RE: Re[4]: 137Cs check sources -Reply

["Marthaller, Chris" <ChrisM@wipp.carlsbad.nm.us>]

Just jumping in on the middle of this thread, so I don't know which
Instrumentation is being talked about, but as I recall, most of the
extremely high range instruments I have had the opportunity to use in my
past lives were GM types.  The only exception was the RO-7 which was an
IC.  Properly calibrated these GM types always agreed within a +/- 20%
of the TLD readings that were returned.  So I believe the main thing
that should come from this thread is use the proper tools for the job at
hand, whether the instrument uses a GM or IC detector.

Chris A. Marthaller RRPT
Phone (505) 234-8661
Sr. Training Coordinator - WIPP 
ChrisM@wipp.carlsbad.nm.us
Obviously, only my own views

I completely agree with Steven:
	The GM tube is indeed paralyzed in the high field.  It really
doesn't matter whether you "paralyze" or "saturate" the electronic
circuitry; the fact is that you are not getting useful information from
the device.  IN fact, you are getting MISLEADING readings
whether paralyzed or saturated.  Whether you are protecting yourself or
others from harmful exposures, it's not the kind of information you'd
like
to have. 

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
H. Gregg Claycamp, Ph.D., C.H.P. 
Assoc. Prof. and Assoc. Chair 
Dept. of Environmental and Occupational
Health University of Pittsburgh 
412-967-6524 FAX:  412-624-1020


On Thu, 1 Oct 1998, Steven Rima wrote:

>      Ted de Castro wrote:
>      
> This particular tube and circuit would top out at about 100 mr/hr but to 
> test its high field response I ran it to about 1 megaR/hr (a cm or so from 
> a 2500 Ci Co-60 source.  It still read 100 mr/hr.
>      
> Any manufacturer who knows anything about GMs and instrumentation will use 
> non paralyzing circuits.  For a while - and long after this problem was 
> identified and solved - they would even advertise this as a spec.  It is so 
> commonplace now that it is hardly even mentioned.
>      
> BUT many who remember how it used to be still bring it up as a problem.
>      
>      
>      Okay, am I missing something here??? An old GM instrument would 
>      saturate and essentially peg low if exposed to a very high radiation 
>      level. Now, at least the one Ted "tested", will read about 100 mR/h 
>      when exposed to MegaR/h levels. I guess I don't understand, why is 
>      this any better than reading zero?!? It is still a potentially BIG 
>      problem if this GM instrument is used in a very high radiation 
>      environment. It doesn't seem to me that this problem is "solved" 
>      whatsoever by the non-paralyzing circuit.
>      
>      Steven D. Rima, CHP, CSP
>      Manager, Health Physics and Industrial Hygiene
>      MACTEC-ERS, LLC
>      steven.rima@doegjpo.com
> ************************************************************************
> The RADSAFE Frequently Asked Questions list, archives and subscription
> information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html
> 

************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html
************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html