[ RadSafe ] GM vs ion chamber for dose measurement

[Aad van der Kooij]

Well, by the time I received this message it was Friday ...

The response of a GM tube can be tuned by using filters around (parts of)the
tube. For example:
http://www.hps.org/hpspublications/articles/allard.html

And yes, you are right about questioning what is going on!
Dose rates are expressed per unit of time and(TGIF)dose in Sv so: Sv.h-1 -)

Enjoy your weekend!

Aad van der Kooij
RPO TU Delft (NL)


-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On Behalf
Of Baumbaugh, Joel SPAWAR
Sent: Thursday, 22 March 2007 18:20
To: radsafe at radlab.nl
Subject: [ RadSafe ] GM vs ion chamber for dose measurement


Yes, I know this isn't Friday, but I have a philosophical question for
the group.

First a short introduction: My current employer appears to be
mandating/authorizing the use of Geiger Mueller (GM) detectors to
measure dose rates in Roentgens (the meters read "capital" R, not "r") .
As many of you know, a given instrument intended for performing
quantitative measurements will respond differently to various energies
of a given radiation, and a GM tube's response is quite energy
dependent.  As the GM's are calibrated with Cs-137 the response to other
x-ray/gamma sources (of other energies/strength) can be quite different
than what one would measure with an ion chamber - i.e. the energy
dependence of the instrument must be known and accounted for in order to
make accurate quantitative measurements.

A short Discussion: I'm not sure that my employer realizes that by
utilizing a GM for measurement of (a wide energy-range of) gamma/x-rays
they are creating the potential for overexposure conditions (where in
some/many cases one should be utilizing an ion chamber).  Exposure vs
dose, Bragg-Gray and kerma considerations put aside for now, I was
taught to always utilize an Ion Chamber if I REALLY wanted to know the
dose-rates (I use a pressurized ion-chamber myself for a wider range of
energy response). After all, last I knew,  "exposure" was still defined
in terms of the amount of ionization charge created in AIR - and I
believe that the fundamental SI unit of exposure still corresponds to
that amount of x-or gamma radiation who's associated secondary electrons
create an ionization charge of on coulomb per kilogram of dry air at
STP.  Anyone know of a GM that utilizes 78% nitrogen and 21% oxygen for
a counting gas?

So, in conclusion: Am I wrong to be questioning what's going on here? Is
the use of GM's for quantitative/qualitative measurement of gamma/x-ray
dose now an accepted protocol?  

Thanks (ahead of time) for your thoughts.

Joel Baumbaugh


Std. Disclaimer... The thoughts, discussion and opinion(s) above are
mine and mine only, not that of my employer and, of course, I am in no
way asserting/suggesting that my employer is doing anything wrong.  This
is only a topic for a philosophical Health Physics discussion and I
would value your opinion(s) on the matter.

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