[ RadSafe ] Cs-137 beam calibrator output

Thompson, Dewey L DThompson3 at ameren.com
Mon Sep 22 20:16:31 CDT 2014


If it is an open beam, I would not expect too horrible of a contribution from Compton Scatter. There will be some, sure. I remember looking at an enclosed box calibrator some years ago, it would not only have the forward scatter but bounce back as well. You could see the effect, the measured dose rate was greater than narrow beam calculations, however it was no 27%.  I don't remember the details now, but I'm thinking it was like 7% to 9%.  Don't hold me to the number, I've slept since then.

A couple of thoughts, well, really one. How confident are you in the gamma constant you are using?  The numbers vary wildly depending on your reference. For example, the IEM website shows a gamma constant for CS~137 as 0,38184 R/C at a meter, that's .229 R/hr if I can still run a calculator.

Ditto to the previous poster on source strength accuracy.

Oh, and I guess I can come up with a second thought. If the source is Cesium salt powder, it can show "hot spots", and not be uniform.

Oh yeah.  A third thought. What instrument was used, and did it have the proper equilibrium shield?

All these errors compound of course.
Thoughts and musings.

Sent from my iPhone

> On Sep 22, 2014, at 6:01 PM, "clayton bradt" <dutchbradt at gmail.com> wrote:
> 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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