$\gamma$-ray spectroscopy of ${}^{209}\mathrm{Tl}$

States in ${}^{209}\mathrm{Tl}$ were populated using a multinucleon transfer reaction with a ${}^{136}\mathrm{Xe}$ beam impinging on a thick ${}^{208}\mathrm{Pb}$ target at $E=785$ MeV. The beam was pulsed at 825-ns intervals in order to perform isomer decay spectroscopy. The known $J^{\pi }=17/2^{+}$ isomer in ${}^{209}\mathrm{Tl}$ was located at 1228(4) keV and measured to have a half-life of $T_{1/2}=146\left(10\right)$ ns. A second isomer with $J^{\pi }=13/2^{+}$ was found to have $T_{1/2}=14\left(5\right)$ ns. The previously suggested low-energy X and Y transitions were found to have energies 57(2) and 47(2) keV respectively, while the measurement of conversion coefficients and a new decay path make the spin assignments below the isomers experimentally firm. Correlating the delayed $\gamma$ transitions with the prompt beam flash allowed the decay of states above the isomer to be found. The longer-lived isomer represents full alignment of the simplest two-particle, one-hole configuration and illuminates the remarkably weak coupling of the proton hole to the ${}^{210}\mathrm{Pb}$ core.

Figure 1

${}^{209}\mathrm{Tl}\gamma$-ray spectrum below the $T_{1/2}=146$ ns isomer. This spectrum was obtained by summing all possible gates in the delayed $\gamma -\gamma -\gamma$ matrix. Breakthrough $\gamma$ transitions from other nuclei could not be completely suppressed but have been identified. The insets shows the new decay path and the new low-energy $\gamma$ rays in the main cascade.

Figure 2

${}^{209}\mathrm{Tl}$ decay scheme below the $T_{1/2}=146$ ns isomer.

Figure 3

The lines represent the theoretical internal-conversion coefficients for different multipole transitions in ${}^{209}\mathrm{Tl}$ [26]. The dots represent the experimental internal-conversion coefficient. All four transitions measured in this work are consistent with pure multipoles.

Figure 4

The time difference between the prompt and delayed transitions with the fitted decay curve of the $17/2^{+}$ and $13/2^{+}{}^{209}\mathrm{Tl}$ isomers shown.

Figure 5

Comparison between the spectroscopic properties of the isomers in ${}^{210}\mathrm{Pb}$ and ${}^{209}\mathrm{Tl}$.

Figure 6

Prompt $\gamma$-ray transitions obtained above the $17/2^{+}{}^{209}\mathrm{Tl}$ isomer, by gating on the 324- and 662-keV delayed transition in the prompt-delayed-delayed $\gamma -\gamma -\gamma$ matrix.

Figure 7

${}^{209}\mathrm{Tl}$ Decay scheme for prompt and delayed transitions.