First identification of excited states in ${}^{169}\mathrm{Ir}$

Gamma rays populating the α-decaying isomeric state in ${}^{169}\mathrm{Ir}$ have been observed for the first time. The experiment employed the recoil-decay tagging method using the JUROGAM γ-ray spectrometer, the RITU gas-filled recoil separator and the GREAT spectrometer located at the RITU focal plane. The γ-ray cascade feeding the isomeric α-decaying state exhibits a rotational structure consistent with a $h_{11/2}$ proton coupled to a triaxially deformed core. The experimental results are compared with predictions from total Routhian surface calculations.

Figure 1

(a) Recoil-α correlated ${}^{169}\mathrm{Ir}$ γ-ray spectrum. Lines marked with a dot represent contamination from the three strongest transitions in ${}^{170}\mathrm{Ir}$, at 218, 267 and 370 keV, respectively [28]. (b) As in (a), but requiring additional correlation with the isomeric ($h_{11/2}$) ${}^{165}\mathrm{Re}$ daughter α decay [7].

Figure 2

Level scheme for ${}^{169}\mathrm{Ir}$ as deduced in the present work. Gamma rays are labeled with transition energies in keV. The intensity of the transitions is proportional to the width of the arrows. Tentative spin and parity assignments are indicated by parentheses. The isomeric $h_{11/2}$ band head is at 153(24) keV above the $s_{1/2}$ ground state [16] (not shown in the level scheme).

Figure 3

Angular intensity ratios as a function of γ-ray transition energy in ${}^{169}\mathrm{Ir}$ (filled circles) defined through Eq. (1). Diamonds represent reference values for stretched $E2$ (286 keV and 463 keV) transitions in ${}^{170}\mathrm{Os}$ and stretched $M1$ and $E1$ (246 keV and 947 keV) transitions in ${}^{169}\mathrm{Re}$ and ${}^{170}\mathrm{Os}$, respectively.

Figure 4

Three $\gamma \text{−}\gamma$ coincidence spectra generated from the recoil-α tagged ($E_{\alpha }=6117$ keV) $E_{\gamma 1}\text{−}{E}_{\gamma 2}$ coincidence matrix. The three gates, indicated by arrows, demonstrate the most salient features of the ${}^{169}\mathrm{Ir}$ level scheme: (a) spectrum in coincidence with the 393 keV transition depopuling Band 2, (b) spectrum in coincidence with the 762 keV transition in Band 1, and (c) spectrum in coincidence with the 937 keV transition connecting Band 2 with Band 1.

Figure 5

Calculated TRSs for the even-$N$ Pt-Ir-Os $N=92\text{−}96$ isotones showing the total energy in the rotating frame as a function of the deformation parameters γ and ${\beta }_2$ at a rotational frequency $\hslash \omega =0.16$ MeV. The top panels show the even-even ${}^{170-174}\mathrm{Pt}$ isotopes. The middle panels show the $(\pi ,\alpha )=(-,-1/2)$ configuration of the odd-$A$ ${}^{169-173}\mathrm{Ir}$ isotopes and the bottom panels display the even-even ${}^{168-172}\mathrm{Os}$ isotopes. The energy difference between contour lines is 100 keV.

Figure 6

Calculated TRSs for the $(\pi ,\alpha )=(+,+1/2)$ $A$-configuration of ${}^{169}\mathrm{Ir}$ showing deformation parameters γ and ${\beta }_2$ at rotational frequency $\hslash \omega =0.0$ (left panel) and $\hslash \omega =0.16$ MeV (right panel). The energy difference between contour lines is 100 keV.

Figure 7

Proton single-particle levels ($e^{\text{'}}$) as a function of the triaxiality parameter γ. The single-particle levels are labeled with their spherical subshell origin as described in the text. The plot is made with ${\beta }_2=0.142$ and $\hslash \omega =0.0$ MeV, using a universal Woods-Saxon potential. The solid lines show levels with $(\pi ,\alpha )=(+,+1/2)$ and $(+,-1/2)$, and the dashed lines those of $(\pi ,\alpha )=(-,+1/2)$ and $(-,-1/2)$. The vertical dashed line indicate the relevant γ deformation for ${}^{169}\mathrm{Ir}$.

Figure 8

Staggering parameter S(I) as a function of spin $I$ for the $[505]11/2^{-}$ bands in the neutron-deficient odd-$A$ iridium isotopes ${}^{169-175}\mathrm{Ir}$. The filled (open) symbols represent the $\alpha =+1/2$ ($\alpha =-1/2$) signature.

Figure 9

Experimental aligned angular momentum $i_x$ as a function of rotational frequency $\hslash \omega$ for the $h_{11/2}$ band in ${}^{169}\mathrm{Ir}$ and ${}^{171}\mathrm{Ir}$. The filled (open) symbols represent the $\alpha =+1/2$ ($\alpha =-1/2$) signature. The same Harris parameters ${\mathfrak{J}}_0=10\hslash {}^2$ ${\mathrm{MeV}}^{-1}$ and ${\mathfrak{J}}_1=50\hslash {}^4$ ${\mathrm{MeV}}^{-3}$ [15] have been used for both nuclides.

Figure 10

Experimental $B(M1;I\to I-1)/B(E2;I\to I-2)$ values as a function of initial spin $I_i$ for the [505]$11/2^{-}$ band in ${}^{169}\mathrm{Ir}$. The measured values are compared with the predictions of the semiclassical model of Dönau and Frauendorf, shown here both for positive and negative γ deformation as described in the text. The slight staggering of the experimental values is due to favored ($\alpha =+1/2$) and unfavored ($\alpha =-1/2$) signature of the strongly-coupled band, represented by filled and open symbols, respectively. A γ deformation of $\gamma \approx -{16}^{°}$ agrees best to the data.