  The cross section for production of Phi mesons in proton-nucleus reactions is
calculated as a function of the target mass. The decay width of the Phi meson
is affected by the change of the masses of the Phi, K+ and K- mesons in the
medium. A strong attractive K- potential leads to a measurable change of the
behavior of the cross section as a function of of the target mass. Comparison
between the kaon and electron decay modes are made.
