UltraLo-1800 Alpha Particle Counter
XIA LLC
Page 66
b)
Guards
Excess guards are typically caused by an unusually radioactive sample that extends under the guard,
in which case you will also see an excess of alphas. But an excessive number of guard events can have
other causes. Any contamination on the sidewalls, for instance, will do it, although this is rare.
c)
Midairs
Midairs are mostly caused by
. As such an excessive number of midairs usually indicates the
presence of radon. This results in a spectrum with the characteristic peaks of radon. If radon is the
culprit, the most likely source is emanation from the sample being measured. Try removing the sample,
purging, and seeing if the signal remains. If it does, there may be radon contamination somewhere in
your gas line or the counter itself. In this case please
If there are no characteristic radon peaks it could be that excess moisture is causing lowered
risetimes, and alphas are showing up as midairs. For details on troubleshooting this scenario, see
Finally, some percentage of cosmogenics show up as midairs (for more information on cosmogenics,
). For information on troubleshooting excess rounds, see the next section.
d)
Rounds
Rounds are caused by cosmogenic radiation, as detailed in
. Excess rounds implies
excess cosmic radiation, which has no easy solution. If you see large numbers of rounds (~8/hour is
normal at sea level) try moving the counter into a basement, if possible. At the moment adding more
material above the counter is the only way to reduce the round rate. XIA is actively researching veto
panel technology, and will alert our customers to any progress.
e)
Noises
You can get a large number of “Noise” triggers if the thresholds are set too low. The thresholds are
set in the diagnostics panel, and they control the level a trace needs to reach in order for
Figure VII-8: Event caused by mechanical vibration of the counter.
