Re: RO-2/Telepole Reading Discrepancy

[james.g.barnes@att.net]

Dear Carl;



I'm assuming the MGC is like the teletector (GM tube on an extendable pole).  My first suspicion is that it is an energy effect common to GM tubes.  The short version is that a GM tube "fires" no matter what the gamma energy is that hits it.  So, to a GM tube, a Cs-137 and Co-60 gamma are the exact same beast.  Depending upon what isotope is used to calibrate the instrument, the instrument will under or over-respond to other isotopes.  This can be mitigated by energy compensating shields, etc., but they are workarounds and not perfect. What isotope do you use to calibrate the instrument?



R02's, on the other hand, are ion chambers.  The output of the detection circuit is proportional to the energy of the gamma it detects.  Thus, it will scale the needle response to the energy of the incident gamma, and gives an inherently more accurate response.



Also, in the teletector design, there as a tiny circuit board within the larger detector which had the high range detector.  If that wasn't oriented perfectly to the source, you'd see all sorts of variation due to probe orientation.  If you can see the dose on an R02, however, I suspect you haven't used that circuit on the extendable; it was for measuring up to 2,000 R/hour.





Jim Barnes, CHP

james.g.barnes@att.net





> We have identified a situation where a survey of the access gate to a demin 

> resin bed with an RO-2 survey meter and a Telepole survey meter are not giving 

> us consistent results.  Readings with several pairs of the instruments has 

> consistently shown an approximate 40% difference in the readings with the 

> Telepole 40% less than the RO-2.  I have my ideas on why this is occurring, 

> however, I am making this query to see if other users have seen this condition 

> and what they think the explanation is.

> 

> The resin bed source is predominantly Co-58 and there is no line-of-sight from 

> the resin bed to the access gate.  The resin bed is located in a concrete 

> cubicle.  The entrance to the cubicle is straight and not a labyrinth.  The dose 

> rate measured by the RO-2 is in the range of 300 to 320 mR/hr.  The RO-2 is made 

> by Eberline and the Telepole by MGP.

> 

> Thanks,

> 

> Carl Onesti

> Point Beach Nuclear Plant

> carl.onesti@nmcco.com

> Phone: 920-755-6170

> Fax: 920-755-6260

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