UltraLo-1800 Alpha Particle Counter
XIA LLC
Page 19
2.
Radon
The other source of background in the UltraLo-1800 is radon gas. Radon is a noble gas, and as such
is nonreactive and capable of diffusing some distance through materials (especially plastics). It’s also
radioactive (in fact, it’s the
second-leading cause of lung cancer in the US
). These two properties
combine to make it a pernicious source of background in any counting experiments. There are two
common isotopes of radon,
220
Rn (also called “thoron” because it comes from the
232
Th chain) and
222
Rn
(sometimes the word “radon” is used solely in reference to this isotope, but as this is potentially
confusing in this manual “radon” will always mean the element with no specific isotope implied). The
decay chains of both are shown in Figure II-10. The two chains have very distinct alpha-decay energies,
and their spectra in equilibrium are shown in
Figure II-11. Outside of equilibrium one or more of the
peaks may be missing, depending on conditions. A spectrum with a peak in any of those locations is
generally indicative of some kind of radon contamination on the sample.
In the UltraLo-1800 radon can show up in two different ways: in the “ongoing exposure” case, by
being continuously present in the counting gas itself; or in the “terminated exposure case” by decaying
Figure II-10: Decays series of
222
Rn (top) and
220
Rn (bottom).
