Re: Ion chamber versus GM

[LIPTONW@detroitedison.com]

A recent incident at the Koeberg Unit 2 nuclear power plant (South Africa)
shows the potential for saturation of a GM detector under high dose rate
conditions.  This took place on May 2, 1997.  Workers were changing out fuel
pool filters.  The measured dose rate with a GM detector was 220 mrem/hr.
During the work, one of the workers noticed that his pocket dosimeter was
off
scale high.  A resurvey with a different instrument showed that the actual
dose rate was 240 rem/hr.  Two of the three workers received exposures above
the 5 rem annual limit.

Even ion chambers can saturate due to recombination of the ions before they
reach the electrode.  They are generally less susceptible than GM detectors,
however.  Special precautions are needed for pulsed fields, such as around
some accelerators.

No matter what your instrument you should:

(1) have a good estimate of the expected dose rate (220 mrem/hr is
unbelievably low for fuel pool filters) and stop work if there's a
significant
deviation with the actual dose rate.

(2) turn on your meter on the most sensitive scale in a known, low dose rate
area.  Then observe the meter as you approach the source, ranging up scales
as
needed.  If the reading starts to decrease as you approach the source, stop
work and reevaluate.

The opinions expressed are strictly mine.
It's not about dose, it's about trust.

Bill Lipton
liptonw@detroitedison.com

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• To: Multiple recipients of list <radsafe>
• Subject: Re: Ion chamber versus GM
• From: "Jesse H. Coleman" <radshoals@airnet.net>
• Date: 30 Jan 98 08:14:01
• Reply-to: UNIX2.DECO.COM:radsafe@romulus.ehs.uiuc.edu
• Sender: radsafe@romulus.ehs.uiuc.edu

I too have seen this distinction between radiation detectors and
measuring devices.  Your use of the ion chamber seems OK to me.
To prevent a violation, consider these sources:

1 - Radiation Detection and Measuriment by Knoll, 1979, on page
151:  "The term ionization chamber has conventionally come to be
used exclusively for the type of detector ..."

2 - Nuclear Radiation Detection by Price, Second edition, 1964
sorry old book, but still good), on page 44 starts a section
called Ionization Chamber Detectors.

3 - Principles of Nuclear Radiation Detection, by Eicholtz and
Poston, 1982, on page 100, "Two broad classifications of
ionization chambers are usually assigned in the discussion of
radiation detectors of this type."

If you call an ion chamber a detector, you are in good company.

Best wishes

Jesse Coleman
RADSHOALS@AIRNET.NET

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