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Re: GM Calibration
This may help explain what you are seeing (I won't delve into whether your
calibration is appropriate or not) by using info from a Ludlum
Measurements, Inc. (you can look up the numbers for your particular
probes):
Model 44-7 End Window GM detector: sensitivity typically 2100 cpm/mR/hr
Cs-137 gamma
Model 44-9 Pancake GM detector: sensitivity typically 3300 cpm/mR/hr Cs-137
gamma
Sensitivity ratio Pancake/End window = 1.6
Malek Chatila <mc02@aub.edu.lb>@romulus.ehs.uiuc.edu on 12/06/99 12:42:09
AM
Please respond to radsafe@romulus.ehs.uiuc.edu
Sent by: radsafe@romulus.ehs.uiuc.edu
To: Multiple recipients of list <radsafe@romulus.ehs.uiuc.edu>
cc:
Subject: GM Calibration
Dear Radsafers,
Allow me to start by first thanking all of those how had responded to my
question concerning the calibration of GM probes. While I am aware of the
limitation and the proper use of the various type of GM probes and their
calibration, i.e. geometry, energy, cpm calibration?. the problem that I am
facing is specifically with Victoreen GM probes coupled to Victoreen THYAC
IV Survey meters. For those who are not familiar with this survey meter,
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
doing this, a 50% error is achieved with the above mentioned instrument,
and hence you cannot adjust the potentiometer. However, as I sated
previously, when the pancake is exposed to its side, a correct reading is
obtained. I also had this same problem with a GM compensated probe with a
THYAC meter. Some of your response reinforced my initial thinking that
Victoreen do expose their GM probes to the side. I have tried to contact
Victoreen few times before I posted my question to radsafe but I did not
receive an answer. I guess I will try again.
As always, thank you for your assistance.
Sincerely,
Malek Chatila
American University of Beirut
Email: mc02@aub.edu.lb
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