Electron multipliers having improved gain stability

An electron multiplier having:

• two or more electron emissive surfaces, a first of the two or more electron emissive surfaces having a first composition, a second of the two or more electron emissive surfaces having a second composition, wherein the first and second compositions differ so as to together limit or overcome an acute gain effect on the electron multiplier due to exposure of the two or more electron emissive surfaces to water molecules,
• and/or
• a single electron emissive surface of mixed composition comprising a first composition component and a second composition component, wherein the first and second composition components differ so as to together limit or overcome an acute gain effect on the electron multiplier due to exposure of the electron emissive surface to water molecules,
• and wherein the first and second compositions differ in a manner such that when the two or more electron emissive surfaces are exposed to water molecules the secondary electron yield of the first electron emissive surface increases, and the secondary electron yield of the secondary electron emissive surface decreases,
• and/or
• wherein the first and second composition components differ in a manner such that when the single electron emissive surface is exposed to water molecules the secondary electron yield contributed by the first composition component increases and the secondary electron yield of the second composition component decreases.