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RE: Puzzler: How Does Humidity Affect Counting Efficiency?
Just guessing, but it seems like the ionization potential for moist air
might be lower than for dry air. If so, it would be easier to ionize the
moist air, so it seems likely that more ionization would be produced in
moist air and given that the moist air is a better conductor this also
would also contribute to an increased count because the ions would more
easily move to be detected. I would guess that the reason that you don't
see this in the beta channel has to do with the energies and discriminator
setting.
A slightly different topic, but related to surprising detector
responses: at one point we were counting black wipes versus the usual
white wipes in a windowless gas flow proportional counter. Initially the
wipe would produce elevated counts, but within minutes the counts dropped
to background levels. These black wipes seemed to pick up a charge during
the wiping process that dissipated within minutes. It was alarming to
think that we'd contaminated our HP office.
Good luck with your sleuthing.
Cindy
At 04:19 PM 5/26/2004 -0600, Neil, David M wrote:
>Err ... I think you have it backward. Dry air has less conductivity, not
>more. That's why the static charges can build up in the first
>place. Moist air bleeds off the charge.
>
>Dave Neil
>
>-----Original Message-----
>From: John_Sukosky@DOM.COM [mailto:John_Sukosky@DOM.COM]
>Sent: Wednesday, May 26, 2004 12:43 PM
>To: Flanigan, Floyd
>Cc: radsafe@list.vanderbilt.edu
>Subject: RE: Puzzler: How Does Humidity Affect Counting Efficiency?
>
>
>
>
>
>
>Floyd,
>
>Thanks. Yes, we considered air conductivity but it would appear to have
>the opposite effect. In other words, dry air would have more conductivity
>(including more static discharges) compared to more humid air. Humid air
>would therefore show a decrease in count rate (not an increase like we're
>measuring).
>
>John
>
>
>
>
>
> "Flanigan,
> Floyd"
>
> <Floyd.Flanigan@nmcco. To:
> <John_Sukosky@DOM.COM>,
> com> cc:
>
> Sent by: Subject: RE:
> Puzzler: How Does Humidity Affect Counting Efficiency?
> owner-radsafe@list.Van
>
> derbilt.Edu
>
>
>
>
>
> 05/26/04 11:30
> AM
>
> Please respond
> to
>
> "Flanigan,
> Floyd"
>
>
>
>
>
>
>
>
>
>I think there is something to think about. If the humidity effects alpha
>but not beta count rate on the Tenelec I tend to lean away from a common
>component culprit. Have you looked at a correlation on air conductivity and
>electrical component proximity? Just a thought.
>
>Floyd W. Flanigan B.S.Nuc.H.P.
>
>-----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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