[ RadSafe ] GM vs ion chamber for dose measurement (UNCLASSIFIED)

Cary Renquist cary_rdsfe at pacbell.net
Fri Mar 23 12:37:21 CDT 2007

The tissue equivalent urem is useful, but...
Caveat Emptor:
The tissue equivalent microrem/h meter is still a
pulse mode device -- 
like a GM. 
Like a GM -- it tends to overestimate the dose from
lower energy sources 
(it is probably much better than a NaI microrem meter,
but not better 
than an ion chamber). 

Measuring Gd-153 (two ~100 keV photons) with a tissue
microrem will give a reading ~50% higher than an ion
chamber (ion
chamber and microrem both calibrated to Cs-137).

Also, there are safety issues for not using pulse-mode
dose rate meters 
-- I'm not going to grab a GM-based dose rate meter or
a pulse-mode urem 
if I am checking out potentially high dose rate

A person with one type of dose rate meter is always
sure of their 
exposure, a person with two is never quite sure. 

Best regards,

-----Original Message-----
From: radsafe-bounces at radlab.nl
[mailto:radsafe-bounces at radlab.nl] On
Behalf Of Borisky, Michael (Civ, ARL/ADLO)
Sent: Friday, 23 March, 2007 06:41
To: Sam Iverstine; radsafe at radlab.nl
Subject: RE: [ RadSafe ] GM vs ion chamber for dose

Classification:  UNCLASSIFIED 
Caveats: NONE

Doesn't anyone appreciate the energy response virtues
and senstivity of
the tissue equivalent microrem/hr meter?

Mike Borisky 
Army Research Lab

-----Original Message-----
From: radsafe-bounces at radlab.nl
[mailto:radsafe-bounces at radlab.nl] On
Behalf Of Sam Iverstine
Sent: Friday, March 23, 2007 8:35 AM
To: radsafe at radlab.nl
Subject: RE: [ RadSafe ] GM vs ion chamber for dose

Great topic. Great link to energy response curves. 
  One's employer could spend less on an ion chamber
than the two days of 
your time and large errors associated with applying
curves and filters, 
in my opinion. But it might be fun to overcome the
This brings me to my question for radsafe: What is the
best hand held 
ion chamber to purchase for mainly radiology uses? The
best I can find 
are the 450 and 451P's by Victoreen. Sometimes these
meters give erratic 
readings due to temperature or mechanical agitation
(dropping). They do 
have a good response rate and accurate digital readout
over all energies 
of interest. The plastic case is water-resistant and
filters fewer low 
energy photons (than a metallic encased meter). But
there must be 
something better in this millennia? 
  Thanks for any response.
  Sam Iverstine, MS, CHP
Aad van der Kooij <A.vanderKooij at TUDelft.nl> wrote:
  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:

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

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

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
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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