Electromagnetic moments of odd-$A$ ${}^{193–203,211}\mathrm{Po}$ isotopes

Hyperfine splitting parameters have been measured for the neutron-deficient odd-mass polonium isotopes and isomers ${}^{193–203}\mathrm{Po}$${}^{g,m}$, ${}^{209,211}\mathrm{Po}$. The measurement was performed at the ISOLDE (CERN) online mass separator using the in-source resonance ionization spectroscopy technique. The magnetic dipole moments $\mu$ and spectroscopic electric quadrupole moments $Q_S$ have been deduced. Their implication for the understanding of nuclear structure in the vicinity of the closed proton shell at $Z=82$ and the neutron mid-shell at $N=104$ is discussed. For the most neutron-deficient nuclei ($A=193,195,197$), a deviation of $\mu$ and $Q_S$ from the nearly constant values for heavier polonium nuclei was observed. Particle-plus-rotor calculations with static oblate deformation describe the electromagnetic moments for these nuclei well, provided a gradual increase of a mean deformation when going to lighter masses is assumed for the polonium nuclei with $A<198$.

Figure 1

The three-step laser-ionization scheme for polonium [16]. The electronic configurations for the levels involved and the laser wavelengths for each step are shown. The hyperfine structures for the 13/2${}^{+}$ and 3/2${}^{-}$ states of odd-$A$ polonium isotopes are shown schematically (not to scale). The hyperfine lines expected for $I=13/2$, $I=3/2$ are also shown.

Figure 2

Schematic of the Windmill detection chamber: (a) frontal view, (b) top view. The beam is implanted into one of 10 carbon foils A (20 $\mu$g/cm${}^2$) mounted on a rotating wheel D which is controled by a stepper motor F. The $\alpha$ decay is monitored using silicon detectors, either at the implantation site C or at the decay position B. The implantation site has annular detector C2 with a hole (hole diameter of 6 mm) in the middle to allow the beam to pass. A retractable, secondary-electron-suppressed Faraday cup E is used for beam tuning and monitoring. Detector C2 is protected by a collimator G. The reference ${}^{241}\mathrm{Am}$ $\alpha$ sources ($<50$ Bq) H are placed between two foil positions on either side of the wheel to monitor the performance of the silicon detectors at the positions B and C in the course of an experimental campaign.

Figure 3

Example of $\alpha$-decay energy spectrum for ${}^{197}\mathrm{Po}$ collected at the Windmill decay station. Decay spectra for ${}^{191}\mathrm{Po}$ are in Ref. [11] and for ${}^{195}\mathrm{Po}$ in Ref. [19].

Figure 4

Example of $\gamma$-ray energy spectrum for ${}^{201}\mathrm{Po}$ collected at the tape station. Decay spectra for ${}^{199}\mathrm{Po}$ are in Ref. [21].

Figure 5

Dependence of the number of the detected photoions (deduced from the number of $\alpha$ particles, $E_{\alpha }^{ls}=6606$ keV, $E_{\alpha }^{hs}=6699$ keV) on the laser frequency detuning. Frequency detuning is shown with respect to the hfs centroids of the high-spin state ${}^{195hs}$Po [$I^{\pi }=(13/2^{+})$] and the low-spin state ${}^{195ls}$Po [$I^{\pi }=(3/2^{-})$]. Solid lines represent a fit to the data (see text). The calculated positions and relative intensities of the individual hyperfine components are shown with vertical bars.

Figure 6

Similar to Fig. 5, but for high-spin states of polonium isotopes. Frequency detuning is shown with respect to the centroids of each hfs.

Figure 7

Magnetic dipole moments for the neutron-deficient odd-$A$ polonium, lead, platinum, and mercury isotopes. Data from this work are shown with filled circles, data from Refs. [35, 37] with the open circles, and magnetic dipole moments for mercury, lead and platinum are taken from Refs. [6, 8, 38, 39, 40, 41, 42, 43, 44, 45]. The lines connecting the data points are to guide the eye only.

Figure 8

Spectroscopic quadrupole moments for ${}^{193–203}\mathrm{Po}$. The PPR-model predictions (see text) are shown with dotted lines.

Figure 9

Deformation parameters extracted from the charge radii, quadrupole moments, and $B\left(E2\right)$ values [49, 50]. For ${}^{191}\mathrm{Po}$ results of the fits with $B>0$ and $B<0$ are shown with the filled and unfilled triangles, respectively.

Figure 10

Comparison of experimental and calculated magnetic moments for odd-$A$ Po low- and high-spin states. Experimental data are shown with the filled circles, magnetic moments calculated in the PPR model with DM deformation are shown with the dotted line, and calculated magnetic moments that take into account mixture of near-spherical and deformed configurations [see Eq. (16)] are shown with the dashed line. The lines connecting the data points are to guide the eye only.