RE: GM Calibration Question

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

Do I understand you to mean that you're trying to calibrate to photons with
an open end-window G-M probe?  If so, you have to know the characteristics
of the source and irradiation geometry with regard to the
secondary-charged-particle complement.  Without knowing more details, this
looks to me to be consistent with electron contamination from a Compton- or
photo-electron surplus, perhaps from a panoramic irradiation geometry.  Does
the apparent dose rate drop when you filter the G-M window with a mm of
plastic?

Bruce Heinmiller CHP
heinmillerb@aecl.ca

> ----------
> From: 	Malek Chatila[SMTP:mc02@aub.edu.lb]
> Reply To: 	radsafe@romulus.ehs.uiuc.edu
> Sent: 	Friday, December 03, 1999 3:59 AM
> To: 	Multiple recipients of list
> Subject: 	GM Calibration Question
> 
> 
> Dear All,
> 
> I have few related questions concerning the calibration of GM probes that
> I
> hope you might clarify for me.
> 
> 1. When calibrating an open-end window GM probe, the probe is placed
> perpendicular to the beam axis, i.e. the window is not directly facing the
> radiation beam, is this correct? 
> 2. When calibrating a pancake GM probe, the pancake window is placed
> directly facing the beam, is this correct?
> 
> The problem that I am having is the following:  when trying to calibrate a
> pancake probes that are coupled to Victoreen's THYAC IV survey meters, the
> meters constantly over read by 50 % when the pancake probe window is
> placed
> facing the radiation beam while an accurate reading is obtained when the
> pancake probe handle is placed perpendicular to the beam axis i.e. the
> window facing down and the beam axis is horizontal.  
> 
> Thank you in advance for your assistance.
> 
> Sincerely,
> 
> Malek Chatila
> American University of Beirut
> Email: mc02@aub.edu.lb
> 
> 
> 
> 
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