In-gas laser ionization and spectroscopy of actinium isotopes near the $N=126$ closed shell

The in-gas laser ionization and spectroscopy (IGLIS) technique was applied on the ${}^{212–215}\mathrm{Ac}$ isotopes, produced at the Leuven Isotope Separator On-Line (LISOL) facility by using the in-gas-cell and the in-gas-jet methods. The first application under on-line conditions of the in-gas-jet laser spectroscopy method showed a superior performance in terms of selectivity, spectral resolution, and efficiency in comparison with the in-gas-cell method. Following the analysis of both experiments, the magnetic-dipole moments for the ${}^{212–215}\mathrm{Ac}$ isotopes, electric-quadrupole moments and nuclear spins for the ${}^{214,215}\mathrm{Ac}$ isotopes are presented and discussed. A good agreement is obtained with large-scale nuclear shell-model calculations by using a ${}^{208}\mathrm{Pb}$ core.

Figure 1

A schematic representation of the general setup for the IGLIS technique. The distance $d$ from the exit of the gas cell to the SPIG for the in-gas-cell method is $d\sim 2$ mm and for the in-gas-jet method is $d\sim 12$ mm.

Figure 2

Alpha spectra of ${}^{215}\mathrm{Ac}$. The corresponding $\alpha$ energies given were extracted from the National Nuclear Data Center data base (NNDC) [9]. The count rate corresponds to the total number of $\alpha$ particles detected during a full HFS scan.

Figure 3

Schematic representation of the electronic hyperfine structure for the odd-odd and odd-even nuclei investigated in the different experimental campaigns. Notice that the splittings of the ground and excited states are not to scale and the transitions wavelengths are given for vacuum.

Figure 4

Hyperfine structure of (a) ${}^{215}\mathrm{Ac}$, (b) ${}^{214}\mathrm{Ac}$, (c) ${}^{213}\mathrm{Ac}$, and (d) ${}^{212}\mathrm{Ac}$ measured by using the in-gas-cell method. The $x$ axes denote the detuning, $w_0-w$, where $w_0$ is the center of gravity for ${}^{227}\mathrm{Ac}$.

Figure 5

(a) Atomic hyperfine structure of ${}^{214}\mathrm{Ac}$ measured by using the in-gas-jet method. In red is the best fit using Voigt profiles to the experimental data. (b) High-energy doublet of ${}^{214}\mathrm{Ac}$, corresponding to the $F=11/2\to F^{\prime }=13/2$ and $F=13/2\to F^{\prime }=13/2$ transitions. A FWHM of $320\left(20\right)$ MHz was obtained. The energy per pulse for this scan was $0.8\mu \mathrm{J}$. The $x$ axis denotes the detuning, $w_0-w$, where $w_0$ is the center of gravity for ${}^{227}\mathrm{Ac}$. The large hyperfine splitting between multiplets corresponds to the splitting of the excited atomic state and is dominated by the magnetic hyperfine coupling constant $a_u$.

Figure 6

Comparison of the collisional broadening and the collisional shift as a function of argon pressure inside the gas cell for the $F=6\to F^{\prime }=7$ transition. The doublet ($F=5\to F^{\prime }=6$ and $F=6\to F^{\prime }=6$; see Fig. 3) was included in the fit due to its presence in the 550 mbar scan.

Figure 7

Left axis, saturation curve for the high-energy singlet ($F=6\to F^{\prime }=7$) in ${}^{215}\mathrm{Ac}$ and right axis, evolution of the Lorentzian broadening ${\mathrm{\Gamma }}_L$ as a function of the laser energy. The Gaussian component of the total broadening was fixed in the fit of the experimental data. The lines connecting the points are only to guide the eyes.

Figure 8

(a)–(c) Change in the signal amplitude for the transitions $F_1=5\to F_2=6$ (blue) and $F_1=6\to F_2=6$ (dashed green) for different laser energy per pulse in ${}^{215}\mathrm{Ac}$. The red curve is the total fit to the experimental data. We observed that, due to the different transition strengths, different resonances are exposed to a different power broadening. The $x$ axis denotes the detuning, $w_0-w$,where $w_0$ is the center of gravity for ${}^{227}\mathrm{Ac}$.

Figure 9

Comparison between the $g$ factors for the actinium isotopes with isotopes of francium and bismuth. The $g_{\text{eff}}$ is the Schmidt value typical for this region [43]. The blue dashed line corresponds to the empirical $g$-factor value from ${}^{209}\mathrm{Bi}$ [51]. Here, only the statistical errors are shown.

Figure 10

Comparison between experimental $g$ factors and electric-quadrupole moments with the respective values extracted from large-scale nuclear-shell-model calculations (SM) and the modified SM (SM Mod) for the isotopes of actinium studied in this work. Here only the statistical errors are shown.

Figure 11

A comparison between experimental $g$ factors and electric-quadrupole moments with the respective values extracted from large-scale nuclear shell-model calculations $N=126$ closed-shell isotones. In the lower panel the comparison between the experimental quadrupole moments with the respective values extracted from shell-model calculations for the $N=125$ isotones is shown. The value $Q=-0.415\left(17\right)\mathrm{b}$ for ${}^{209}\mathrm{Bi}$ was taken from Ref. [54] and was used to correct the electric-quadrupole moment of ${}^{208}\mathrm{Bi}$ based on the correction for the previous value from ${}^{209}\mathrm{Bi}$ [55]. In the same way as in the case of $h_{9/2}$(${}^{215}\mathrm{Ac}$), the $g$ factor for the ground state in ${}^{211}\mathrm{At}$ was taken from $g$ factors of the high-spin isomeric state $I={(21/2)}^{-}$ [9]. Only the statistical errors are shown.