XIA UltraLo-1800 User Manual page 21

Figure II-13: The "band" of midairs (black), starting from ceilings (red) and going toward alphas (blue). There are a number of
midairs outside this band that are caused by other phenomena, most are likely misclassified cosmogenics (green). At low
energy the timing parameters separating rounds and midairs become difficult to determine accurately.
Radon in the counting gas gives rise to a very specific signal. First, the decay of the radon atom itself
occurs anywhere within the volume with equal probability. Because of the geometry of the counter,
most of the resulting tracks will have a risetime shorter than a typical alpha and longer than a typical
ceiling event. We call these events "mid-airs", and they form a band of increasing energy and risetime
that spans between events that occur just off of the electrodes to events that occur right next to the
tray (see Figure II-13). Some of these radon decays will have similar risetimes as alphas and thus be
classified as alphas (after all, they are alpha decays occurring near the sample tray). We can estimate
this rate, and it's about 0.00025 alphas/cm
change over time or between different environments if the counter is set up and maintained correctly.
(Counters have been run in underground labs with external radon activities of over 500 Bq/m
increase in midairs.) If you see more activity than around 5 midairs per hour, see
Finally, because thoron has a precise decay chain and every alpha decay has a specific energy, any
signal from thoron will create a specific spectrum. For an example of how this looks in the UltraLo-1800,
see Figure II-12. The rate of thoron decay in the chamber is so low that this spectrum takes many days to
appear and will only be noticeable with very low-count samples. However, familiarity with this spectrum
(and the spectrum of
contamination, such as emission from a sample or terminated contamination.
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2
/hr in the Full configuration. The innate background will not
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Rn, also shown in Figure II-11) could help diagnose other forms of radon
UltraLo-1800 Alpha Particle Counter
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with no
Excess Events.
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