Mass spectrometry and decay spectroscopy of isomers across the $Z=82$ shell closure

Recent results from a measurement campaign studying the isomerism in neutron-deficient Tl isotopes are presented. The measurements make use of a nuclear spectroscopy setup coupled to the high-resolution Penning-trap mass spectrometer ISOLTRAP at CERN's radioactive ion-beam facility ISOLDE. The mass values of ${}^{190,194}$Tl are improved and a mass-spin-state assignment is carried out. An additional mass measurement of the grandparent nuclide ${}^{198}$At allows the deduction of the spin-state ordering in ${}^{190}$Tl. As a result, the excitation energies of the isomers in both Tl isotopes are determined for the first time to $E_{\mathrm{ex}}{(}^{194}\text{Tl})=260\left(15\right)$ keV and $E_{\mathrm{ex}}{(}^{190}\text{Tl})=89\left(12\right)$ keV. Furthermore, this allows anchoring of the ground-state and isomer masses of ${}^{194}$Bi, ${}^{202}$Fr, and ${}^{206}$Ac, which are linked by two independent $\alpha$-decay chains.

Figure 1

Scheme of the ISOLTRAP setup [6, 7], with (1) the RFQ cooler and buncher, (2) the MR-TOF MS with Bradbury-Nielsen gate, (3) the preparation Penning trap, (4) the precision Penning trap, and (5) the decay-station extension. Inset: Schematic top view of the decay station.

Figure 2

Resonances obtained for ${}^{194}$Tl with the TOF-ICR method and an excitation time of 3 s in the precision trap. The solid lines are fits of the theoretical line shape to the data. The resonance corresponds to (a) the isomer (protons on) and (b) the ground state (protons off). For details see text.

Figure 3

A $\beta$-triggered $\gamma$-ray energy spectrum for ${}^{194}$Tl between an HPGe detector and the scintillator at the implantation point after 1.5 h of implantation. The peak at 820 keV does not fit any known transition from the decay of ${}^{194}$Tl.

Figure 4

A $\beta$-triggered $\gamma$-ray energy spectrum for ${}^{190}$Tl for one HPGe detector at the implantation point after $2\times 15$ min of implantation.

Figure 5

Independent $\alpha$-decay chains for the two states in ${}^{190}$Tl. The state ordering in ${}^{190}$Tl and all mother nuclei as well as their masses can be extracted from the direct mass measurements of ISOLTRAP [thick horizontal (red) lines] and the known $\alpha$ energies [dashed ((blue) lines [26, 27, 34, 35]. Energies are given in keV.

Figure 6

Hyperfine-structure scan for ${}^{198}$At resulting from a dedicated experiment [37] with the use of the Windmill $\alpha$-decay setup [38]. For individual states the counts at a fixed $\alpha$ energy were measured while varying the wave number.

Figure 7

Magnetic moments for even-$A$ neutron-deficient Tl isotopes [1, 2, 3, 42, 43, 44, 45]. Shaded horizontal bands indicate empirical moments calculated using the additivity relation from [46].

Figure 8

Systematics of the excitation energy of the isomeric states in even-$A$ neutron-deficient Tl isotopes (black diamonds) compared to energy-level spacings in neutron-deficient Pb isotopes (magenta and blue triangles). Data are taken from [28], [48], [49], and this work (green diamonds). Note that the excitation energies of the isomers in Tl isotopes with $N<109$ (dotted line) are extrapolated values. The (red) circles represent the energy difference between the Fermi level and the neutron $i_{13/2}$ level as obtained from a spherical HFB calculation with the SKM* interaction using the hfodd code [50, 51].