Evolution of the Secondary Emission Efficiencies of various materials measured in the CERN SPS secondary beam lines

About 180 Secondary Emission Monitors (SEM) are used in the transfer lines to and from the CERN SPS for the measurement of profile, position and intensity of injected and ejected beams. Important changes, up to 50%, in the Secondary Emission Efficiencies (SEE) of Aluminium have been observed in the region of the foil where the beam impinges. Tests have been performed with different types of foils: Aluminium, Titanium, Gold, Gold coated Aluminium and Titanium. The foils have been exposed to a large integrated number of particles, close to 1x1020 p/cm2 over two years, and their SEE measured regularly. The measurements of the long-term variations of the SEE of the various foils are presented.