Re: CALIBRATION OF CONTAMINATION DETECTORS

["James G. Barnes" <mail15077@pop.net>]

We look at the deposited activity reported by the manufacturer, then determine 
the equivalent number of counts per minute (net) using the probe/counter.  We 
then calculate an efficiency for the probe or counter based on this simple 
approach.

We use isotopes that reasonably match the energy emissions of the isotopes we 
are frisking for.  We use sources that match the diameter of the smear paper to 
eliminate as many of the geometric factors as possible.

Alpha calibration may be a bit trickier, but that is due to self-absorption in 
the smear media.  We generally ignore this as we don't consider the loss that 
significant for counting smears with hand held instruments.


Jim Barnes, CHP
RSO
Rocketdyne Division; Rockwell International

>      RadSafers:
>      
>      I am not sure if the following topic has been discussed on Radsafe.  
>      When a detector is calibrated to measure surface contamination, which 
>      value do you use. 1) Deposited activity, 2) 4Pi emission rate supplied 
>      by the manufacturer, or calculate from 2 Pi, which includes 
>      backscatter and 3) 2 Pi emission rate supplied by the manufacturer 
>      including back scatter.  I am only interested in sources used for 
>      calibration of portable instruments such as pancake GM detectors, 
>      Alpha Scintillators and personnel contamination monitors used for 
>      measuring loose and/or fixed surface contamination. I have yet to see 
>      any reference in a standard as to which value should be used for 
>      calibration.  It is inferred that all release limits are based upon 
>      4Pi emissions or total activity, but have never seen that in writing.  
>      Any help or information will be appreciated.
>      
>      Thanking you in advance
>      
>      Riasp Medora
>      riasp_medora@fernald.gov
> 
>