The magnetic moment of the first excited state of 19O

Abstract

The magnetic decoupling method, applied to highly ionized nuclear-excited 180 atoms, has yielded for the 198 MeV 2 + state the value Jg I = 0.35 ± 0.04. In conjunction with the recently determined negative sign, this value agrees well with weak/-/coupling predictions.The study of hyperfine interactions in isolated ions has been extended in recent years to the highest ionization states of light atoms (Z = 6 -12) [1,2]. The large magnetic fields associated with the single-electron ionic ground-state [3] have enabled the determination of nuclear magnetic moments for levels with lifetimes in the 10 -12 s range [4].Indeed, the g-factor of the 180 (2 + ) level was the first to be determined in such an environment [5 ]. In that measurement, the time-integral version of the PAC technique was applied to magnetically separated 1807+ ions recoiling into vacuum. As the fraction of ions in the ground state (with which the large HF field of 86 MG is associated [3] ) is not very well known, the g-factor could only be determined within rather broad limits (t ql = 0.20 -0.36). As these limits depart considerably from the/-/coupling prediction of -0.76 for 2 a pure d5/2 neutron configuration, a determination of the sign and precise absolute magnitude were called for. * On leave from the Nuclear Physics Department, WeizmannInstitute, Rehovot, Israel.A recent measurement utilizing the transient field effect on recoil in polarized iron has shown the sign to be negative [6].The following describes a PAC measurement of Igl by means of the magnetic decoupling method, in which the HF-interacting ionic system is subjected to an external magnetic field, applied along the beam direction. A preliminary result has been reported in ref. [16]. In the field range intermediate between free HF coupling and the Paschen-Back effect (which corresponds to an unperturbed correlation) the perturbation is governed by the ratio of the nuclear and electron magnetic moments. The method has recently been successfully implemented in determining the gfactor of the 160 (3-) level in ions of low charge [7]. It has the advantage over the time-integral version of PAC (as far as data interpretation is concerned) of reduced sensitivity [4] to the abundances of the perturbing electron configurations in the ionic ensemble. The 180 first 2 + level was populated in the reaction 4He(180, a)180 using a 1806+ beam of 200 nA at a bombarding energy of 44 MeV from the K61n FN 143