RE: GM Calibration

["Heinmiller, Bruce" <heinmillerb@aecl.ca>]

Although G-M tubes give no energy information on the electrons entering the
gas, for a given photon fluence incident on the tube wall, there is very
definitely energy "information", insofar as the number of electrons capable
of reaching the gas is higher for higher-energy Compton electrons generated
from higher-energy photon interactions in the tube wall.  Energy
compensation for lower energies notwithstanding, I believe that's how G-M
tubes are able to measure exposure (or exposure rate) over a wide
photon-energy range.  SCPE of the incident field is not relevant provided
the tube wall is thick enough to re-establish SCPE in a volume element in
the tube wall near the gas, and in that sense it is an issue.  While the
response (ignoring dead-time) is purely determined by the number of ionizing
electrons entering the gas, I submit that this number has no meaning with
regard to exposure unless SCPE exists in the tube wall near the gas.  It is
the number of electrons entering the gas under SCPE conditions in the wall
near the gas that I refer to as the SCPE complement of electrons.

Bruce Heinmiller CHP
heinmillerb@aecl.ca

> ----------
> From: 	Lieskovsky, Miro[SMTP:MLieskovsky@nbpower.com]
> Reply To: 	radsafe@romulus.ehs.uiuc.edu
> Sent: 	Tuesday, December 07, 1999 10:46 AM
> To: 	Multiple recipients of list
> Subject: 	RE: GM Calibration
> 
> 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.
> 
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