Prompt and delayed spectroscopy of ${}^{199}\mathrm{At}$

The neutron-deficient nucleus ${}^{199}\mathrm{At}$ has been studied through $\gamma$-ray and electron spectroscopy, using the recoil-decay tagging technique. Two experiments were conducted, using a gas-filled recoil separator with a focal-plane spectrometer alone and together with a germanium-detector array at the target position. The resulting level scheme for ${}^{199}\mathrm{At}$ includes a new isomer with a half-life of 0.80(5) $\mu$s and a spin and parity of ($29/2^{+}$). The $13/2^{+}$ isomer, which de-excites via an $M2$ transition to the $9/2^{-}$ ground state, was measured to have a half-life of 70(20) ns. Our earlier version of the level scheme for ${}^{197}\mathrm{At}$ has been updated as well.

Figure 1

The $\alpha$-particle energy spectrum obtained using the ${}^{82}\mathrm{Kr}+{}^{120}\mathrm{Sn}$ reaction. A maximum correlation time of 21 s was set between the implantation of the recoil and its subsequent $\alpha$ decay.

Figure 2

(a) Spectrum of singles prompt $\gamma$ rays tagged with the $\alpha$ decay of ${}^{199}\mathrm{At}$, collected from the ${}^{120}\mathrm{Sn}$(${}^{82}\mathrm{Kr},p2n$)${}^{199}\mathrm{At}$ reaction. (b) Spectrum of $\gamma$ rays in coincidence with the 600-keV doublet, obtained from the ${}^{150}\mathrm{Sm}$(${}^{52}\mathrm{Cr},p2n$)${}^{199}\mathrm{At}$ and ${}^{120}\mathrm{Sn}$(${}^{82}\mathrm{Kr},p2n$)${}^{199}\mathrm{At}$ reactions. The dashed lines are inserted to guide the eye for some relevant coincidence-anticoincidence relations.

Figure 3

Delayed $\gamma$ rays tagged with the ${}^{199}\mathrm{At}$ $\alpha$ decay, detected in the clover detectors. (a) Spectrum of delayed singles $\gamma$ rays obtained with lower beam intensity. (b) Spectrum of $\gamma$ rays in coincidence with the 163-keV transition detected in the planar detector. The 484-keV peak, denoted by a diamond, belongs to ${}^{200}\mathrm{Po}$. The data are obtained from the ${}^{165}\mathrm{Ho}$(${}^{40}\mathrm{Ar},6n$)${}^{199}\mathrm{At}$ reaction; a maximum correlation time of 5 $\mu$s was set between the recoil and its isomeric decay.

Figure 4

The proposed level scheme of ${}^{199}\mathrm{At}$. The intensities of the 54-, 70-, 133-, 163-, 187-, 202-, and 573-keV transitions below the isomeric states are not to scale.

Figure 5

A spectrum of delayed $\gamma$-ray transitions detected in the planar detector within 5 $\mu$s after the detection of the ${}^{199}\mathrm{At}$ recoil in the DSSSD, obtained with lower beam intensity. The inset presents the time distribution between the implantation of a ${}^{199}\mathrm{At}$ recoil and the observation of the 163-keV $\gamma$ ray. The filled circles denote the astatine x-ray peaks and the diamonds denote peaks belonging to the polonium isotopes. A spectrum acquired within the time interval of 5–10 $\mu$s has been subtracted. The data are obtained from the ${}^{165}\mathrm{Ho}$(${}^{40}\mathrm{Ar},6n$)${}^{199}\mathrm{At}$ reaction.

Figure 6

Time-difference spectrum between the 163-keV $\gamma$-ray transition observed in the planar detector and the K-conversion peak of the 573-keV transition observed in the PIN-detector array. The solid line presents the decay curve fitted from 60 ns on. The dashed line, with the intensity compressed by a factor of 20, shows the total projection of the time spectrum for comparison. The inset presents the delayed conversion electron spectrum tagged with the ${}^{199}\mathrm{At}$ $\alpha$ decay. The K-, L-, and M-conversion peaks of the 573-keV transition are marked in the spectrum together with the K-conversion peak of the 434-keV transition. The data are obtained from the ${}^{165}\mathrm{Ho}$(${}^{40}\mathrm{Ar},6n$)${}^{199}\mathrm{At}$ reaction.

Figure 7

A prompt $\gamma$-ray spectrum tagged with the $\alpha$ decay of ${}^{197}\mathrm{At}$, including escaped $\alpha$ particles. Peaks containing events from the contaminating ${}^{194}\mathrm{Po}$ are denoted with diamonds.

Figure 8

The level scheme of ${}^{197}\mathrm{At}$. The intensity of the isomeric 311-keV transition is not to scale.

Figure 9

Level energies of negative-parity states in odd-mass astatine nuclei compared with the yrast states in their even-mass polonium isotones. ${}^{199}\mathrm{At}$ has a neutron number of 114. For the polonium data, denoted by closed symbols, see the studies [3, 31, 32] and references therein. The data for the heavier astatine nuclei are taken from the studies [5, 7, 8, 9, 10].

Figure 10

$B(M1)/B(E2)$ values of the $i_{13/2}$ band in comparison with theory and the corresponding band in ${}^{193}\mathrm{Bi}$ [33]. The theoretical estimate, denoted by the dashed line, was calculated using the Dönau-Frauendorf model [35, 36], assuming ${\beta }_2=-0.2$ and $K=13/2$. The datasets have been shifted slightly along the spin axis for visual purposes.

Figure 11

Experimental kinematic moments of inertia of the rotational band feeding the 13/$2^{+}$ isomer ${}^{197}\mathrm{At}$ and in ${}^{199}\mathrm{At}$ in the studied astatine nuclei compared with neighboring oblate deformed nuclei [3, 31, 33, 37]. High-spin data, which exhibit a back bend, have been omitted for ${}^{193}\mathrm{Bi}$, for purposes of low-spin comparison.

Figure 12

Level energies of the 29/$2^{+}$, 25/$2^{+}$, and 23/$2^{-}$ states in the odd-mass astatine nuclei, denoted by open symbols, compared with the ${11}^{-}$ and $9^{-}$ states in their polonium isotones, denoted by closed symbols. ${}^{199}\mathrm{At}$ has a neutron number of 114. The data point indicated with an arrow is floating in the level scheme and has been lifted 30 keV in the figure for visual purposes. For the polonium data see the studies [3, 31, 32] and references therein.