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Re: Sr-90 Source Activity/Geometry



At 13:07 24.03.1999 -0600, you wrote:
>We recently purchased a Sr-90 source from Amersham.
>The certificate of calibration lists the activity as 802 Bq, which is for
>the Sr-90 only.
>Our crew is having a discussion about what the activity, in dpm, would be
>for 2 pi and 4 pi geometry.
>Also, if you are calibrating a CAM with this source, would you use the 2 pi
>or 4 pi dpm to determine efficiency?
>Anyone care to comment?
..........................................

I have some problems to understand your question. As far as I understand
you purchased a well calibrated Sr-90 source, with a certified value of
activity for Sr-90. Due to the short half life of Y-90 this will be in
secular equilibrium. The activity in dpm will be the same, whatever
measurement method you will use - only the cpm will vary, depending on your
geometry and your measurement device. The ratio of cpm to dpm or vice versa
is your calibration factor and shows your measurement efficiency. Your
source has to be in the same chemical form and - if applicable - the same
geometry as your sample. You cannot use a point source to calibrate an
instrument for measurement of a large area sample, unless you use very
complicated mathematical methods. In usual analysis you wait for the
equilibrium to be established between Sr-90 and Y-90. If you intend to
measure Y-90 after separation, this would be a different case and you would
have to determine the calibration factor or your efficiency by treating
your standard solution the same way as you perform the radiochemical
analysis. 

The problems associated with measurement of solid precipitated sources of
Sr-90 has led me to use exclusively liquid scintillation spectrometry for
determination of Sr-90, because there you have a well defined 4 pi-
geometry with excellent counting efficiencies.


Franz 


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

Office:
Hofrat Dr. Franz Schönhofer
Federal Institute for Food Control and Research
Department of Radiochemistry
Kinderspitalg. 15
A-1095 Vienna
Austria
Tel.: +43-1-40 490 27820
e-mail: schoenhofer@baluf.via.at
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