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Re: Puzzler: How Does Humidity Affect Counting Efficiency?
Hi all,
Most don't realize it, but at a given temperature and pressure humid air is
less dense than dry air. It is not a big factor, but it exists. I first
heard about it in junior high from a science teacher who was an ex-military
pilot. Planes have a noticeably longer take off roll on humid days since
they have to reach slightly higher speed to take off..
If the distance between the window of your Tennelec and the sample is
significant, that could be a factor for alpha's but not beta's.
Dale
----- Original Message -----
From: "Wes Van Pelt" <WesVanPelt@att.net>
To: <John_Sukosky@DOM.COM>; <radsafe@list.Vanderbilt.Edu>
Sent: Thursday, May 27, 2004 9:17 AM
Subject: RE: Puzzler: How Does Humidity Affect Counting Efficiency?
> 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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