Re: Calibratiion using a 25 mm source vs. a 47 mm smear

[Chris Alston <alstonc@odrge.odr.georgetown.edu>]

Cindy

You don't say what probes you're calibrating, so I'll assume that they are
end-window or pancake GM's, and that the source (planchet?) will be at one
(1) cm from the window. If you were to take a look at the geometry of the
2Pi emissions from the source, you'd probably agree that more of the
particles from a smaller source would be captured by the detector, than
would be captured from a larger source. So, you'd be overestimating your
efficiency (not insignificantly, though I have no hard numbers; feel like a
little trig?). Probably, as you say, using the smaller source could be
rationalized, in consideration of all the imponderables of the distribution
of activity on the wipes, etc. But, it's the principle of the thing. I
think that most people would agree that it's best to use standard sources
that replicate, as closely as possible, the type of samples you're counting.

Some good references are:
NCRP Report 58, A Handbook of Radioactivity Measurements Procedures, 2nd
ed., (reprint 1989)
ANL-88-26, Operational Health Physics Training, Moe HJ and Vallario EJ, (1988)

Happy trails
cja
alstonchris@netscape.net

At 11:13 AM 12/16/99 -0600, you wrote:
>A RP Calibration Tech brought up the issue of using a 25 mm source to
calibrate
>instruments reading a 47 mm smear.  The question is:  What is the impact ?  I
>have searched the archives and haven't found the answer to the question.
>References are also requested so that the justification (or lack thereof)
can be
>documented.  My gut feeling tells me that it won't make a difference due
to the
>distribution, method of smearing, etc., but I can't seem to find the
reference
>to back me up.
>
>Please respond to me directly at ledbetterc@nimo.com unless you feel the list
>would benefit.
>
>Cindy Ledbetter
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