Collective excitations in the transitional nuclei ${}^{163}\mathrm{Re}$ and ${}^{165}\mathrm{Re}$

Excited states in the neutron-deficient nuclei ${}_{75}^{163}{\mathrm{Re}}_{88}$ and ${}_{75}^{165}{\mathrm{Re}}_{90}$ were populated in the ${}^{106}{\mathrm{Cd}(}^{60}\mathrm{Ni},p2n\gamma )$ and ${}^{92}{\mathrm{Mo}(}^{78}\mathrm{Kr}$, $3p2n\gamma )$ fusion-evaporation reactions at bombarding energies of 270 and 380 MeV, respectively. γ rays were detected at the target position using the JUROGAM spectrometer while recoiling ions were separated in-flight by the RITU gas-filled recoil separator and implanted in the GREAT spectrometer. The energy level schemes for ${}^{163}\mathrm{Re}$ and ${}^{165}\mathrm{Re}$ were identified using recoil-decay correlation techniques. At low spin, the yrast bands of these isotopes consist of signature partner bands based on a single $\pi h_{11/2}$ quasiproton configuration. The bands display large energy splitting consistent with the soft triaxial shape typical of transitional nuclei above $N=82$. The configurations of the excited states are proposed within the framework of the cranked shell model.

Figure 1

(a) $\gamma$ rays following the ${}^{60}\mathrm{Ni}+{}^{106}\mathrm{Cd}$ reaction and correlated with any recoil implantation detected in the GREAT DSSD located at the focal plane of the RITU separator. Prominent $\gamma$ rays originating from ${}^{163}\mathrm{Ta}$ [24] and ${}^{163}\mathrm{W}$ [25] are labeled with their transition energies. (b) $\gamma$ rays correlated with recoil implantations followed by the characteristic $\alpha$ decay from the ${\text{11/2}}^{-}$ isomer in ${}^{163}\mathrm{Re}$ within the same DSSD pixel of the GREAT spectrometer. The recoil-$\alpha$ correlation time was limited to 214 ms.

Figure 2

Level scheme deduced for ${}^{163}\mathrm{Re}$. Transition and level excitation energies are given in keV and their relative intensities are proportional to the widths of the arrows. Dashed lines and parentheses indicate tentative assignments. The excitation energy of the ${\text{11/2}}^{-}$ state was measured to be 115.1(40) keV in previous work by Davids et al. [26].

Figure 3

$\gamma$-ray spectra extracted from a $\gamma \gamma$ coincidence matrix correlated with recoil implantations followed by the $\alpha$ decay from the ${\text{11/2}}^{-}$ isomer in ${}^{163}\mathrm{Re}$ detected within the same DSSD pixel of the GREAT spectrometer. The time for recoil-decay correlations was limited to 214 ms. Spectra show coincidences with (a) the 577-keV transition, (b) the 688-keV transition, and (c) the 466-keV transition.

Figure 4

$\gamma$-ray spectra extracted from a $\gamma \gamma$ coincidence matrix correlated with recoil implantations followed by the $\alpha$ decay from the ${\text{11/2}}^{-}$ isomer in ${}^{163}\mathrm{Re}$ detected within the same DSSD pixel of the GREAT spectrometer. The time for recoil-decay correlations was limited to 214 ms. Spectra show coincidences with (a) the 733-keV transition, (b) the 792-keV transition, and (c) the 251-keV transition.

Figure 5

$\gamma$-ray spectra extracted from a $\gamma \gamma$ coincidence matrix correlated with recoil implantations followed by the 5520-keV unresolved $\alpha$ decays from ${}^{162}\mathrm{W},{}^{165m}\mathrm{Re}$, and ${}^{166}\mathrm{Re}$ detected within the same DSSD pixel of the GREAT spectrometer. The time for recoil-decay correlations was limited to 5 s. Spectra show coincidences with (a) the 337-keV transition, (b) the 202-keV transition, and (c) the 672-keV transition. Transitions established in ${}^{165}\mathrm{Re}$ (see Fig. 6) are labeled.

Figure 6

Energy spectra of $\alpha$ particles from the ${}^{78}\mathrm{Kr}+{}^{92}\mathrm{Mo}$ reaction. (a) $\alpha$ particles detected between 500 and 5000 ms after a recoil implantation in the same pixel of the GREAT DSSD. (b) Second-generation $\alpha$ decays following a recoil implantation and the subsequent decay of the 5520-keV unresolved $\alpha$-decay peaks. (c) The same conditions as for (b), with the additional constraint that the decay chain is in delayed coincidence with the 449-keV transition in ${}^{162}\mathrm{W}$ detected at the target position. (d) The same conditions as for (b), with the additional constraint that the decay chain is in delayed coincidence with the 539-keV transition detected at the target position. The second-generation $\alpha$ decay of ${}^{161m}\mathrm{Ta}$ indicates that the 539-keV $\gamma$ ray is emitted by ${}^{165}\mathrm{Re}$.

Figure 7

Level scheme deduced for ${}^{165}\mathrm{Re}$ from an $\alpha$(5520 keV)-correlated $\gamma \gamma$ matrix. Transition energies are given in keV and their relative intensities are proportional to the widths of the arrows. Dashed lines and parentheses indicate tentative assignments. The excitation energy of the ${\text{11/2}}^{-}$ state was measured to be 48(40) keV in previous work by Poli et al. [34].

Figure 8

Evolution of low-lying excited states in neutron-deficient transitional Re isotopes as a function of neutron number. Excitation energies of the ${\text{9/2}}^{-}$, ${\text{13/2}}^{-}$, ${\text{15/2}}^{-}$, ${\text{17/2}}^{-}$, and ${\text{19/2}}^{-}$ levels are given in keV and stated relative to the ${\text{11/2}}^{-}$ state.

Figure 9

Staggering parameter $S\left(I\right)$ as a function of spin $I$ for [514]${\text{9/2}}^{-}$ bands in the neutron-deficient odd-$A$ rhenium isotopes ${}^{163}\mathrm{Re}$ (band 1), ${}^{165}\mathrm{Re},{}^{167}\mathrm{Re}$ [11], ${}^{169}\mathrm{Re}$ [35, 36], and ${}^{171}\mathrm{Re}$ [37, 38]. Filled (open) symbols represent the $\alpha =-1/2$ ($\alpha =+1/2$) signatures, respectively.

Figure 10

Calculated total Routhians $E^{\prime }$ as a function of $\gamma$ deformation for single- and three-quasiparticle configurations in (a) ${}^{163}\mathrm{Re}$ and (b) ${}^{165}\mathrm{Re}$. Routhians are calculated at a rotational frequency of 0.2 MeV and assume a prolate-oblate energy difference $V_{\mathrm{po}}=-0.4$ MeV and quadrupole deformation parameters of ${\varepsilon }_2=0.117$ and ${\varepsilon }_2=0.142$ for ${}^{163}\mathrm{Re}$ and ${}^{165}\mathrm{Re},$ respectively. Deformation parameters are taken from Ref. [50]. Harris parameters are fixed at ${\mathcal{J}}_0=26{\hslash }^2{\mathrm{MeV}}^{-1}$ and ${\mathcal{J}}_1=32{\hslash }^4{\mathrm{MeV}}^{-3}$ [44]. The $e,f,eEF,fEF,eAB$, and $fAB$, configurations have negative parity [dark-gray (blue) lines], while the $eAE,fAE,eAF$, and $fAF$ configurations have positive parity [light-gray (red) lines]. Solid (dashed) lines represent the $\alpha =-1/2$ ($\alpha =+1/2$) signature, respectively.