Laser-assisted decay spectroscopy for the ground states of ${}^{180,182}\mathrm{Au}$

A study of the ground states of the laser-ionized and mass-separated odd-odd isotopes ${}^{180,182}\mathrm{Au}$ was performed using the Resonance Ionization Laser Ion Source, Windmill detection setup and ISOLTRAP Multi-Reflection Time-of-Flight Mass Spectrometer at ISOLDE, CERN. A complex fine-structure $\alpha$-decay pattern of ${}^{180}\mathrm{Au}$ was deduced, providing insight into the low-lying levels in the daughter nucleus ${}^{176}\mathrm{Ir}$. An $\alpha$-decay branching ratio of $b_{\alpha }\left({}^{180}\mathrm{Au}\right)=0.58\left(10\right)\%$ and a half-life of $T_{1/2}=7.2\left(5\right)$ s have also been derived, allowing for the calculation of the reduced $\alpha$-decay widths and determining the degree of hindrance of respective $\alpha$-decay branches. From complementary first in-source laser spectroscopy measurements of the hyperfine structure in atomic transitions of ${}^{180,182}\mathrm{Au}$, the nuclear magnetic moments of $\mu \left({}^{180}\mathrm{Au}\right)=-0.83\left(9\right){\mu }_N$ and $\mu \left({}^{182}\mathrm{Au}\right)=1.66\left(9\right){\mu }_N$ were extracted with an inclusion of a correction for the hyperfine anomaly. Based on the observed hyperfine structure patterns, and on the comparison of the measured and calculated $\mu$ values, a preferred ground-state spin and parity $I^{\pi }\left({}^{180}\mathrm{Au}^{\mathrm{gs}}\right)=\left(1^{+}\right)$ is proposed, and the earlier assignment of $I^{\pi }\left({}^{182}\mathrm{Au}^{\mathrm{gs}}\right)=\left(2^{+}\right)$ is confirmed. For ${}^{180}\mathrm{Au}$, the most probable proton-neutron Nilsson configuration of $\pi 3/2^{-}\left[532\right]\otimes \nu 5/2^{-}\left[512\right]$ suggests the same proton state as in the heavier deformed odd-odd nuclei ${}^{182,184}\mathrm{Au}$.

Figure 1

(a) Singles $\alpha$-decay spectrum registered in Si1 and Si2 at $A=180$; the peaks are marked with their energies in keV. The quoted $\alpha$-decay energies for ${}^{180}\mathrm{Au}$ include the correction for $\alpha +$ conversion electrons summing in silicon detectors. The overlapped blue histogram is the result of GEANT4 simulations, see Sec. 3a4 for details. (b) $\alpha -\gamma$ coincidences for $\alpha$ decay from (a) for $\gamma$-ray energies up to 250 keV with a prompt coincidence timing gate $\mathrm{\Delta }T(\alpha -\gamma )<250$ ns. (c) Projection on the $\gamma$-ray axis for events inside the blue region in (b). The $\gamma$-ray peaks in (b) and (c) are marked with their energies in keV. Additionally, the number of counts in each peak is given in brackets in (c).

Figure 2

A decay scheme for ${}^{180}\mathrm{Au}$ proposed in this study. The $\alpha$-decay energies $E_{\alpha }$, relative intensities $I_{\alpha }$, reduced $\alpha$-decay widths ${\delta }_{\alpha }^2$ and hindrance factor values ${\mathrm{HF}}_{\alpha }$ for the parent isotope ${}^{180}\mathrm{Au}$ and levels and transitions in the daughter nucleus ${}^{176}\mathrm{Ir}$ deduced in this work are shown alongside some previously-known data from Ref. [20]. The half-life value of 8.1(3) s is taken from [30]. Hindrance factors were extracted relative to the average value of ${\delta }_{\alpha }^2=66\left(6\right)$ keV for the unhindered $\alpha$ decays from ${}^{179,181}\mathrm{Au}$, see Sec. 4b for details. A value $\mathrm{\Delta }=9.5\left(7\right)$ keV is discussed in Sec. 3a2. The 5675-keV and 5686-keV $\alpha$ decays are shown by a dashed line as tentative.

