Re: wrong DOT label

[Dave Derenzo <dave@uic.edu>]

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
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Dave Derenzo, RSO (dave@uic.edu)
UIC Radiation Safety Section, M/C 932
Phones: Voice (312) 996-1177  Fax: (312) 996-8776

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