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

TL;DR

This study investigates the decay properties of long-lived states in $^{186}$Tl, including the first observation of $eta$-decay from the $2^-$ state, extending the level scheme and analyzing decay mechanisms.

Contribution

It provides the first observation of $eta$-decay from the $2^-$ state in $^{186}$Tl and extends the level scheme with new decay data and state energy placements.

Findings

First $eta$-decay observation from the $2^-$ state in $^{186}$Tl.

Extended the level scheme of $^{186}$Tl with new decay pathways.

Compared retardation factors with neighboring isotopes, highlighting shell effects.

Abstract

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.