Mass measurements of neutron-rich indium isotopes toward the $N=82$ shell closure

Precise mass measurements of the neutron-rich ${}^{125–130}\mathrm{In}$ isotopes have been performed with the TITAN Penning trap mass spectrometer. TITAN's electron beam ion trap was used to charge breed the ions to charge state $q=13+$ thus providing the necessary resolving power to measure not only the ground states but also isomeric states at each mass number. In this paper, the properties of the ground states are investigated through a series of mass differentials, highlighting trends in the indium isotopic chain as compared to its proton-magic neighbor, tin ($Z=50$). In addition, the energies of the indium isomers are presented. The ($8^{-}$) level in ${}^{128}\mathrm{In}$ is found to be 78 keV lower than previously thought and the ($21/2^{-}$) isomer in ${}^{127}\mathrm{In}$ is shown to be lower than the literature value by more than 150 keV.

Figure 1

Layout of the TITAN experiment, showing the three ion traps used for this experiment: the RFQ cooler and buncher, the EBIT charge breeder and the measurement Penning trap. Arrows show the path of the incoming ISAC/TIS beam as well as the singly charged ion (SCI) beam and the highly charged ion (HCI) beam. See text for details.

Figure 2

Time-of-flight spectrum of the beam after extraction from the EBIT (dark gray). The red peak at 13+ indicates the ions selected, using the Bradbury Nielsen gate, for injection into MPET.

Figure 3

Time of flight resonance for ${}^{128}\mathrm{In}^{13+}$, showing the ground state and isomer. The excitation time was 125 ms. The line shows the fit to the data.

Figure 4

Two-neutron separation energies for the isotopic chains of Cd, In, Sn, and Sb. The filled circles show the values derived from this experiment, while other values are taken from the AME2016 [31].

Figure 5

The empirical pairing gap, ${\mathrm{\Delta }}_n^3$, for indium and tin isotopes in the mass region of interest. Filled circles show the pairing gaps derived from our measurements, while other values are taken from the AME2016 [31].

Figure 6

$\delta V_{pn}$ for indium and tin isotopes in the mass region of interest. Filled circles show the values derived from our measurements, while other values are taken from the AME2016 [31].

Figure 7

Energy levels of ground states and isomers measured in this experiment (solid red line) compared to the literature values (dashed black line) taken from ENSDF [32]. The size of the shaded bar represents the error on the value. The spin and parity assignments are also taken from ENSDF.