First identification of rotational band structures in ${}_{75}{}^{166}\mathrm{Re}_{91}$

Excited states in the odd-odd, highly neutron-deficient nucleus ${}^{166}\mathrm{Re}$ have been investigated via the ${}^{92}\mathrm{Mo}({}^{78}\mathrm{Kr},3p1n){}^{166}\mathrm{Re}$ reaction. Prompt $\gamma$ rays were detected by the JUROGAM II $\gamma$-ray spectrometer, and the recoiling fusion-evaporation products were separated by the recoil ion transport unit (RITU) gas-filled recoil separator and implanted into the Gamma Recoil Electron Alpha Tagging spectrometer located at the RITU focal plane. The tagging and coincidence techniques were applied to identify the $\gamma$-ray transitions in ${}^{166}\mathrm{Re}$, revealing two collective, strongly coupled rotational structures, for the first time. The more strongly populated band structure is assigned to the $\pi h_{11/2}\left[514\right]9/2^{-}\otimes \nu i_{13/2}\left[660\right]1/2^{+}$ Nilsson configuration, while the weaker structure is assigned to be built on a two-quasiparticle state of mixed $\pi h_{11/2}\left[514\right]9/2^{-}\otimes \nu \left[h_{9/2}{f}_{7/2}\right]3/2^{-}$ character. The configuration assignments are based on the electromagnetic characteristics and rotational properties, in comparison with predictions from total Routhian surface and particle-rotor model calculations.

Figure 1

The level scheme of ${}^{166}\mathrm{Re}$ deduced from the present work. The energy is in keV. The states and $\gamma$-ray transitions below ($6{}^{+}$) have been described in Ref. [15].

Figure 2

(a) $\gamma$-ray energy coincidence spectrum obtained from gating on the 121-keV $\gamma$-ray transition in band (1) from the clover data. (b) $\gamma$-ray energy coincidence spectrum obtained from a gate on the 225-keV $\gamma$-ray transition in band (1) taken at target-degrader distances from 1000 to $8000 \mu \mathrm{m}$. (c) Spectrum obtained from the triple-coincidence cube using a sum of double gates on the 212/225-keV and 301/296-keV $\gamma$-ray transition pairs in band (1). $+,\gamma$-ray transitions not assigned in the level scheme in Fig. 1.

Figure 3

(a) $\gamma$-ray energy coincidence spectrum from the clover detectors obtained by gating on the 126 keV $\gamma$-ray transition in band (2). (b) $\gamma$-ray energy coincidence spectrum obtained from a gate on the 267-keV $\gamma$-ray transition in band (2) taken at target-degrader distances from 1000 to $8000 \mu \mathrm{m}$. $+,\gamma$-ray transitions not assigned in the level scheme in Fig. 1. The peaks marked with stars are corresponding to transitions in band (1).

Figure 4

The TRS calculations for ${}^{166}\mathrm{Re}$ at $\hslash \omega =0.0$, 0.2, and 0.4 MeV (left, middle, and right column, respectively) with the following proton and neutron (parity, signature) levels closest to the Fermi surface blocked: $\pi (-,-1/2) \nu (+,+1/2)$, top row; $\pi (-,-1/2) \nu (-,+1/2)$, middle row; and $\pi (+,-1/2) \nu (+,+1/2)$, bottom row. The energy difference between successive contour curves is 100 keV. The solid circles in the figures denote the minima of the surfaces.

Figure 5

Experimental quasiparticle (a) Routhians and (b) aligned angular momenta for bands (1) and (2) in ${}^{166}\mathrm{Re}$, as well as the systematic comparisons in ${}^{164,165}\mathrm{W}$ [2, 3] and ${}^{167}\mathrm{Re}$ [31].

Figure 6

Cranked Routhians using the universal Woods-Saxon potential for (a) quasineutrons and (b) quasiprotons in ${}^{166}\mathrm{Re}$ with the deformation parameters ${\beta }_2=0.17,{\beta }_4=0.009$, and $\gamma =-1.6^{\circ }$ from TRS predictions. Different lines represent different parity and signature $\left(\pi ,\alpha \right)$: solid (+, +1/2); dotted (+, $-1/2$); dot-dashed $(-,+1/2)$; dashed $(-,-1/2)$. The labels for the configurations are given in Table 2.

Figure 7

$B\left(M1\right)/B\left(E2\right)$ ratios for bands (1) and (2) in ${}^{166}\mathrm{Re}$. Experimental values are compared with theoretical predictions from semiclassical and particle-rotor model calculations. For band (2) the semiclassical calculations with and without the signature-dependent term in Eq. (3) are shown. See text for further details.

Figure 8

Calculated energy staggering using the PRM with and without residual interaction, as a function of spin $I$ for bands (1) and (2) in ${}^{166}\mathrm{Re}$. Solid black circles represent the observed energy staggering.