Static quadrupole moment of the Kπ = 14+ isomer in 176W

The investigation of high-K isomeric states in the deformed nuclei of the A∼180 region has found renewed interest in recent years. Much experimental and theoretical work was devoted to understand the mechanisms which govern their decay to lower-lying states, particularly the anomalous strong decays to low-K states. Other questions of great importance are the quenching of the pairing correlations and the shape polarization effects in the high-seniority multi-quasiparticle excitations. Our interest focused on the 41 ns Kπ=14+ 3746 keV isomeric state with anomalous decay in 176 W. On the basis of a precise g-factor measurement we assigned to this isomer a pure four-quasiparticle configuration, composed by two protons in the 7/2+[404] and 9/2-[514] orbitals and two neutrons in the 7/2+[633] and 5/2-[512] orbitals. In the present work the measurement of its static quadrupole moment has been performed. Prior to our experiment, static quadrupole moments have been measured only for three high-K isomeric states of seniority ≥ 4 in the A∼180 region: 16+ in 178 Hf, 35/2- in 179 W and 25+ in 182 Os. A deformation very similar to that of the ground state has been deduced for the 16+ isomer in 178 Hf, while for the high-K isomers in 179 W and 182 Os significantly smaller deformations were reported. The quadrupole interaction of the 14+ isomeric state in 176 W has been investigated in the electric field gradient (EFG) of the polycrystalline lattice of metallic Tl by applying the time-differential perturbed angular distribution method. For W impurities in Tl host the EFG strength and its temperature dependence have been recently reported. The isomer was populated in the 164 Dy(16 O,4n)176 W reaction using a 83 MeV 16 O pulsed beam (pulse width 1.5 ns, repetition period 800 ns) delivered by the XTU-Tandem of Laboratori Nazionali di Legnaro. The target consisted of 0.5 mg/cm2 metallic 164 Dy on thick Tl backing in which both the recoiling 176 W nuclei and the projectiles were stopped. The target has been heated at 464 K in a special oven. This temperature was chosen in order to ensure an EFG strength convenient for the observation of the quadrupole interaction pattern on time scale given by the isomeric lifetime. The gamma-rays were detected by Ge detectors of 25% efficiency placed at the angles 0 angle and 90 angle with respect to the beam direction. The 240, 351, 440, and 558 keV gamma-lines of the 176 W yrast band which collects practically all the isomeric decay branches have been analysed. The experimental modulation ratio is presented together with the least-squares fit. A quadrupole frequency νQ=92(10) MHz has been determined, which corresponds, with the EFG calibration, to a spectroscopic quadrupole moment Qs=6.3(1.4) eb. Assuming the strong coupling scheme, a value of Q0 7.7(1.8) eb is obtained for the intrinsic quadrupole moment of the 14+ isomer. This value fits very well into the systematics of the ground-state quadrupole moments of W nuclei what indicates that no shape polarization occurs in the multiquasiparticle structure of 176 W. (authors)