RE: wrong DOT label

[Glen.Vickers@ucm.com]

Dave,

Your observation about the absolute lower end of energy response is right.
At our commercial nuclear reactor, the preponderance of our emmissions are
in excess of 60-80 keV.  We find in the energy ranges and geometries we
encounter that the compensated GM typically responds higher than a typical
ion chamber.  The NRC used an instrument with a small diameter compensated
GM tube the last time we had package and vehicle dose rates checked.  It
wasn't much of a suprise when their measurements were essentially the same
as ours.

The isotopic mix at a given facility certainly is a key variable in choosing
the most appropriate instrument.  What instruments do others use?

Sincerely,
Glen Vickers

> -----Original Message-----
> From:	Dave Derenzo [SMTP:dave@uic.edu]
> Sent:	Wednesday, May 17, 2000 7:49 AM
> To:	Multiple recipients of list
> Subject:	Re: wrong DOT label
> 
> Dear Glen and Irene,
> 
> Using an energy compensated GM detector is certainly a good idea.
> However, 
> the following specs are interesting.
> 
> Eberline HP-270 Energy Compensated GM Detector- There is scant information
> 
> in their current catalog which simply states "Energy Range: 30 keV to 6 
> MeV" with no mention of accuracy.  A much older catalog from the 1970s 
> however indicates energy response of +/- 20% from 40 keV to 1.25 MeV.  It 
> also includes an energy response curve that seems to indicate the detector
> 
> under-responds by about 50% at 40 keV and drops off rapidly below that
> value.
> 
> Ludlum 44-38 Energy Compensated GM Detector - The current catalog states 
> "Within +/- 15% of true value from 50 keV -1.25 MeV.  There is no energy 
> response curve.
> 
> I am pretty sure that other energy compensated GM detectors have similar 
> specs.  So, if you are measuring a bremmstrahlung spectrum with such a 
> detector, you might expect a reading that is too low by maybe 50%.
> 
> A similar look at the energy response info for ion chambers yields the 
> following:
> 
> Eberline RO-20 - +/- 30% from 8 keV to 6 Mev and +/- 15% from 33 keV to 6 
> MeV, implying there is greater error in the range of 8 to 33 keV.  There
> is 
> no energy response curve in the catalog.  A look at the old catalog for 
> different models such as the RO-2 indicates a fairly flat response that 
> drops off starting at about 15 keV with the slide open, and drops off 
> starting at about 35 keV with the slide closed.
> 
> Victoreen 471 - Within 10% for gamma and x-rays from 6 to 300 keV with
> beta 
> cap off: within 10% from 25 keV to 2 MeV - and to 105% of N-16 gamma rays 
> (most prominent gamma is 6.13 MeV) as tested at the University of 
> Lowell.  The energy response curve shows under-response at low energies.
> 
> So which is more conservative, the G.M. detector that you can get closer 
> but that could significantly under-respond, or the ion chamber which is 
> more accurate but cannot be brought close to the surface?  Tough call if 
> you are trying to measure exposure rates from P-32 bremmstrhalung that is 
> partially attenuated by the vial shield.  All things considered, I think 
> that you could very easily get a +/- 50% to 100% difference in readings
> for 
> such a situation, and that you need to interpret readings very carefully 
> before making a report to a regulatory agency alleging 
> non-compliance.  Both parties have probably made their best efforts to
> make 
> the measurement.  Like my former boss used to say, "Where are the 
> bodies?"  My opinion is that unless there was a significant safety problem
> 
> caused by this difference, give the shipper the benefit of the doubt.
> 
> Dave Derenzo
> 
> At 09:28 PM 05/16/2000 -0500, you wrote:
> >The comments about the type of instrument used, large ion chamber vs.
> small
> >diameter compensated GM are certainly valid.  If someone were to be
> >non-conservative and use a large volume detector, aren't they taking risk
> >upon themselves?
> >
> >We use a small diameter, compensated GM detectors for all of our package
> >measurements.  I realize, that a small-diameter pulse counter may read
> higher
> >than an ion chamber in low energy fields and different geometries, but I
> >ensure that I never get any calls about exceeding dose rate limits.
> >
> >There is no cost difference in shipping White-I vs. Yellow-II, so I'd
> think
> >someone would use the most conservative meter?
> >
> >Glen Vickers
> >glen.vickers@ucm.com
> >************************************************************************
> >The RADSAFE Frequently Asked Questions list, archives and subscription
> >information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html
> 
> 
> Dave Derenzo, RSO (dave@uic.edu)
> UIC Radiation Safety Section, M/C 932
> Phones: Voice (312) 996-1177  Fax: (312) 996-8776
> 
> ************************************************************************
> The RADSAFE Frequently Asked Questions list, archives and subscription
> information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html


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