Decay modes of the $9/2^{-}$ isomeric state in ${}^{183}\mathrm{Tl}$

The internal transition decay and $\alpha$ decay of the $T_{1/2}=53.3\mathrm{ms}, 9/2^{-}$ isomeric state in the neutron-deficient isotope, ${}^{183}\mathrm{Tl}$, have been studied using the Resonance Ionization Laser Ion Source and the Windmill detection setup at the ISOLDE facility at CERN. Clean samples of ${}^{183m}\mathrm{Tl}$ were produced by selective laser ionization and subsequent mass separation. An internal transition cascade of 356.5–272.2 keV $\gamma$ rays has been identified. Multipolarities of these transitions have been determined by means of simultaneous $\gamma$-ray and conversion-electron spectroscopy. Improved data for the fine structure of the ${}^{183m}\mathrm{Tl}\to {}^{179}\mathrm{Au}$ decay were deduced. This involves a 6058 keV $\alpha$-decay transition, which populates a previously unknown state in ${}^{179}\mathrm{Au}$, that is tentatively assigned as a ($9/2^{-}$). It is interpreted as a coupling of the $1h_{9/2}$ proton-intruder with the excited $0^{+}$ state in the ${}^{178}\mathrm{Pt}$ core.

Figure 1

Systematics of $1/2^{+}, 3/2^{+}$, and $9/2^{-}$ states in neutron-deficient odd-mass Tl isotopes. The $3/2^{+}\to 1/2^{+}$ transitions have $M1+E2$ mixed character and their mixing ratios $\delta$ are given. The $9/2^{-}\to 3/2^{+}$ transitions have $E3$ multipolarity. Data determined in the present work are highlighted with red color, other data are taken from Evaluated Nuclear Structure Data File.

Figure 2

(a) Singles $\gamma$-ray energy spectra detected within the ‘pulse’ gate and within the ‘background’ gate (red shaded spectrum). (b) Difference of two previous spectra. (c) Background-subtracted $\gamma$-ray energy spectrum, detected in prompt coincidence with the 272.2 keV transition.

Figure 3

(a) Singles conversion-electron energy spectra detected within the ‘pulse’ and ‘background’ (red shaded spectrum) gates. (b) Background-subtracted conversion-electron energy spectrum detected in prompt coincidence with the 272.2 keV transition. (c) Background-subtracted conversion-electron energy spectrum detected in prompt coincidence with the 356.5 keV transition.

Figure 4

Time distribution of the 272.2 keV $\gamma$-ray events with respect to the proton-pulse impact. A ‘pulse’ gate was used to identify radiation that arises from the decay of ${}^{183m}\mathrm{Tl}$ and a ‘background’ gate for a subtraction of long-lived activities and room background. Vertical dashed lines indicates the range, where the fit with an exponential decay was possible, see text for details.

Figure 5

Decay scheme of ${}^{183m}\mathrm{Tl}$ deduced in the present work. Note that excitation energy of the 127(17) keV state in ${}^{179}\mathrm{Au}$ has been determined as the difference in $\alpha$-particle energies and therefore it has a large experimental uncertainty. The same applies for the 407(17) keV state. For the $\alpha$ decay branching ratio, a weighted average of the value determined here and that taken from [28] is given. The half-life for ${}^{183m}\mathrm{Tl}$ is taken from [9], since the value determined here is less accurate, see the text for details. The data for $3/2^{+}, 3/2^{-}$, and $5/2^{-}$ states in ${}^{179}\mathrm{Au}$ are taken from [19]. Values deduced or improved in the present work are highlighted with red color.

Figure 6

Singles $\alpha$-particle energy spectra detected within the ‘pulse’ and ‘background’ (red shaded spectrum) gates. $\alpha$ decays of ${}^{183m}\mathrm{Tl}$ are annotated with energies given in keV. The ${}^{241}\mathrm{Am}$ peak is from calibration standard that was placed inside of the vacuum chamber of the Windmill system.

Figure 7

Spectrum of $\gamma$ rays, detected in prompt coincidence with the 6058 keV $\alpha$ decay. Only events that occurred within the ‘pulse’ gate were accepted.

Figure 8

Partial $\alpha$-decay schemes for ${}^{183m}\mathrm{Tl}$ and ${}^{182}\mathrm{Hg}$. Hindrance factors (HF) for $\alpha$-decay transitions are given. The data for ${}^{182}\mathrm{Hg}$ are taken from [36].