Measurements of the Electric Quadrupole Moment of Nb and Zr Isotopes with Modulated Adiabatic Fast Passage after Recoil Implantation into hcp Co

Abstract

We report a new method for the measurement of quadrupole moments of radioactive nuclei which were not accessible with other techniques until now: modulated adiabatic fast passage on oriented nuclei (MAPON) after recoil implantation into hcp Co. Quadrupole moments of Zr and Nb isotopes were determined which are interesting in the context of the proton and the neutron effective charge at A ϳ 90. [S0031-9007(97) PACS numbers: 21.10. Ky, 76.60.Gv, 76.80. + y Electric quadrupole moments of nuclear states are a measure for the nonsphericity of the electric charge density, which may originate from single-particle and collective nuclear properties. Thus, measurements of electric quadrupole moments may yield essential nuclear structure information. The standard method for the measurement of electric quadrupole moments of radioactive nuclei is laser spectroscopy (LS). Until now, the application of LS has been restricted mainly to nonrefractory elements. This is due to the fact that at on-line mass separators-such as ISOLDE at CERN-radioactive ion beams with high intensity are available only for nonrefractory elements. For only selected systems, laser desorption of daughter isotopes-which are not available in a primary beamhas been applied successfully; see, e.g., Ref.[1]. In recent years, diverse attempts have been started to improve the intensities of refractory elements at mass separators, such as the use of He-jet systems after nuclear reactions [2] or the development of on-line laser ion sources [3]. However, the intensities as obtained until now are still relatively small. Therefore another method would be desirable, with which quadrupole moments of radioactive nuclei can be measured, independent of the fact whether or not these are refractory. Here we introduce such a method, which is applicable to all elements throughout the nuclear chart: modulated adiabatic fast passage on oriented nuclei (MAPON) after recoil implantation into hcp-Co single crystals. The MAPON technique was introduced by Callaghan et al. [4,5]. It allows the determination of quadrupole splittings which are small in comparison to the magnetic inhomogeneous broadening. The power of the MAPON technique has not been recognized for long. This is due to the fact that mostly polycrystalline Fe had been used as host lattice for MAPON measurements. Because of the cubic symmetry the quadrupole interaction-it originates from an unquenched orbital momentum-is small. In addition, it is inhomogeneously broadened, the broadening being of the same order of magnitude as the total splitting. This is due to the fact that, as detected recently [6], the electric field gradient (EFG) at the impurity site in Fe depends strongly on the direction of magnetization with respect to the crystallographic axes. Therefore single crystal hcp Co as host lattice was supposed to be superior: The EFG originating from the noncubic lattice symmetry is well defined and, in addition, much larger than the EFG in Fe. In this case, however, the radioactive nuclei have to be implanted. ...