Magnetic response of closed-shell +/-1 nuclei in Dirac-Hartree approximation

We formulate a systematic approximation to the Dirac-Hartree description of closed-shell +/-1 nuclei using the closed-shell basis. Expectation values of one-body operators include random-phase-approximation-like loop corrections to the single particle (or hole) results. We use a nonspectral form of the Hartree propagator which allows inclusion of positive- and negative-energy continuum contributions exactly. The loop corrections quench the single-particle isoscalar magnetic moments back to the Schmidt values, resulting in good agreement with experiment. Elastic M1 electron scattering form factors are also quenched at low q. However, the loop corrections vanish at q~=1.2 fm^-1 and change sign so as to enhance the single-particle result at still higher momentum transfers. This raises the possibility that experimental signatures of the strong relativistic nuclear dynamics characteristic of Dirac-Hartree models may be present in elastic M1 form factors for closed-shell +/-1 nuclei.