Spectroscopy of the long-lived excited state in the neutron-deficient nuclides ${}^{195,197,199}\mathrm{Po}$ by precision mass measurements

Direct mass measurements of the low-spin $3/2^{-}$ and high-spin $13/2^{+}$ states in the neutron-deficient isotopes ${}^{195}\mathrm{Po}$ and ${}^{197}\mathrm{Po}$ were performed with the Penning-trap mass spectrometer ISOLTRAP at ISOLDE-CERN. These measurements allow the determination of the excitation energy of the isomeric state arising from the $\nu i_{13/2}$ orbital in ${}^{195,197}\mathrm{Po}$. Additionally, the excitation energy of isomeric states of lead, radon, and radium isotopes in this region were obtained from $\alpha$-decay chains. These excitation energies complete the knowledge of the energy systematics in the region and confirm that the $13/2^{+}$ states remain isomeric, independent of the number of valence neutrons.

Figure 1

The neutron spherical shell-model orbitals, their energies taken from experimental data on ${}^{207}\mathrm{Pb}$ [11], as well as the neutron number corresponding to the filled orbital(s).

Figure 2

Schematic view of the ISOLTRAP setup with the $\alpha$-decay station. Inset: time-of-flight spectrum of mass $A=195$. Full separation of ${}^{195}\mathrm{Po}^{+}$ from ${}^{195}\mathrm{Tl}^{+}$ isotopes was achieved after 1000 revolutions in the MR-ToF MS.

Figure 3

The hyperfine structure of the 843-nm atomic transition for the $3/2^{-}$ and $13/2^{+}$ nuclear states in (a) ${}^{195}\mathrm{Po}$ and (b) ${}^{197}\mathrm{Po}$ [26, 32]. The dashed lines indicate the wave numbers used for the mass measurements. Note that the two hyperfine spectra were recorded in a different experiment, and hence the laser resolution achieved in the present work is different.

Figure 4

The $\alpha$-decay energy spectra for ${}^{195}\mathrm{Po}$, panels (a) and (b), and ${}^{197}\mathrm{Po}$, panels (c) and (d), measured with the two silicon detectors surrounding the implantation point in DSS2.0. The spectra were recorded at different RILIS wave numbers: Panels (a) and (c) were taken at 11853.75 and 11853.95 ${\mathrm{cm}}^{-1}$ respectively, while panels (b) and (d) were taken at 11853.83 ${\mathrm{cm}}^{-1}$.

Figure 5

ToF-ICR resonances of ${}^{195(g,m)}\mathrm{Po}$ with 2 s excitation time in red circles (ground state) and blue triangles (isomeric state). The red (blue) dashed line represents the center frequency of the ground (excited) state. Quasipure samples of either state were measured thanks to selective enhancement from RILIS and different half-lives (see text for details). The solid lines represent the fit of the theoretical line shape [41] to the experimental data.

Figure 6

ToF-ICR resonance of ${}^{197(g,m)}\mathrm{Po}$. The low-spin ToF- ICR resonance was recorded with 5 s excitation and the high-spin with 3 s. The two resolved resonances in the red circle ToF-ICR spectrum are isomer (left) and ground (right) states. The pure isomeric state is represented by the blue triangles on the ToF-ICR spectrum. The red (blue) dashed line is the center frequency of the ground (excited) state. The solid lines are fits of the experimental data to the theoretical line shape [41].

Figure 7

Center frequency ${\nu }_c$ resulting from a fit of a mixed resonance with a single-resonance curve, as a function of the isomeric state ratio $R_m$. It was obtained from simulated data of ${}^{195}\mathrm{Po}$ for different excitation times. The purple dashed straight line represents the pure cyclotron frequency of the ground state where the ratio of ground-to-isomeric state is 1:0.

Figure 8

Simplified $\alpha$-decay scheme from low- and high-spin states in ${}^{203,205}\mathrm{Ra}$ to ${}^{191,193}\mathrm{Pb}$. Using the excitation energies from this work in ${}^{195,197}\mathrm{Po}$ and the $Q_{\alpha }$ values [43, 44], the excitation energy in each linked isotope is determined. The red and blue lines represent the ground and excited states, respectively.

Figure 9

The excitation energy of the $13/2^{+}$ state in odd-$A$ polonium isotopes in the neutron-deficient lead region. The excitation energies determined indirectly by using the new mass data and $\alpha$-decay $Q_{\alpha }$ values are also shown. The results obtained from ISOLTRAP (red) show that the $13/2^{+}$ configuration does not become the ground state. Data from Refs. [43, 44] are shown with other colors.