Reduced transition probabilities along the yrast line in ${}^{166}\mathrm{W}$

Lifetimes of excited states in the yrast band of the neutron-deficient nuclide ${}^{166}\mathrm{W}$ have been measured utilizing the DPUNS plunger device at the target position of the JUROGAM II $\gamma$-ray spectrometer in conjunction with the RITU gas-filled separator and the GREAT focal-plane spectrometer. Excited states in ${}^{166}\mathrm{W}$ were populated in the ${}^{92}\mathrm{Mo}({}^{78}\mathrm{Kr},4p$) reaction at a bombarding energy of 380 MeV. The measurements reveal a low value for the ratio of reduced transitions probabilities for the lowest-lying transitions $B(E2;4^{+}\to 2^{+})/B(E2;2^{+}\to 0^{+})=0.33\left(5\right)$, compared with the expected ratio for an axially deformed rotor ($B_{4/2}$ = 1.43).

Figure 1

$\gamma$-ray coincidences correlated with implanted nuclear recoils detected in the DSSDs of the GREAT spectrometer. Spectra for three different target-to-degrader distances of (a) 5, (b) 1000, and (c) 8000 $\mu \mathrm{m}$ with the ten JUROGAM II Ge detectors at ${133}^{\circ }$. Each spectrum is in coincidence with the 480 keV (${12}^{+}\to {10}^{+}$) transition, which was chosen to eliminate the contributions from energy doublets in the side bands. The fully Doppler-shifted (s) and degraded (d) components of the 252-, 326-, and 424-keV $\gamma$-ray transitions are labeled.

Figure 2

(a) Partial level scheme showing levels and transitions in the yrast band of ${}^{166}\mathrm{W}$. The width of the arrows is proportional to the relative intensities of the $\gamma$-ray transitions. The levels are labeled by their spins, parities, and excitation energies. All energies are stated in keV. (b) Partial level scheme for the yrast band in ${}^{166}\mathrm{W}$ with the levels labeled by their spins, parities, and mean lifetimes in picoseconds. The width of the arrows is proportional to the reduced transition probabilities of the $\gamma$-ray transitions with values given in Weisskopf units (Wu).

Figure 3

Lifetime determination using the differential decay curve method (DDCM). The individual mean lifetimes and error bars are represented as the solid and dashed horizontal lines, respectively for (a) the $2^{+}$ state and (b) the $4^{+}$ state in ${}^{166}\mathrm{W}$. The decay curves extracted from coincidence spectra gated above the (c) $2^{+}$ state and (d) $4^{+}$ state. The line drawn through the experimental points is the fit to the decay curve. All spectra are collected from the ten detectors at ${133}^{\circ }$. (e) The numerator of Eq. (1) and the derivative of the decay curve for the $2^{+}$ state and (f) the $4^{+}$ state.

Figure 4

(a) Ratios of the $4^{+}$ and $2^{+}$ state excitation energies ($E_{(4/2)}$) for the even-mass W isotopes. Measured reduced transition probabilities $B\left(E2\right)$ for the W isotopes for (b) the $2^{+}\to 0^{+}$ transitions, (c) the $4^{+}\to 2^{+}$ transitions, and (d) the $6^{+}\to 4^{+}$ transitions. (e) Theoretical reduced transition probabilities $B\left(E2\right)$ for the $2^{+}\to 0^{+}$ transitions in the even-mass W isotopes. Model predictions using the finite-range droplet model (open diamonds), Woods-Saxon model (filled diamonds), and dynamical microscopic model (open circles) are displayed. The values for ${}^{166}\mathrm{W}$ correspond to neutron number $N=92$. Data for the heavier isotopes were obtained from Refs. [4, 25, 26, 27, 28]. All $B\left(E2\right)$ values are given in Weisskopf units (Wu).

Figure 5

Ratios of reduced transition probabilities measured for the even-mass W isotopes (a) $B$($E2:4^{+}\to 2^{+})/B(E2:2^{+}\to 0^{+}$), (b) $B$($E2:6^{+}\to 4^{+})/B(E2:2^{+}\to 0^{+}$), and (c) $B$($E2:6^{+}\to 4^{+})/B(E2:4^{+}\to 2^{+}$). In each case, the theoretical ratio for a collective rotor is indicated by a dashed line [36].