γ -ray spectroscopy of Tl209

Abstract

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 ${J}^{\ensuremath{\pi}}=17/{2}^{+}$ isomer in $^{209}\mathrm{Tl}$ was located at 1228(4) keV and measured to have a half-life of ${T}_{1/2}=146(10)$ ns. A second isomer with ${J}^{\ensuremath{\pi}}=13/{2}^{+}$ was found to have ${T}_{1/2}=14(5)$ 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 $\ensuremath{\gamma}$ 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.