LEMS: application of the method to study the static quadrupole moment of the isomer in 179W

Abstract

The method of the level mixing spectroscopy (LEMS) was applied for the first time for the study of the static quadrupole moments of high-K isomers in the A ≈ 180 mass region. Results from a preliminary experiment for the static quadrupole moment of the 35 2 − (750 ns) isomer in 179 W give a limit for its upper value Q < 7b, corresponding to a quadrupole deformation of β 2 < 0.343.An important parameter to learn more about the nuclear deformation at high spin is the static quadrupole moment. The method of the level mixing spectroscopy (LEMS) [1] has proven to be a powerful tool to study the quadrupole interaction of a nuclear isomeric state with the electric field gradient (EFG) of a host material, especially at high spin. Recently the quadrupole moments of a number of high-spin isomers have been determined in the lead region [2,3,4].The LEMS method can be explained as follows: an aligned ensemble of highly excited nuclei is produced in a heavy-ion fusion-evaporation reaction. The reaction products recoil into a host material. Usually this is a material which has an axially symmetric EFG, V zz , poly-or single crystal. Single crystals are mounted so that the EFG axis is misaligned with the beam axis by a large angle, β > 35 • . The orientation of the isomeric nuclear states is then perturbed by the combined interaction: magnetic dipole interaction provided by an on-line superconducting magnet, whose field is parallel to the beam line, and an electric quadrupole interaction provided by the non-cubic environment of the lattice in which the nuclei are stopped. The anisotropy of the γ-radiation is measured as a function of the external magnetic field. The result is called LEMS curve. Three regimes can be distinguished. At zero and low magnetic fields only the quadrupole interaction is present and perturbs the initial orientation of the nuclear spins as the EFG axis is not parallel to the alignment axis of the spins. The spins are more isotropically distributed and consequently the γ-rays more isotropically emitted. At large magnetic fields we can neglect the quadrupole interaction. The nuclear spins make a Larmor precessionaround the magnetic field axis. Consequently, the orientation of the ensemble does not change and the initial anisotropy is measured. In the intermediate region where both the quadrupole and the magnetic interaction play a significant role, there is a smooth transition from the low magnetic field regime to the high magnetic field regime. The magnetic field at which the transition occurs is sensitive to the ratio eQV zz /g, where Q and g are the nuclear quadrupole moment and g-factor, respectively.So, if the magnetic moment (or the g-factor) of an isomeric state and the EFG of the host material are known, we can deduce the static quadrupole moment by fitting the experimental data with a theoretical curve which accounts for the above features.We report first results about the application of the method for the study of the quadrupole moments of high-spin high-K isomers in the A ≈ 180 mass region.Isomer...

Study of the static quadrupole moment of theK= 35/2 isomer in

“…Despite the physical importance of the quadrupole moments of high-K isomers, only a few of them have been measured at this point. Experimental techniques include Coulomb excitation [1], TDPAD [2] and level mixing spectroscopy (LEMS) [3]. In the lifetime range where LEMS is sensitive (50 ns-500 ms), only one quadrupole moment measurement was prior to the present experiment on the 179 W(K = 35/2) isomer.…”