[ RadSafe ] Cs-137 beam calibrator output

[JPreisig at aol.com]

Radsafe,
 
     Have you included proper buildup factor????
 
     Run the problem on MCNP.  See what values you  get.
 
     Joe Preisig
 
 
 
In a message dated 9/22/2014 9:18:00 P.M. Eastern Daylight Time,  
DThompson3 at ameren.com writes:

Clayton.

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.

Dewey
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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