RE: GM Calibration Question

["Feldman, Gerald" <gfeldman@msx.ndc.mc.uci.edu>]

Malek:

My experience with this instrument matches that of Joey Mitchel, and I agree
that you should call Victoreen and ask them how to calibrate the instrument.
Then ignore part or all of what they say and calibrate the unit in a way
that makes sense!

When I called Victoreen a few years ago, the tech with whom I spoke told me
to have the beam normal to the SIDE of the TWGM probe.  I then asked to
speak to the tech's supervisor and he told me that that was incorrect and
that the beam should be normal to the BACK of the probe.  (Just try
surveying a counter-top in that position.) {;-(

I suggest that you calibrate a GM in the orientation in which it will be
used.  For air ionization chambers, follow the orientation recommended by
the manufacturer.  

A good resource to get you started is NCRP Report No. 112, Calibration of
Survey Instruments Used in Radiation Protection for the Assessment of
Ionizing Radiation Fields and Radioactive Surface Contamination. 

Gerald Feldman
gfeldman@uci.edu
 

-----Original Message-----
From: Malek Chatila [mailto:mc02@aub.edu.lb]
Sent: Friday, December 03, 1999 1:26 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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