Appendix D Optimizing the CANBERRA iSERIES for Air Sample Counting
Sample Collection Time
Air samples are typically collected for as little as a few minutes to as much as several
days. As stated earlier in this application note, early versions of the algorithm assumed
that the Thoron progeny (ThC and ThC') were present if the Radon progeny (RaC')
existed. Further study has shown that in many instances there will be a significant
amount of RaC' present without significant amounts of ThC or ThC' being present.
The thoron progeny interferences have half-lives considerably longer than those of the
radon progeny. It typically takes much longer for thoron to build-up on a filter than
does for radon. The ThB activity collected on the filter would increase with time until
equilibrium is reached.
In Figure 12, the obvious peak is from radon progeny (RaC' Po-214). Even with 24
hours of sampling, the thoron daughters are small in comparison. Figure 15 a filter that
was exposed for approximately one hour. It has virtually no thoron progeny.
Figure 12 24 Hour PTFE Air Filter First Count
The same sample counted 100 minutes later (Figure 13) shows the amount of thoron
present. Even with a 24 hour collection, this sample still shows a comparatively small
amount of thoron compared with the radon after 100 minutes of decay.
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