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Re:Sr-90 Source Activity/Geometry



For your CAM calibration, efficiency should be determined using the source's
total activity (i.e. 4 pi efficiency).  This provides indication of the entire
"detection system's" efficiency and subsequently the beta activity being
deposited on the CAM's filter.  Most instrument vendor manuals list 2 pi
efficiency values.  However, in the field, 4 pi efficiency must be used (because
your sample of radioactive material is emitting radiation isotropically).

You also need to correct (i.e., double) your source activity for the Y-90
activity inherently present.

In addition, you may have to make another correction, depending on the physical
make-up of your calibration source, in order to account for possible backscatter
radiation.  For example, Sr-90/Y-90 activity electroplated on stainless steel
will inflate your detector's efficiency by approximately 25% due to the
back-scattered beta particles finding the detector.  Back-scattering does not
occur (to any significance) on CAM air filter sample media.  However, if your
source media closely matches you sampling media, then no such correction is
necessary. 

Rodney Bauman, CHP, RRPT
Weldon Spring Site
rodney_bauman@wssrap-host.wssrap.com

____________________Reply Separator____________________
Subject:    Sr-90 Source Activity/Geometry 
Author: <radsafe@romulus.ehs.uiuc.edu>
Date:       3/24/99 1:06 PM

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?

Bill Miller
INEEL


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