RE: Puzzler: How Does Humidity Affect Counting Efficiency?

[John_Sukosky@DOM.COM]

Had the service rep in on Friday to check out the Tri-Carb.  I showed him

the data and went over the Radsafe feedback with him on the tritium check

source results proportional to humidity.  First, he said that he had never

heard of anyone having this problem before.  Of course, he also has never

heard of anyone calibrating at 10% humidity and operating at 60% humidity!

He called the Technical Expert at the main office and relayed his thoughts

on what might be happening. They think that air humidity is condensing on

the glass vials while they're in the refrigerated Tri-Carb cabinet.  The

water on the outside of the glass is luminescencing while it's counting.

According to the expert, it's easier for the Tri-Carb to discriminate

non-tritium counts at lower count rates than at higher count rates.  I'm

not sure of the technical reason for this.



One guess from me: the Tri-Carb has two PM tubes and the sample is counted

between them.  It may be that it by having the PM tubes simultaneously

counting the sample, counts coming from non-uniform condensed humidity on

the outside of the vial do not match the timing criteria that it expects to

see for tritium counts.  Of course, I would think that even if the

condensed humidity on the outside of the vial luminescents, the light

spectrum would be quite different than from the tritium light spectrum

coming from inside the vial and easy to discriminate at any count rate.

The problem in both cases may be that it can't completely discriminate

against the false counts due to spill-over into the tritium portion of the

spectrum.  But I still can't figure out why it only happens at higher count

rates.



Anyone have any thoughts on this?  Does it seem plausible to you?  I do

have to confirm that humidity is making it's way into the cabinet.  In very

humid conditions, when we open up the top to put in a sample, it's actually

raining inside (water dripping off the inside of the cabinet).





John M. Sukosky, CHP

Dominion

Surry Power Station

(757)-365-2594 (Tieline: 8-798-2594)



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