Re: Puzzler: How Does Humidity Affect Counting Efficiency?

[Kai Kaletsch <eic@shaw.ca>]

Could it have something to do with your source? Pb-210 does not give off

alpha, so you are relying on the progeny (Po-210) to stick around long

enough to decay and give off an alpha. Could Po-210 escape from your source?



Also, I would expect a good alpha counter never to change efficiency (unless

it's broken). That's what makes gross alpha counting so useful for field

use. The efficiency of a good alpha counter should only be given by

geometry: If the alpha particle hits the detector it gets counted with 100%

efficiency, if it misses the detector it doesn't get counted. If this is not

the case, the window is too thick, the detector is too far away from the

source, or the unit is not plateaued properly.



Regards,

Kai



Kai Kaletsch

Environmental Instruments Canada Inc.

http://www.eic.nu



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

From: <John_Sukosky@DOM.COM>

To: <radsafe@list.vanderbilt.edu>

Sent: Wednesday, May 26, 2004 7:31 AM

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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