ABSTRACT
Radioactive sources are favored for use in IMS analyzers because they provide stable and reliable operation, with ionization chemistry that is well suited for most current applications of IMS. Furthermore, radioactive foils do not require an external power supply and have no moving parts or maintenance requirements. At present, the most widely used and best understood of all ion sources for IMS is still the long-favored radioactive 63Ni source, which is also widely used in electron capture detectors (ECDs) for gas chromatography (GC). The preferred radioactive source is 10 mCi (3.7*108 Bq) of 63Ni coated as a thin layer on a metal strip, generally nickel or gold.2 The maximum energy of the electrons emitted from the 63Ni source
is 67 keV, with an average energy near 17 keV. Almost all the energy of this source is dissipated in air at an ambient pressure within 10 to 15 mm, from the surface of the metal, establishing guidelines for the optimum diameter when the source is used as a cylinder, a common geometry in IMS analyzers.3 The electrons emitted from the 63Ni produce ions and secondary electrons (see Chapter1), and this process is repeated until the secondary electrons are no longer energetic enough to ionize the gas molecules of the supporting atmosphere. The formation of an ion pair requires about 35 eV, so each beta particle emitted from the source could ideally produce on average about 250 ions if we assume that 50% of the electrons are directed into the foil.4,5 Negative ions may be produced also through electron attachment processes, which, in most cases, proceed efciently when the electrons are at thermal energy, thus further increasing the ion yield.
