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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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