The Nuclear Science Laboratory maintains
and operates a SNICS II (Source of Negative Ions by Cesium Sputtering)
sputter ion source, manufactured by National Electrostatics Corp. (NEC).
This source has been in operation since 1989, and is the primary ion
source for the FN Tandem accelerator, producing all the ion beams for
injection into the FN Tandem with the exception of helium beams.
The theory of operation of the SNICS II sputter ion source is actually quite simple. A reservoir of cesium is heated to approximately 120 °C, so that cesium vapor is formed. This vapor rises from the reservoir in vacuum to an enclosed region between the cathode, which is cooled, and the ionizer, which is heated. Some of the cesium condenses onto the cool surface of the cathode, while some of the cesium comes in contact with the surface of the ionizer and is immediately "boiled away". The positively charged cesium ions leaving the ionizer are accelerated toward and focused onto the cathode, sputtering material from the cathode at impact. Some of the sputtered material gains an electron in passing through the cesium coating on the surface of the cathode, and forms the negatively charged beam. Since the entire source is operated at approximately 80 kV below ground, this negative beam is accelerated out of the source and is available for injection into the FN Tandem accelerator.
A wide variety of ion
beams can be produced with this source, depending upon the nature
of the cathode used. The cathode is a cylindrical section of copper,
which has a small cavity drilled into the cylinder on axis. Typically,
a powder containing the isotope for which beam is desired is tamped
into the cavity, to be sputtered out of the cavity by the cesium beam.
Whenever possible, the cathodes are prepared in advance, but preparing
a new cathode from fresh powder is a relatively simple task. Cathode
lifetime varies with isotope, with some lasting for a week or more of
continuous duty. However, cathodes can be changed in a relatively short
time, so that many cathodes can be used to complete an experiment, if
required.
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