Laser-assisted decay spectroscopy and mass spectrometry of ${}^{178}\mathrm{Au}$

A comprehensive study of the isotope ${}^{178}\mathrm{Au}$ has been made at the CERN-ISOLDE facility, using resonance laser ionization. Two long-lived states in ${}^{178}\mathrm{Au}$ were identified—a low-spin ground state and a high-spin isomer—each of which were produced as pure beams. Using the ISOLTRAP precision Penning trap, the excitation energy of the isomeric state in ${}^{178}\mathrm{Au}$ was determined to be $E^{*}=189\left(14\right)\mathrm{keV}$. The $\alpha$-decay fine structure patterns of the two states were studied using the Windmill decay station, providing information on the low-lying states in the daughter nucleus ${}^{174}\mathrm{Ir}$. Nuclear spin assignments of $I\left({}^{178}\mathrm{Au}^g\right)=(2,3)$ and $I\left({}^{178}\mathrm{Au}^m\right)=(7,8)$ are made based on the observed $\beta$-decay feeding and hyperfine structure intensity patterns. These spin assignments are used for fitting the hyperfine structures of the two states from which values for the magnetic dipole moments are extracted. The extracted moments are compared with calculations using additivity relations to establish the most probable configurations for ${}^{178}\mathrm{Au}^{g,m}$.

Figure 1

(a) The three-step laser-ionization scheme used to produce the gold ions (see text for details); (b) the hyperfine structure spectrum of ${}^{178}\mathrm{Au}$ (rate of photo-ions measured by the MR-ToF MS as a function of the wave number of the first-step laser, before tripling, used in the ionization process). Peaks 1, 2, 5, and 6 (red data points) belong to ${}^{178}\mathrm{Au}^m$ and peaks 3 and 4 (blue data points) to ${}^{178}\mathrm{Au}^g$; (c) the hyperfine levels (not to scale) for ${}^{178}\mathrm{Au}^m$ ($\mu >0$) and ${}^{178}\mathrm{Au}^g$ ($\mu <0$). The relative spacing and ordering of the hfs levels for ${}^{178}\mathrm{Au}^{g,m}$ indicate the difference in the magnitude and sign of their respective magnetic dipole moments (see Sec. 3e). The transitions between hyperfine states for ${}^{178}\mathrm{Au}^g$ are indicated by the blue arrows. Due to Doppler broadening, transitions A and B overlap and their combined intensity is shown as peak 3 in panel (b); similarly transitions C and D overlap and combine to make peak 4. For ${}^{178}\mathrm{Au}^m$, all four transitions are well separated; the transition label numbers correspond to the peak numbers in panel (b).

Figure 2

Time-of-flight ion-cyclotron resonances of the two long-lived states in ${}^{178}\mathrm{Au}$ measured with ISOLTRAP. The blue, filled symbols are the data points for ${}^{178}\mathrm{Au}^g$, and the red, hollow symbols are for ${}^{178}\mathrm{Au}^m$. The fits of the theoretical line shapes [35] are also shown by the solid blue line for ${}^{178}\mathrm{Au}^g$ and by the dashed red line for ${}^{178}\mathrm{Au}^m$. The data were collected separately using the different RILIS settings (see text for details).

Figure 3

Decay spectra for ${}^{178}\mathrm{Au}^g$ (left) and for ${}^{178}\mathrm{Au}^m$ (right). Panels (a) and (b): singles $\alpha$-decay spectra, measured in $\mathrm{Si}1+\mathrm{Si}2$; the blue histogram in panel (b) shows the results from geant4 simulations of a single 5925-keV $\alpha$ decay followed by internal conversion instead for a 56.8-keV $M1$ transition. Panels (c) and (d): the $\alpha \text{−}\gamma$ coincidences measured within the time interval $\mathrm{\Delta }T(\alpha \text{−}\gamma$) $⩽$ 210 ns for ${}^{178}\mathrm{Au}^{g,m}$, respectively. The vertical dashed red lines indicate the $E_{\alpha }$ centroids of the $\alpha \text{−}\gamma$ coincidence groups. Panels (e) and (f): projections upon the $E_{\gamma }$ axis for the $E_{\alpha }=5720\text{−}5880$ and 5800–5960 keV regions of panel (c) and (d) (areas shaded in pink), respectively. The inset of panel (e) shows the projection on the $E_{\gamma }$ axis for the $E_{\alpha }=5820\text{−}5860\mathrm{keV}$ region of panel (c). The inset of panel (f) shows the overlap of the spectra in panels (e) and (f) for the $E_{\gamma }=58\text{−}93\mathrm{keV}$ region, shown in blue and red, respectively.

Figure 4

Projections on the $E_{\alpha }$ axis for the $\alpha \text{−}\gamma$ groups of ${}^{178}\mathrm{Au}^g$ data shown in Fig. 3, for the (a) 82.8-keV, (b) 90.0-keV, (c) 98.3-keV, (d) 115.7-keV, and (e) 174.8-keV $\gamma$-ray transitions. Vertical lines indicate $\alpha$-decay energies of interest (see text for details).

Figure 5

Decay schemes of ${}^{178}\mathrm{Au}^{g,m}$ deduced in this work. The $\alpha$-decay energies ($E_{\alpha }$), intensities ($I_{\alpha }$), reduced $\alpha$-decay widths (${\delta }_{\alpha }^2$), and hindrance factor values (${\text{HF}}_{\alpha }$) are shown. The ${\text{HF}}_{\alpha }$ values were calculated relative to the average ${\delta }_{\alpha }^2$ value for the unhindered $\alpha$ decays of ${}^{177}\mathrm{Au}$ ($11/2^{-}\to 11/2^{-}, {\delta }_{\alpha }^2=82\left(14\right)\mathrm{keV}$ [39]) and ${}^{179}\mathrm{Au}$ ($1/2^{+}\to 1/2^{+}, {\delta }_{\alpha }^2=58\left(4\right)\mathrm{keV}$ [40]). A value of $\mathrm{\Delta }=129\left(17\right)\mathrm{keV}$ is determined, based on the decay data and mass measurements presented in the current work.

Figure 6

Projections on the $E_{\alpha }$ axis for the $\alpha \text{−}\gamma$ groups of ${}^{178}\mathrm{Au}^m$ shown in Fig. 3, for the (a) 56.8-keV, (b) 67.4-keV, (c) 91.2-keV, (d) 139.2-keV, (e) 421.5-keV, and (f) 472.1-keV $\gamma$-ray transitions. Vertical lines indicate $\alpha$-decay energies of interest (see text for details).

Figure 7

The decay curves for ${}^{178}\mathrm{Au}^g$ ($•$) and ${}^{178}\mathrm{Au}^m$ ($\blacksquare$), fitted with exponential functions.

Figure 8

Background-subtracted singles $\gamma$-decay spectra measured by the LEGe following the ${\beta }^{+}/\mathrm{EC}$ decays of (a) ${}^{178}\mathrm{Au}^g$ and (b) ${}^{178}\mathrm{Au}^m$. The relevant peaks are labeled with their energies (in keV), along with the spins and parities of the initial and final states in ${}^{178}\mathrm{Pt}$ and their intensities corrected for $\gamma$-ray detection efficiency, normalized to the intensity of the 170.4-keV $2^{+}\to 0^{+}$ transition.