Novel Compact Cesium Ion Source for Mass Spectrometry

In secondary ion mass spectrometers analysis of solid materials is accomplished by a sputtering process in which an energetic beam of primary ions is directed at the target to be analyzed. Each impact of a primary ion ejects a small number of atoms from the target surface. A fraction of these ejected atoms are ionized in the ejection process and can be mass-analyzed and therefore identified and quantified in a secondary mass spectrometer. The efficiency of secondary ion formation can be increased by using chemically active primary ion species such as Cesium (+) which are used to enhance negative ion yields.Cesium (+) ions are typically produced by a thermal surface ionization process in which cesium vapor impacts upon a heated tungsten surface causing the cesium atoms to evaporate as ions. There are two designs in current use. In the older design, vapor from cesium metal in a heated stainless steel reservoir diffuses through a heated tungsten frit and ions are extracted from the front surface of the frit. The performance of this source is limited by a number of design factors. In a newer design cesium vapor from the decomposition of heated cesium chromate flows through an annular opening onto a heated flat solid tungsten ionizer. Here the cesium flow rate can be greater. However, manufacturing tolerances are tight and quality control appears to be a problem with this design. Additionally, source lifetime is limited to a few hundred hours.Researchers at Arizona State University have developed a compact ion source, which produces cesium ions from a solid cesium salt by thermal surface ionization from a novel design configuration. The primary novel features of the source include compactness and consequent low power operation; ease and low cost of fabrication; long source lifetime from a small charge of cesium salt; and good ion optical properties.The source has the following benefits: The orifice ionizer produces a extraordinarily high flux density of cesium atoms resulting in a high current density of Cs+ ions without the wastage that accompanies the tungsten frit ionizer. The heat-resistant ionizer portion of this new source is very inexpensive to fabricate due to the materials it is made of. Additionally, a resistance heater can be wound directly into the source.