Laser-assisted nuclear decay spectroscopy of ${}^{176,177,179}\mathrm{Au}$

A study of the laser-ionized and mass-separated neutron-deficient isotopes ${}^{176,177,179}\mathrm{Au}$ was performed using the Resonance Ionization Laser Ion Source and the Windmill detection setup at ISOLDE, CERN. New and improved data on complex fine-structure $\alpha$ decays of the three isotopes were deduced, providing insight into the low-lying levels in the daughter nuclei ${}^{172,173,175}\mathrm{Ir}$. New information on the properties of $\beta$-decay daughter products ${}^{177,179}\mathrm{Pt}$ was also obtained. From the first in-source laser spectroscopy measurements of the hyperfine structure in the atomic 267.6-nm transition of ${}^{176}\mathrm{Au}$, the nuclear magnetic moments for both high- and low-spin $\alpha$-decaying states were deduced. Together with the values determined from the additivity relations, they were used to propose the most probable spins and configurations for both states. The $\alpha$-decay branching ratios were determined as $b_{\alpha }\left({}^{176}\mathrm{Au}^{\text{ls}}\right)=58\left(5\right)\%$ and $b_{\alpha }\left({}^{176}\mathrm{Au}^{\text{hs}}\right)=29\left(5\right)\%$.

Figure 1

(a) $\alpha$-decay energy spectrum registered in $\mathrm{Si}1+\mathrm{Si}2$ for the runs at mass $A=179$. The peaks are labeled with their energies in keV and the nuclides they originate from. The fitting of the peaks for ${}^{179}\mathrm{Au}$ with the Crystal Ball function is shown by the blue dashed line, and the sum of the individual curves is given by the red line. (b) $\alpha \text{−}\gamma$ coincidences from panel (a) for $\gamma$-ray energies up to 250 keV with a coincidence timing gate $\mathrm{\Delta }T\left(\alpha \text{−}\gamma \right)\le 250\mathrm{ns}$. (c) Projection on the $E_{\gamma }$ axis from within the blue-colored shape “A” in panel (b). Panel (d) is the same as panel (c) but for the events within the rectangle “B” from panel (b).

Figure 2

Projections on the $\alpha$-energy axis from the $\alpha \text{−}\gamma$ coincidences plot in Fig. 1 using a $\pm 1.5$-keV gate on the $\gamma$ ray indicated in the top left of each panel. The vertical red dotted line indicates the centroid of the 5718-keV peak in coincidence with the 105.3- and 131.9-keV $\gamma$ decays.

Figure 3

The decay scheme for ${}^{179}\mathrm{Au}$ deduced in this study. Shown are the $\alpha$-decay energies $E_{\alpha }$, relative intensities $I_{\alpha ,\mathrm{rel}}$, reduced $\alpha$-decay widths ${\delta }_{\alpha }^2$, calculated using the Rasmussen approach [29], and hindrance factors $H{F}_{\alpha }$ relative to the unhindered g.s.$\to \mathrm{g}$.s. 5848-keV decay. The 26.1-keV transition was not seen in our study, but its existence was proven in Ref. [28]; see the main text for details. The values taken from the literature are labeled with the respective references in square brackets and were used for calculations of ${\delta }_{\alpha }^2$ values shown in the figure. The $\gamma$ rays de-exciting the 102.3- and 175.6-keV states in ${}^{175}\mathrm{Os}$ are known, but not shown in the plot for clarity.

Figure 4

The time distribution for $\alpha$ decays of ${}^{179}\mathrm{Au}$ as measured by $\mathrm{Si}1+\mathrm{Si}2$ detectors at the implantation position and for the 153.7-keV $\gamma$ decay, following $\beta$ decay of ${}^{179}\mathrm{Au}$. A sequence of 36-s-long implantation-decay cycles was implemented in which the beam was implanted for 30.89 s (shaded in light blue) followed by the closure of the ISOLDE beam gate for 5.11 s (shaded in brown), during which the pure decay of the implanted activity was measured. At the end of each 36-s cycle, the wheel of the WM was rotated and a fresh foil placed into the implantation position for the next measurement cycle. The decay part of each distribution was fitted with an exponential function, from which the values of $T_{1/2}\left({}^{179}\mathrm{Au}\right)=7.3\left(3\right)\mathrm{s}$ and 7.1(12) s are calculated for $\alpha$ and $\gamma$ decays, respectively.

Figure 5

Background-subtracted $\gamma$-ray singles spectrum in LEGe for the runs at $A=179$, with transitions related to the daughter products of ${}^{179}\mathrm{Au}$ marked with their energies in keV. Several known $\gamma$ rays in ${}^{179}\mathrm{Ir}$ are also labeled. The 221-keV transition has a long-half-life, and its origin could not be established in the present study.

