[ RadSafe ] Fwd: Cs-137 beam calibrator output

Chris Alston achris1999 at gmail.com
Mon Sep 22 19:31:09 CDT 2014


I think that the key to the conundrum lies in the term "nominal".
Some suppliers of sources to the manufacturers of these devices are
not as careful as they might be in quantifying the activities.  You
might try estimating the half-value thickness for the beam, as a way
of getting a handle on how hard it is; i.e., how much is 662 keV, and
how much is Compton garbage.

---------- Forwarded message ----------
From: clayton bradt <dutchbradt at gmail.com>
Date: Mon, Sep 22, 2014 at 7:00 PM
Subject: [ RadSafe ] Cs-137 beam calibrator output
To: radsafe at health.phys.iit.edu

My apologies if this duplicates an earlier post. Having received no bounce
notice for the previous one, I am re-sending from a different email account.


Original post:

I’ve been reviewing data for a beam calibrator containing a Cs-137 source
with nominal activity 600 mCi on 1/18/2005.  The measured output at 1 meter
along the beam center line was 0.2443 R/h on the same date.  Comparing that
with the calculated exposure rate at 1meter from a 600 mCi point source I
get 0.193 R/h, assuming a Gamma constant for Cs-137 of 0.322 R/h at 1 meter
per Ci.   I am surprised at such a large difference (27%) between the
theoretical value and measurement.  Should I be?  I know that scatter from
the collimator will affect the actual exposure rate, but this much?

Any help from people with more experience with exposure rate calibrations
on this list will be most appreciated.
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