Figure 3

Projections on the $\alpha$-energy axis from $\alpha -\gamma$ coincident plot in Fig. 1, using a $\pm 1.5$ keV gate encompassing the $\gamma$ ray indicated in the top left of each projection.

Figure 4

The time distribution for $\alpha$ decays of ${}^{180}\mathrm{Au}$ as seen in Si1 and Si2 at the implantation position. A sequence of implantation-decay cycles was implemented, in which the beam was implanted for 30.9 s (shaded in blue), followed by the closing of the ISOLDE beam gate for 5.1 s (shaded in brown). At the end of each 36 s cycle, the wheel of the Windmill was rotated to place a fresh foil into the implantation position. The decay part of the distribution was fitted with an exponential function, from which a value of $T_{1/2}\left({}^{180}\mathrm{Au}\right)$ = 7.2(5) s was deduced. By extrapolating the decay curve beyond 36 s, the amount of ${}^{180}\mathrm{Au}$ activity, removed by the wheel movement, can be estimated. The latter is needed for the $b_{\alpha }\left({}^{180}\mathrm{Au}\right)$ determination.

Figure 5

A background-subtracted $\gamma$-ray energy spectrum detected by the LEGe detector at $A$=180. Transitions associated with the decay of excited levels in ${}^{180}\mathrm{Pt}$ are indicated (energies in keV). The expected position of the unobserved 346.3 keV $6_1^{+}\to$ $4_1^{+}$ decay is indicated in brackets.

Figure 6

The three-step resonance photo-ionization scheme used by RILIS to produce ${}^{180,182}\mathrm{Au}$ ions. The hfs for the 267.6-nm transition for both isotopes is shown (not to scale). $I$ is the nuclear spin, while $J$, $F$ and $J^{\prime }$, $F^{\prime }$ are the electron spin and the total angular momenta of the atom for the $6s{}^2{S}_{1/2}$ g.s. and excited $6p{}^2{P}_{1/2}$ state, respectively. The four allowed atomic transitions between $F_{1,2}$ and $F_{1,2}^{\prime }$ sublevels are indicated by the colored arrows. Note the inverted positions of the sublevels $F_{1,2}$ and $F_{1,2}^{\prime }$ in ${}^{180,182}\mathrm{Au}$, which reflects the opposite signs of their respective magnetic moments.

Figure 7

Examples of the hfs spectra (filled black squares) using the 267.6-nm transition for: (a) ${}^{182}\mathrm{Au}$ as measured by the MR-ToF MS; (b) and (c) ${}^{180}\mathrm{Au}$ by using the MR-ToF MS or Windmill decay station, respectively. The zero frequency corresponds to the wave number 37358.9 ${\mathrm{cm}}^{-1}$. Expected theoretical positions and intensities for the four hfs components, assuming $I$ = 2 for ${}^{182}\mathrm{Au}$ and $I$ = 1 for ${}^{180}\mathrm{Au}$, are shown by the narrow vertical black lines. The Voigt-profile fits for each experimental hfs spectrum are shown by red lines (spin assignments $I({}^{182}\mathrm{Au}$) = 2 and $I({}^{180}\mathrm{Au}$) = 1), and by blue line for $I({}^{180}\mathrm{Au}$) = 2. Fitting with $I$ = 3 is not shown on (b) and (c) as it is visually indistinguishable from that of $I$ = 2.

Figure 8

A Nilsson level diagram relevant to ${}^{180}\mathrm{Au}$ for (a) neutrons (82 $<N<$ 126); (b) protons (50 $<Z<$ 82). The shaded region indicates the region of quadrupole deformation 0.2 $<\beta <$ 0.3. The dash-dotted green lines show the positions of respective Fermi surfaces for neutrons and protons.