Figure 6

Typical examples of the hfs spectra (filled symbols) measured by $\alpha$ decay counting with WM, by using the 267.6-nm transition for (a) ${}^{177}\mathrm{Au}^{\text{g,m}}$ (data are from Refs. [15, 18]) and (b) ${}^{176}\mathrm{Au}$ from the present study. The zero frequency corresponds to the centroid position of the hfs for the reference isotope ${}^{197}\mathrm{Au}$. For ${}^{176}\mathrm{Au}$, the solid lines depict Voigt-profile fits to the data with the assumptions of $I=9$ for the high-spin state in ${}^{176}\mathrm{Au}$ and $I=3$ for the low spin; the respective theoretical positions of the hfs components are shown by the vertical black lines. Fits with different assignments ($I=7$, 8 for high spin and $I=2$, 4, 5 for low spin) are indistinguishable from those shown in the figure. The four theoretically expected hfs components for low-spin ${}^{176}\mathrm{Au}$ were not fully resolved, and therefore only two broader peaks were observed, each of them involving two overlapping hfs components.

Figure 7

(a) Combined singles $\alpha$-decay spectrum from $\mathrm{Si}1+\mathrm{Si}2$ with the lasers tuned to the ${}^{177}\mathrm{Au}^{\text{g}}$ hfs frequencies. (b) The same, but with the lasers tuned to the ${}^{177}\mathrm{Au}^{\text{m}}$ frequency range. The peaks are labeled with their energies in keV and the nuclides they originate from. The tentative peaks are labeled with their energies in brackets. A small peak at 5486 keV is due to the weak calibration ${}^{241}\mathrm{Am}$ sources installed on both sides of the WM wheel. The origin of a weak $\approx 5330$-keV decay observed in both panels of the figure could not be established. The red lines in both panels are the Crystal Ball fits, similar to those of ${}^{179}\mathrm{Au}$.

Figure 8

The decay schemes for ${}^{177}\mathrm{Au}^{\text{m,g}}\to {}^{173}\mathrm{Ir}^{\text{m,g}}\to {}^{169}\mathrm{Re}^{\text{m,g}}$. Shown are the $\alpha$-decay energies $E_{\alpha }$, reduced $\alpha$-decay widths ${\delta }_{\alpha }^2$, calculated using the Rasmussen approach [29], relative intensities $I_{\alpha ,\mathrm{rel}}$, half-life values $T_{1/2}$, and $\alpha$-decay branching ratios $b_{\alpha }$. The values taken from literature are labeled with the respective references in square brackets; they were used for calculating reduced widths. See the main text for the discussion of masses and excitation energies of some states shown in the plot. The de-excitation path of the known 424.4-keV state in ${}^{173}\mathrm{Ir}$ is not shown for clarity.

Figure 9

(a) Combined singles $\alpha$-decay spectrum at $A=176$, from $\mathrm{Si}1+\mathrm{Si}2$ with the lasers tuned to the ${}^{176}\mathrm{Au}^{\text{hs}}$ hfs frequencies. (b) The same, but with the lasers tuned to the ${}^{176}\mathrm{Au}^{\text{ls}}$ frequency range. The peaks are labeled with their energies in keV and the nuclide they originate from. The shifted and normalized 5753-keV peak of ${}^{176}\mathrm{Pt}$ is shown by the blue histogram in panel (b); see Sec. 4c3 for discussion of this procedure

Figure 10

Decay schemes for ${}^{176}\mathrm{Au}^{\text{hs}}$ (a) and for ${}^{176}\mathrm{Au}^{\text{ls}}$ (b). For each $\alpha$-decay branch, the $\alpha$-decay energies $E_{\alpha }$, relative intensities $I_{\alpha ,\mathrm{rel}}$, and reduced widths ${\delta }_{\alpha }^2$, calculated using the Rasmussen approach [29], are shown. The values taken from literature are labeled with respective references in square brackets, e.g., evaluated data in Ref. [49] for the high- and low-spin states in ${}^{172}\mathrm{Ir}$. The 5798- and 6157-keV decays of ${}^{176}\mathrm{Au}^{\text{ls}}$ were not observed in this study; their existence was assumed from Ref. [12], with the relative intensives for f.s. decays of ${}^{176}\mathrm{Au}^{\text{ls}}$ derived as explained in Sec. 3c2. The energies for $\gamma$ rays from excited states in ${}^{172}\mathrm{Ir}$ are from the SHIP study [12], except for the 500.3(3)- and 475.4(5)-keV decay, taken from Ref. [47]. The excitation energy of the 25-keV state was deduced from the energy difference of the 500.3- and 475.4-keV $\gamma$ rays; see Sec. 4c3 for discussion of this procedure.

Figure 11

A comparison of the reduced $\alpha$-decay widths for g.s. $\to \mathrm{g}$.s. decays of even-even platinum and mercury isotopes with the data for ${}^{171,173,175,176,177,179}\mathrm{Au}$. All values were calculated using the Rasmussen approach [29], with literature data taken from ENSDF. The open and closed red triangles represent the decays of isomeric $11/2^{-}$ and $1/2^{+}$ ground states in the odd-$A$ gold isotopes, respectively. The new data for ${}^{176}\mathrm{Au}^{\mathrm{ls},\mathrm{hs}}$ Au from this study are shown by the pink triangles.