RE: Puzzler: How Does Humidity Affect Counting Efficiency?

["Wes Van Pelt" <WesVanPelt@att.net>]

John and Radsafers,



Well, here is my second guess at the solution to this puzzler.



But first, review the facts:



The counting efficiency went up over 2 or 3 months as humidity went up. 

The backgrounds stayed the same.

The correlation holds true for three TOTALLY DIFFERENT types of instruments:

liquid scintillation, 

	semiconductor detector (HPGe), and

	gas flow proportional.

All are pulse counting instruments with a lower level discriminator or a

counting window (region of interest). 

All use electricity to generate a high voltage (on the PM tube and gas

chamber) or a voltage (across the semiconductor detector). 



The only connection among these three counters is the electric power supply.

My guess is that, over several months, the voltage or current supplied to

the three instruments changed.



I suggest you measure and record the voltage supplied to these instruments

over a period of time. Perhaps the voltage drops as your air conditioning

units come on line as the season gets warmer. You could also get an

Uninterruptible Power Supply (UPS) or isolation transformer that regulates

the output voltage to a constant value (e.g., 117 volts ac) even if the

input voltage drops.



Best wishes,

Wes



Wesley R. Van Pelt, PhD, CIH, CHP

Wesley R. Van Pelt Associates, Inc.



-----Original Message-----

From: owner-radsafe@list.vanderbilt.edu

[mailto:owner-radsafe@list.vanderbilt.edu] On Behalf Of John_Sukosky@DOM.COM

Sent: Wednesday, May 26, 2004 8:31 AM

To: radsafe@list.vanderbilt.edu

Subject: Puzzler: How Does Humidity Affect Counting Efficiency?



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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