g-factor of the 7/2+ isomeric bandhead in 175 W

Considerable effort is presently devoted to the investigation of the high-K isomers of multi-quasiparticle intrinsic structure systematically found in the deformed nuclei with Z=72-76 of the A ≅ 180 mass region. The configuration assignments for these isomers are based on measured static moments, as well as, on experimental branching ratios in the associated bands, from which (gK - gR)/Q0 are derived. In the multi-quasiparticle state g-factor calculations, values taken from neighbouring odd-mass nuclei are generally used for the proton and neutron deformed single-particle g-factors. A good knowledge of these quantities is required for reliable high-K state g-factor evaluations. In the present work we report on the g-factor measurement for the low-lying Jπ = 7/2+ isomer bandhead in 175 W described by the neutron 7/2+[633] Nilsson orbital. The isomeric state was populated in the 164 Dy(16 O,5n)175 W reaction using the 83 MeV pulsed 16 O beam (pulse width 1.5 ns, repetition period 800 ns) delivered by the LNL XTU-Tandem. The target consisted of 0.5 mg/cm2 metallic 164 Dy on thick Pb backing which stopped the recoiling 175 W nuclei and the 16 O beam. The target was placed in an external magnetic field of 27.2(6) kG whose direction was periodically reversed. The 7/2+ isomeric state with T1/2 = 216(6) ns and Ex=234.9 keV de-excites by a dipole transition of 130.9 keV to the 5/2- level. The angular distribution of the 130.9 keV gamma-ray has been observed time-differentially by using two planar Ge detectors placed at ± 135 angle with respect to the beam direction. The background corrected time spectra I(t,θ) obtained for the magnetic field direction up and down were used to form the experimental modulation ratio Rexp(t)=[I↑(t,θ) - I↓(t,θ)]/[I↑(t,θ) + I↓(t,θ)]. The modulation pattern revealed Larmor oscillations with an amplitude strongly attenuated in time. The observed damping of the anisotropy has been attributed to quadrupole interactions with the electric field gradients (EFG) created by the radiation induced defects near the 175 W nuclei in the cubic lattice of the metallic Pb host. Similar effects have been previously reported for Po isomers implanted in Pb. In the least-squares fit we used the expression Rtheo(t) = b2 E2(IωQ t) cos2(θ-ωL t) derived for the combined interaction of the externally applied magnetic field and the weak EFG due to radiation damage. A value of g=--0.187(6) was determined for the g-factor. Up to now g-factors were reported for the 7/2+[633] neutron state only in four rare-earth nuclei. The present result is the first g-factor determination for this Nilsson orbital in the Z=72-76 region characterized by the occurrence of high-K multi-quasi-particle states. (authors)