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.
Categories: nucl-ex