Calibratiion using a 25 mm source vs. a 47 mm smear -Reply

[Richard Machado <MACHRM@doe-md.gov>]

We had a similar concern with electroplated alpha spec. calibration
sources.  The were plated very nearly to the edge, in contrast to the
laboratory prepared samples, which were plated to within about 2
millimeters of the edge.  Not much difference, or so it seemed.  

We have not been able to prove it but we think it explains the bias in our
chemical recovery.  (We average about 104% when we would expect
right at 100% or slightly less)  The smaller diameter plates are more
efficient with respect to the detector.  The calibration sources, plated
further out, must give a slightly lower efficiency relative to the samples.

I suspect the 25 mm cal. source would give a higher efficiency and
therfore underestimate the activity on a 47mm smear.

Richard Machado
Health Physicist (BWXT of Ohio)
machrm@doe-md.gov 
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