RE: GM Counters in High Backgrounds

["Pollan, Paul B." <PBPOLLAN@southernco.com>]

 Dru, certainly one would think the pulses would be additive, but in
actuality, the smear in the lowered background reads between 1K and 1.5k
cpm.  The instrument is calibrated to NBS standards and the CF is fairly
linear (0.1).  This appears to be an anomaly that the "road" HPs seem to see
everywhere they go when working with fairly high level smears.  When I speak
of high level, I am talking about the 200K-500Kdpm/100cm2 smears (CF
accounted for) taken usually on the platform and tools by primary Steam
Generator workers.  I have witnessed such occurrences on a regular basis
during Outages.  An example is a 200kdpm/100cm2 smear counted in the high
background actually reads between 180k to 250kdpm/100cm2 on a frisker in a
1kdpm background.  I have yet to account for this.  The Eberline folks don't
understand it either.  It's a standing joke in Containment during Outages
when a person gets a 12kdpm/100cm2 count in a 10kdpm background and calls it
"2k" that the individual is stuck with that name for the whole Outage until
he/she counts the smear in a much lower background and find it reading
between 12k and 15kdpm/100cm2.

I appreciate your input - Paul.
> -----Original Message-----
> From:	Dru Carson [SMTP:carsond@ludlums.com]
> Sent:	Wednesday, October 13, 1999 8:30 AM
> To:	Multiple recipients of list
> Subject:	RE: GM Counters in High Backgrounds
> 
> 
> 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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