Decay studies of the long-lived states in $^{186}$Tl

Decay spectroscopy of the long-lived states in $^{186}$Tl has been performed at the ISOLDE Decay Station at ISOLDE, CERN. The $\alpha$ decay from the low-spin ($2^−$) state in $^{186}$Tl was observed for the first time and a half-life of 3.4$^{+0.5}_{−0.4}$ s was determined. Based on the $\alpha$-decay energy, the relative positions of the long-lived states were fixed, with the ($2^−$) state as the ground state, the 7$^{(+)}$ state at 77(56) keV, and the 10$^{(−)}$ state at 451(56) keV. The level scheme of the internal decay of the $^{186}$Tl(10$^{(−)}$) state [$T_{1/2}$=3.40(9) s], which was known to decay solely through emission of 374-keV $\gamma$-ray transition, was extended and a lower limit for the $\beta$-decay branching $b_\beta$ >5.9(3)% was determined. The extracted retardation factors for the $\gamma$ decay of the 10$^{(−)}$ state were compared to the available data in neighboring odd-odd thallium isotopes indicating the importance of the $\pi d_{3/2}$ shell in the isomeric decay and significant structure differences between $^{184}$Tl and $^{186}$Tl.