In-source Laser Spectroscopy of Neutron-rich Thallium Isotopes in the vicinity of N = 126

The properties of thallium isotopes beyond the neutron shell closure at N=
126 remained unknown. Experimental data is urgently needed to test different
nuclear models around the doubly-magic nucleus, 208Pb. This work focuses on
studies of shell effects in ground-state properties around N = 126, by in-source
resonance-ionisation laser spectroscopy measurements of 207−209Tl at a radioactive
isotope production facility, ISOLDE, CERN.

The application of the Laser Ion Source and Trap structure at ISOLDE heavily
suppressed the dominant isobaric contamination, francium. Due to this, the first-
ever laser spectroscopy measurements of the 207mTl and 209gTl were performed at
the ISOLDE Decay Station. The changes in the mean-square charge radii and the
magnetic dipole moments are deduced from the measured hyperfine structures.
The characteristic kink phenomenon in the charge radii, as well as the odd-
even staggering, have been confirmed for the thallium isotopes when crossing
the N = 126 shell closure. A sudden jump of the magnetic dipole moment at
207gTl has been observed along the isotopic chain. The experimental results are
compared to state-of-the-art calculations in the framework of the relativistic mean
field approach, and from the self-consistent theory of finite Fermi systems based
on the energy density functional by Fayans. In the case of the Fayans functional,
the pairing interaction with the dependence on the density gradient is the key to
providing agreement with the experimental results. In addition, the inclusion of
the phonon coupling appears to be crucial for reproducing the values of magnetic
dipole moment of thallium isotopes.