Re Interferences with Pb212 Analyses? Problem Solved

["Leo M. Lowe" <llowe@senes.on.ca>]

The problem with the anomalously elevated Pb-212 levels in the NORM samples
has been solved.

The samples were being analyzed by gamma spec.  The problem originated in
the automatic software program that analyzed the gamma spectra. When
analyzing for Th-228 by gamma spec, the counting program uses Pb-212 as a
surrogate (and reports it so) unless that peak is to low and/or uncertain
according to some internal criteria.  In such cases, it switches to a gamma
that is directly from Th-228 for the analysis (apparently without
outputting a warning). Unfortunately, the Th-228 gamma in these samples had
much interference from the uranium-238 series radionuclides, which were
significantly above background.  This was then incorrectly reported as the
elevated Th-228 (Pb-212).  The bottom line is that the Th-232 series
radionuclides are in equilibrium as expected.

Which all goes to prove that automatic reliance on a "canned" program can
sometimes lead to incorrect results.  One interesting point however is that
another lab had also reported confounding  results in the Th-232 series for
the same material.

Thanks to all those who contacted me directly with suggestions.  Your
assistance was much appreciated.

Leo M. Lowe, Ph.D.
llowe@senes.on.ca

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