The Nuclear Structure 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.