Puzzler: How Does Humidity Affect Counting Efficiency?

[John_Sukosky@DOM.COM]

We've had problems controlling the environmental conditions in our Counting

Lab due to a degradation of the room's environmental control system.

Counting equipment calibration and setting of statistical ranges occurred

during the winter months.  As the temperature changed and humidity

increased in the past few months, here in Virginia, our source checks on

some instruments have gone into the warning (> 2 Sigma) and Control (> 3

Sigma) levels.  We analyzed the data with temperature, humidity and

barometric pressure.  There is no correlation with temperature or

barometric pressure but almost a perfect correlation with humidity.  When

humidity goes up, count rate goes up at the same rate.  When humidity goes

down, count rate goes down at the same rate.  Here's a list of affected

instruments and source check results:



Instrument   Type                            Humidity/Count Rate



Tri-Carb     Liquid Scintallation Counter    Strong Correlation



Tennelec     Gas Flow Proportional Counter   Alpha - Strong Correlation

                                             Beta - No Correlation



HPGe         High Purity Germanium           Strong Correlation





Does anyone have any ideas on what the mechanism could be for this effect?

We suspect that there is some common electronic component between all three

of these instruments that's affected by humidity. What we can't understand

is why the Tennelec Alpha channel is strongly affected by humidity but the

beta channel shows absolutely no correlation with humidity???  It's using

the same source (Pb-210) that's counted at the same time for checking both

alpha and beta channels.



Thanks for any ideas on this.  We're really perplexed on this one!





John M. Sukosky, CHP

Dominion

Surry Power Station

(757)-365-2594 (Tieline: 8-798-2594)



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