Experimental study of ${\frac{1}{2}}^{+}$ isomers in ${}^{199,201}\mathrm{At}$

Using fusion-evaporation reactions, a gas-filled recoil separator, and recoil-electron and recoil-electron-$\alpha$ tagging techniques, a new isomeric ${\frac{1}{2}}^{+}$ state [$T_{1/2}=45\left(3\right)\mathrm{ms}$] in ${}^{201}\mathrm{At}$ is identified, and an earlier reported corresponding state [$T_{1/2}=273\left(9\right)\mathrm{ms}$] in ${}^{199}\mathrm{At}$ is confirmed. The ${\frac{1}{2}}^{+}$ state is suggested to originate from an intruder $\pi {\left(s_{1/2}\right)}^{-1}$ configuration. In addition, nuclear structure of states below and above this ${\frac{1}{2}}^{+}$ state are studied in both nuclei. The isomer decays through a cascade of an $E3$ transition followed by a mixed $M1/E2$ transition to the ${\frac{9}{2}}^{-}$ ground state, and it is interpreted to be fed from nearly spherical ${\frac{3}{2}}^{+}$ and ${\frac{5}{2}}^{+}$ states originating from $\pi {\left(d_{3/2}\right)}^{-1}$ and $\pi {\left(d_{5/2}\right)}^{-1}$ configurations, respectively.

Figure 1

Raw $\alpha$-particle energy spectra in DSSD obtained with the reactions used in these studies.

Figure 2

(a) Recoil-electron tagged $\gamma$-ray spectrum observed in the planar detector. Conversion-electron spectra: (b) Electron in coincidence with the 269-keV $\gamma$ ray observed in the planar germanium detector. (c) Same as panel (b), but with the electron in coincidence with the 190-keV $\gamma$ ray. In panels (b) and (c), background subtraction has been introduced.

Figure 3

Prompt $\gamma$ rays detected in the JUROGAM2 array. (a) Recoil tagged prompt $\gamma$ rays. (b) R-$e{}^{-}$ tagged prompt $\gamma$ rays, where the electron is in fast coincidence with the 190- or 269-keV delayed $\gamma$ ray detected in the focal plane Clover array. (c) R-$e{}^{-}$ tagged prompt $\gamma$ rays in fast coincidence with the 173-keV prompt $\gamma$ ray. In panels (b) and (c), background subtraction has been introduced.

Figure 4

Level scheme of the ${\frac{1}{2}}^{+}$ structure in ${}^{201}\mathrm{At}$. The intensities of transitions below the ${\frac{1}{2}}^{+}$ state and the lower 173-keV transition are not to scale. For the inset, see Sec. 4.

Figure 5

Logarithm of the time difference between the recoil implantation and the observation of the following electron in the same pixel of the DSSD. (a) All observed recoil-electron chains. (b) Electrons in coincidence with the 190- or 269-keV $\gamma$ ray observed at the focal plane. (c) Same as panel (b), but the recoil is correlated with the 173- or 433-keV prompt $\gamma$ ray observed in the JUROGAM2 array. The half-life of the ${\frac{1}{2}}^{+}$ state of $45\left(3\right)\mathrm{ms}$ was obtained using the logarithmic time-scale method [24].

Figure 6

(a) Conversion-electron energy spectrum related to R-$e{}^{-}\text{−}{}^{199}\mathrm{At}$ $\alpha$ chains. The lowest energy electron peak corresponds to the $L+M+\cdots$-converted transition of the ${\frac{1}{2}}^{+}$ isomer. The peak at $\sim 160$ keV corresponds to the sum of the previous transition and the $K$-conversion of the subsequent 141-keV transition. (b) $\gamma$ rays in coincidence with the 103-keV conversion electron observed using the planar detector.

Figure 7

R-$e{}^{-}\text{−}{}^{199}\mathrm{At}$ $\alpha$ tagged energy spectrum of prompt $\gamma$ rays detected in the JUROGAM2 array. (a) Electron energy of $163\mathrm{keV}$ demanded. (b) $\gamma$ rays in coincidence with the 156-keV prompt $\gamma$ ray with background reduction. Transitions in brackets most likely belong to ${}^{199}\mathrm{At}$ Band 1 [27] or ${}^{199}\mathrm{Po}$ [25].

Figure 8

Level scheme of the ${\frac{1}{2}}^{+}$ structure in ${}^{199}\mathrm{At}$. The intensity of the 156-keV transition and the transitions below the ${\frac{1}{2}}^{+}$ state are not to scale. For the inset, see Sec. 4.

Figure 9

Logarithm of the time difference between recoil implantation and the observation of the subsequent electron in the same pixel of the DSSD. In addition to the R-$e{}^{-}\text{−}{}^{199}\mathrm{At}$ $\alpha$ chain, the 156- or 410-keV prompt $\gamma$ ray must be observed in the JUROGAM2 array. In this analysis, the 1-s recoil-electron time gate was omitted. The logarithmic time-scale method [24] yields a half-life of $273\left(9\right)\mathrm{ms}$.

Figure 10

Energy systematics of the ${\frac{1}{2}}^{+}$ isomer originating from the $\pi {\left(s_{1/2}\right)}^{-1}$ configuration in nuclei close to magic number $Z=82$. For comparison, also the behavior of ${\frac{9}{2}}^{-}$ states in Tl isotopes originating from the $\pi \left(h_{9/2}\right)$ structure is shown. The inset shows the level structure feeding the ${\frac{1}{2}}^{+}$ state with respect to the ${\frac{1}{2}}^{+}$ level energy. Neutron numbers for nuclei studied in this experiment are 114 and 116. Data for Bi isotopes are taken from Refs. [1, 4, 25, 29, 30, 31], for At from this work and Refs. [3, 4, 28, 32], for Fr from Refs. [5, 6, 7], and for Tl isotopes from Refs. [29, 30, 33, 34, 35, 36].