precision by Penning trap mass spectrometry, which also resolved some low-lying isomers that could not be resolved in this work. The earlier Penning trap measurements serve as excellent cross-checks for the MR-TOF-MS measurements, and in some cases the MR-TOF-MS measurements improve the literature uncertainties of higher-lying isomer masses and excitation energies. A new isomeric state for ${}^{128}\mathrm{In}$, recently reported for the first time by the JYFLTRAP group, is also confirmed by the TITAN MR-TOF-MS, with a measured excitation energy of 1813(17) keV.

Figure 1

Schematic overview of the TITAN experimental devices used for this work.

Figure 2

Mass spectrum for $A=128$ with lasers on (tuned on indium, solid line) and one laser transition blocked (dashed line). When the laser is blocked, the indium peaks disappear, providing peak identification verification.

Figure 3

Isomer excitation energies measured in this work, plotted in comparison to those reported in ENSDF [37] and the recent Penning trap measurements from TITAN [8] and JYFLTRAP [9]. Bands indicate the uncertainty. Note that isomeric states not measured in this work, including those with $t_{1/2}<$ 1 ms and those that could not be fit separately from the ground state, are not included here. The utility of the MR-TOF-MS as a tool for measuring isomer excitation energies is clear, improving literature uncertainties in several cases and observing a number of isomers not measured previously by the TITAN Penning trap due to the the MR-TOF-MS's sensitivity, background handling abilities, and nonscanning measurement technique.

Figure 4

Systematics of the ${1/2}^{-}$ isomer excitation energy for even-$N$ indium isotopes, all of which have ${9/2}^{+}$ ground states. Data are taken from ENSDF [37] and recent precision mass measurements [8, 43, 50, 51].

Figure 5

Comparison of neutron-rich indium TITAN MR-TOF-MS mass measurements from this work and the previous TITAN Penning trap measurements (Babcock2018) [8] to predictions from theoretical mass models [53, 54, 55, 56, 57]. All masses are plotted relative to the Ame2016 values [3], which include extrapolations for $N\ge 84$. The solid vertical line indicates the major shell closure at $N=82$.