Re: Puzzler: How Does Humidity Affect Counting Efficiency?

["Ian Adam" <Ian.Adam@icr.ac.uk>]

Sorry- mis-posted this the first time!



John



One possibility that occurs to me is that of leaking high value

resistors and cables. Surface effects due to ageing, surface

degradation, contact with skin oils/perspiration, or accumulation of

dust can all work towards surface absorbing of humidity and reduction of

resistance.



Depending where this is in the circuit it can lead to a change in HT,

either in the HT generation circuits or in the

control/feedback/regulator circuits.



Another possibility is that a change might occur in the e and delta-e

(threshold and window width) that could lead to a change in counts

depending on where the window was looking.



In these cases the check would be to re-peak the system occasionally to

see if windows are drifting.



Ian





Ian Adam

Radiation Safety Officer

The Institute of Cancer Research

Cotswold Road

Sutton

Surrey

SM2 5NG

Tel: 020 8722 4250

Mob: 07791 702 230

Fax: 020 8722 4300

EMail: Ian.Adam@icr.ac.uk 





>>> <John_Sukosky@DOM.COM> 26/05/2004 14:31:09 >>>









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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