  We report the first measurement of the diffusion coefficients of francium and
rubidium ions implanted in a yttrium foil. We developed a methodology, based on
laser spectroscopy, which can be applied to radioactive and stable species, and
allows us to directly take record of the diffusion time. Francium isotopes are
produced via fusion-evaporation nuclear reaction of a 100 MeV 18-O beam on a Au
target at the Tandem XTU accelerator facility in Legnaro, Italy. Francium is
ionized at the gold-vacuum interface and Fr+ ions are then transported with a 3
keV electrostatic beamline to a cell for neutralization and capture in a
magneto-optical trap (MOT). A Rb+ beam is also available, which follows the
same path as Fr+ ions. The accelerated ions are focused and implanted in a 25
um thick yttrium foil for neutralization: after diffusion to the surface, they
are released as neutrals, since the Y work function is lower than the alkali
ionization energies. The time evolution of the MOT and the vapor fluorescence
signals are used to determine diffusion times of Fr and Rb in Y as a function
of temperature.
