Re: Calibratiion using a 25 mm source vs. a 47 mm smear

["William E. Cox" <COXWX@naesco.com>]

Cindy,

NCRP 112 discusses calibration source size in section 7.2.4.  If you are looking
for point source contamination then a small efficiency source may be
appropriate.  However, when you are looking for distributed contamination, you
should use a distributed efficiency source.  This is also discussed in ANSI
42.25 (Cal and Use of Proportional Counters).  You can get significantly
different efficiencies (delta ~20%) for the same nuclide on a given system using
different size sources.

Another thing to watch out for is the true active area of the 25 mm source.  In
some instances, these may have much smaller active areas (e.g., 2 mm) this is
normally stated on the source certificate.

How you characterize the distribution of contamination on a smear is another
matter, however I would think that it would tend to be more distributed than
point like.

Billy Cox
Seabrook Station
Health Physics
(603) 773-7319
coxwx@naesco.com


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