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

[Franz Schoenhofer <schoenho@via.at>]

Cindy,

Unfortunately you do not give any information about what kind of measuring
device (detectors) you use and what kind of radionuclides you have to
measure, except that they are smears. And - most important - there is no
information about what purpose your measurements are for.

Nevertheless I try to answer in such a way, that you could cover most cases. 

If your measurements are to show compliance with maximum levels set by the
authorities you could simply estimate the error introduced by using not
exactly the same geometry. If it might be in the order of maybe 10% (why
not determine that experimentally by a few measurements?) and your readings
are two orders of magnitude lower or higher than the maximum permisssible
level, then you should not face any problems to make a valid report. If
your readings are close to the maximum permissible level, then you might
run into a problem. Is there any reason, why you cannot use smear samples
the same size as your reference standards are? A small difference in smear
sample area will make an even smaller difference in efficiency - both
regarding removal of surface contamination and counting efficiency. 

I would like to refer to gamma-spectrometry, where geometry (three
dimensional), but also density of the sample and especially elemental
composition of the sample plays a big role in calibration. There are very
sophisticated methods known to correct for the influence of these
conditions. I have worked quite a lot in that business, but I always took a
pragmatic approach: We tried to keep as many factors constant as possible
(using for calibration of milk powder spiked milk powder in exactly the
same geometry, for water a spiked water sample etc.) In case we had a
matrix different from our many different calibration sources, we estimated
the error and added it to the overall error of our measurements. We have in
Austria extremely strict procedures for accreditation of laboratories, but
this procedure was accepted. 

Finally I would like to recommend to purchase calibration sources for the
geometry in question. An excellent source for such sources is for instance
ANALYTICS, they have made for us quite a lot of gamma-spectrometry
calibration sources in many different geometries.

In case you want to have more information I would appreciate your feed
back. It would be a pleasure for me too to discuss such questions. You may
of course contact me at my private e-mail address.

Best regards,

Franz








At 11:13 16.12.1999 -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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Franz Schoenhofer
Habicherg. 31/7
A-1160 Vienna
Austria
Tel.: +43-1-495 53 08
Fax.: same number
mobile phone: +43-664-338 0 333
e-mail: schoenho@via.at

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