Operation - Primary beams
The primary beams most commonly used in SHRIMP are composed of oxygen and cesium ions. These primary species are used because they produce a chemical enhancement of ion yields. The electropositive Cs enhances yields of electronegative elements like O and S, while electronegative oxygen enhances yields of metals. The most efficient setup is to use a negative-ion primary beam [O] to sputter positive secondaries, and a positive ion beam [Cs] to sputter negative secondaries.
Oxygen ions are produced in a duoplasmatron ion source. This source is based around an electric glow discharge between a hollow cathode and an anode plate. The electrons in the discharge attach themselves to oxygen atoms and molecules to form the negative ion beams.
Cesium ions are produced in a Cs gun. A Cs-bearing charge is heated to evaporate the cesium which is then passed through a superheated frit, which causes electrons to be lost from the atom and hence ionizes the cesium.
Kimball Physics Inc. Cs gun ion source