We report a study of implantation and optical spectroscopy of foreign atoms in a solid-helium matrix. With a sample of more than ${10}^9$ Ba atoms in 1 ${\mathrm{cm}}^3$, a storage time in excess of 5 hours has been demonstrated. Laser-induced fluorescence studies favor the assumption that the isotropy of the trapping site for the implanted atoms is preserved during the liquid-to-solid phase transition.

• Received 13 September 1993

DOI:https://doi.org/10.1103/PhysRevB.49.3645