RE: GM Calibration

["Lieskovsky, Miro" <MLieskovsky@nbpower.com>]

Bruce,
I agree with your posts that the overresponse is due to the secondary
electrons from air reaching the GM's sensitive volume.  

I think that the SCPE is not an issue with the GM tubes because there is no
information on imparted energy in the count / pulse.  Or existence of SCPE
simply does not matter for the GM tubes.
  
The primary electrons (when measuring betas) or the secondary electrons
(predominantly from the tube wall when measuring photons) are important. As
you point out the GM tube detects (almost) every single electron that enters
its sensitive volume.  The resulting pulse is of the same size regardless of
the  imparted energy from the electron and that is why no information on the
amount of absorbed energy can be extracted.

  
The so called 'energy compensated GM tubes'  are designed to give a count
rate proportional to the dose rate for a wide photon energies. (And, yes
they usually underrespond below few tens of keV, overrespond at around
130keV when compared to Cs or Co.)  The design  and the size of the tube are
the ultimate factors for the number of count rates per unit dose rate. Some
GM based meters also "integrate" the counts to provide the dose. 

Miro

Miroslav Lieskovský 
Health Physicist 
NB POWER - PLGS
P.O. BOX 600
Lepreau, N.B.
Canada, E5J 2S6 
tel. (506) 659 7421
fax. (506) 659 6507
mlieskovsky@nbpower.com <mailto:mlieskovsky@nbpower.com> 








 

-----Original Message-----
From: Heinmiller, Bruce [mailto:heinmillerb@aecl.ca]
Sent: December 6, 1999 12:12 PM
To: Multiple recipients of list
Subject: RE: GM Calibration


See my previous post on this subject.  If you're exposing the pancake probe
window directly, and from the information you've provided, I submit that you
have an UNKNOWN radiation field.  In particular, you have a mixed field of
photons for which the probability of interaction in the G-M tube is very
low, and an electron field for which the probability of interaction in the
G-M tube is very high (if they're allowed to reach the G-M tube).  There are
several reasons why the electron fluence may not represent the
secondary-charged-particle-equilibrium (SCPE) complement.  Because, roughly
speaking, SCPE is a necessary condition for exposure-rate measurement, SCPE
is established by ensuring as much as possible that every electron ionizing
the G-M tube gas was created in the G-M tube wall or FILTER, and not some
place else.  Thus, you need a filter over the G-M tube window both for
calibration and for any use of the instrument as an exposure-rate meter.
The filter should be sufficiently thick to stop all electrons incident on
it.  At the same time, it has to be thick enough that it serves as a source
of Compton- and photo-electrons, the number of which reaching the G-M tube
gas is assumed to be proportional to the exposure rate.  Even at that, this
assumption may not be valid for low-energy photon fields unless the filter
is made of material appropriate for energy compensation.  However, for
Cs-137 (Ba-137m), lack of energy compensation will not be the primary reason
for an over-response.  My guess is that you have an electron surplus (with
respect to SCPE) incident on the G-M tube window.  This hypothesis is
testable (if it is accepted that very-low-energy photons are a non-issue) in
the manner that I indicated in my previous post.

Bruce Heinmiller CHP
heinmillerb@aecl.ca

> when calibrating these probes-meters, the probe is exposed to a known
> radiation field (Cs-137), then an internal correction factor is selected
> by
> adjusting the potentiometer.   Whoever, this internal correction factor
> can
> be used only as long as the percent error in the reading is plus or minus
> 35%.   Now common sense would tell you to expose the pancake probe window
> directly to the radiation beam i.e. the way it will be used, whoever, by
> 
> Malek Chatila
> American University of Beirut
> Email: mc02@aub.edu.lb
> 
> 
> 
> 
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