Stability, or pressure stability, is the ability to maintain a constant pressure given a constant gas load. Different cathode materials provide varying levels of stability and pumping speed. Titanium cathodes remain stable when pumping reactive gases. Unfortunately, while pumping noble gases (argon being the most typical) with titanium cathodes, instabilities result. Noble gases do not react with titanium and are simply buried in titanium cathodes. This increasingly happens in the centres of the cells. Upon added sputtering of the cathode, there is the potential for these gases to be released back in the vacuum environment. Alternatively, cathodes made of tantalum are used. Although they do not retain noble gases, they do aid in stable pumping. Ions contacting the tantalum cathode are reflected back into the ion pump as high energy neutrals. These molecules usually are repelled with enough energy to bury themselves in other areas of the pumping element (e.g. within the anode structure). At that location no or only very little sputtering occurs and therefore once implanted atoms are not released again and remain in the solid.