Stable analogues of a toxic fission product (Cs) were introduced into magnesium aluminate spinel single crystals by room temperature ion implantation. Changes in the atomic concentration of Cs atoms upon annealing at high temperature were monitored by Rutherford backscattering (RBS) experiments using a nuclear microprobe. Results show that Cs atoms are inhomogeneously distributed in the crystal after annealing; they are collected in islands with a size of a few tens of micrometers. The remaining crystal exhibits a very small Cs concentration due to the exfoliation of the implanted layer.