Re: Counting Wet Smears

[Bob Hearn <rah@america.net>]

"How wet is wet?" is a very significant point. You may have factors of
variability related to the pH of the water/filter/sample material,
solubility of the sample deposition, degree of sorption of the wet
deposition into the media, effect of variations in thickness of water layer,
surface tension effects...

Not to be a nay sayer, but recognize that a combination of factors will
warrant a degree of conservatism that will be difficult to establish since
the range of variables cannot all be controlled or calibrated. Caution at best!

Good luck!

Bob Hearn

At 02:45 PM 10/29/96 -0600, you wrote:
>Good afternoon,
>
>    If it were critical to count the wet smears, I think the approach I would 
>take would be to characterize several dry smears to get a very accurate 
>determination of the activity.  Then I would measure the activity with my
field 
>equipment to establish a "dry" reading.  Then, I would moisten the smears and 
>re-read them.  The difference between the wet and dry readings would give me a 
>correction factor for the instrument.
>
>    When checking them in the field, I'd read the wet smears and apply the 
>correction factor to get field results.  I'd then take the smears back to the 
>lab, dry them out, and count them at my leisure.
>
>    One could also think about using NaI (maybe a 3" X 3"), but I would be 
>afraid that the background would obliterate any chance of seeing low levels of 
>contamination on smears.
>
>
>Jim Barnes, CHP
>Radiation Safety Officer
>Rocketdyne Division; Rockwell Aerospace
>
>    >      I am looking for information on methods to count wet smears.  Much 
>    > of 
>    >      the work performed at Hanford is out-of-doors and can't necessarily 
>    >      stop just because of a little rain or dew.  Currently, smears are 
>    >      dried before counting.  However, if there is a justifiable
technique 
>    >      for counting a wet smear, I would like to hear about it.
>    >      
>    >      In addition, are there any definitions of "how wet is wet"?  At
what 
>    >      point is a smear considered a wet smear?
>    >      
>    >      The isotopes of concern are Co-60, Eu-152, and Cs-137.  The smear 
>    >      media is masslin and cloth technical smears.
>    >      
>    >      Please send replies to me directly at ml_johnson@ccmail.pnl.gov.
>    > 
>
>
>
>