Re: Puzzler: How Does Humidity Affect Counting Efficiency?

[daleboyce@charter.net]

I used to do a lot of low background counting and radon was definitely a

variable in gas flow counting.  We saw effects for both beta and alpha, but

we had some really low background beta counters.  However, the correlation

was with barometric pressure changes.  It was most apparent when doing real

time radon monitoring with ZnS cells.



The background went up after a drop in pressure. Radon that has accumulated

in the soil is sucked out by the pressure change. As I remember, we observed

a time lag that we attributed to ingrowth of the lead and bismuth daughters.

If you have the capability I'd go back and look to see if a low pressure

front passed through a few hours before you noticed the background increase.

There is also seasonal variation in radon.



As Wes points out people frequently notice radon (daughters) after a rain

storm.  Rain usually comes along with a low, and it washes the daughters out

of suspension in the air.  This sometimes causes problems with portal

monitoring after a light rain.   As I remember heavy rains tend  to wash the

daughters away.



We did not see it in our low level gamma facility, but it had a HEPA

filtration on the input and high rate of air turnover. So there was little

chance of radon build up in the room.  The air source was from the roof.



I can't say I ever noticed an effect in LSC, but I didn't look too hard at

that.



Dale





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

From: "Wes Van Pelt" <WesVanPelt@att.net>

To: <John_Sukosky@DOM.COM>; <radsafe@list.Vanderbilt.Edu>

Sent: Wednesday, May 26, 2004 11:16 AM

Subject: RE: Puzzler: How Does Humidity Affect Counting Efficiency?





>

> John Sukosky and All,

>

> I always like a good puzzler.

>

> Well, here is a (wild) guess. When the humidity is high it may have rained

> recently. After a rain, radon emanation from the soil may be increased.

With

> increased radon emanation the background could increase.

>

> From your description, it seems that the counting efficiency correlates

with

> humidity.  Does background count rate also correlate with humidity?

>

> Best regards,

> 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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