Smears (AKA Swipes) -Reply

[ABELQUIE@ORAU.GOV]

 Tony wrote:

 Does anyone know of any studies on taking smears/swipes?  I am
trying to
put together a Statistics class for technicians who are hostile to training. 
You know the types: "This won't help me do a better survey . . ."  The
thought
came to me that no one has studied the manner of smear taking.  Many
plants use
1 3/4 inch smears, but some still use 1" smears.  The wider smear allows
the
surveyor to place an extra finger on the backside surface and thus
swipe more
area if they use the same 16 to 18 inch "S" shape.  I would sure
appreciate any
input here.


Tony-

A few studies have been reported in the HP Journal describing the
tremendous variability (in collection efficiency) inherent in smear
sampling.  References include:
1) Campbell et al. "Wipe Testing for Surface Contamination by Tritiated
Compounds" Health Phys. 64(5):540:544, 1993
2) Klein et al. "Detecting removable surface contamination" Health Phys.
62(2):186-189; 1992

Recognizing this variability, the guidance in ISO-7503 ("Evaluation of
Surface contamination - Part 1:Beta emitters (maximum beta energy
greater than 0.15 MeV) and alpha emitters, 1988) states that "the smear
material should be chosen to suit the surface to be checked (for example
filter paper for smooth surfaces, cotton textile for rough surfaces)" and
adds that if the collection efficiency is not determined experimentally, a
value of 10% should be used. Of course, collection efficiency is also
affected by  "human factors" such as finger pressure on smear, size of
"S", etc.  

In practice, the collection efficiency is frequently ignored and is implicitly
taken to be unity (i.e., the smear is counted and only the detector 
efficiency, not collection efficiency, is used to evaluate the amount of
removable surface activity).   

The MARSSIM recognizes the limitation in using smears to quantify
removable activity.  Section 8.5.3 (note: there are two sections identified
as 8.5.3, removable activity is discussed in 2nd section) states  "...
measurements of smears are very difficult to interpret quantitatively. 
Therefore, the results of smear samples should not be used for
determining compliance.  Rather, they should be used as a diagnostic tool
to determine if further investigation is necessary."  

However, I realize that smears are certainly necessary for evaluating
compliance with surface activity limits on packages, etc. for
transportation. 

Hope this helps.

Regards,
Eric Abelquist
abelquie@orau.gov