Lifetimes in ${}^{211}\mathrm{At}$ and their implications for the nuclear structure above ${}^{208}\mathrm{Pb}$

Lifetimes of excited states in ${}^{211}\mathrm{At}$ were measured using the electronic $\gamma -\gamma$ fast timing technique. The nucleus of interest was populated in a ${}^{208}\mathrm{Pb}({}^6\mathrm{Li},3n){}^{211}\mathrm{At}$ fusion-evaporation reaction at the FN Tandem accelerator of the Institute for Nuclear Physics, University of Cologne. The lifetimes of the $17/2_1^{-}$ and $23/2_1^{-}$ states were determined, together with an upper limit for the $13/2_1^{-}$ state. The experimental results are compared to two shell-model calculations, one using a semiempirical interaction for three particles in a single $j=9/2$ shell and the other using the modified Kuo-Herling interaction in a multi-$j$ model space.

Figure 1

The PRD curve of the set-up measured with a ${}^{152}\mathrm{Eu}$ source.

Figure 2

Right: Partial level scheme of ${}^{211}\mathrm{At}$ relevant for the analysis adopted from Ref. [6]. Left: Full projections of HPGe-LaBr-LaBr (blue) and HPGe-HPGe-LaBr (red) coincidence spectra.

Figure 3

The figure indicates the basic steps for extracting the lifetime of the $13/2^{-}$ state. (a) Top: LaBr (blue) and HPGe (red) detector projection in HPGe-LaBr gates, where the gate in the LaBr spectrum is set on the feeder. The vertical lines indicate the gate used to produce the time difference spectra. The corresponding peak-to-background ratio is 11.5. (a) Bottom: The fitted time response of the background (dashed line), together with the PRD curve and the obtained centroid difference from (b). (b) Time-difference spectra for the 254- and 1067-keV feeder-decay combination. (c) Same as (a) but the gate in the LaBr spectrum is set on the decay transition and the analysis of the background is done around the feeder. The corresponding peak-to-background ratio is 10.3.

Figure 4

Analogous to Fig. 3 but for the $23/2^{-}$ state. The corresponding peak-to-background ratios are 4.7 and 2.1.

Figure 5

(a) LaBr (blue) and HPGe (red) detector projection in HPGe-LaBr gates. (b) Time-difference spectrum (see text for details). The line indicates the fit to the data used to extract the lifetime of the $17/2^{-}$ state.