RE: GM Counters in High Backgrounds

["Dru Carson" <carsond@ludlums.com>]

The instrument model, probe type, probe area, efficiency, and size of the
wipe smear, in the example are irrelevant. The only variables you need are
background and gross count from the sample. Subtract gross count from
background to obtain net count from the sample.

Background = 1000 cpm
Gross count = 1500 cpm
Net count = 500 cpm

If the background was lowered to 50 cpm, the gross count would be 550 cpm.
The net count remains the same (500 cpm.

At these count rates, dead time loss is not a factor so the instrument
should read 550 cpm. If you have seen instruments that show a non-linearity,
I would question the validity of the instrument calibration. Each range is
calibrated independently so that it will remain linear in cpm.

Dru Carson

p.s. can anybody accurately read 50 cpm on an analog meter?


These are my opinions and do not reflect
the management of Ludlum Measurements
in fact, they probably wish I would shut up!

-----Original Message-----
From: radsafe@romulus.ehs.uiuc.edu
[mailto:radsafe@romulus.ehs.uiuc.edu]On Behalf Of Rodney Bauman
Sent: Tuesday, October 12, 1999 10:12 AM
To: Multiple recipients of list
Subject: Re:GM Counters in High Backgrounds


I used to have some of the same arguments with the house and contract RTs.
However, our arguments usually centered around dose rates (e.g., RO-2
readings)
instead of count rates.  The techs would always argue that a net reading
would
increase if counted/surveyed in a lower background area.  That is, the high
background levels "hid" most of the net activity which would only become
apparent once counted/surveyed in a low background area.

However, the fact is that net activity is net activity.  High background
rates
will inevitably increase your background variance and, coupled with the high
background rate, increase your background decision level; which, in turn,
will
increase your net activity MDA.  Based on the location's background
variance,
the measuring of 500 net cpm activity may not be possible to a high level of
confidence - and that may be the problem.  But if possible, then 1500 gross
cpm
in a 1000 cpm background should produce approximately 550 gross cpm in a 50
cpm
background.

Rodney Bauman, CHP, RRPT
rodney_bauman@wssrap-host.wssrap.com

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