RE: Data on Porous Surfaces

["Abelquist, Eric" <ABELQUIE@ORAU.GOV>]

Randy-

ISO-7503 (1988) presents an excellent approach to the problem you
mention.  This standard allows the overall (or total) efficiency to be
divided into two components: instrument efficiency and surface
efficiency.  The instrument efficiency is a 2 pi efficiency and accounts
for such factors as the calibration geometry and detector window
thickness.  The surface efficiency is the fraction of the radiation from
the total surface activity that is emitted from the surface (in the
direction of the detector), and is nominally 0.5.  Backscatter increases
surface efficiency, while self-absorption decreases the value
(particularly for alpha radiation).  In fact, ISO-7503 recommends a
nominal value for alpha surface efficiency of 0.25.  [This approach has
been adopted in the recently finalized MARSSIM manual.]

We performed experiments for the NRC in support of their decommissioning
rulemaking that evaluated the effects of various surface conditions on
detectability of commercially available instrumentation, with emphasis
on surface efficiencies.  Results have been published in NUREG-1507,
which is available on the NRC web page.  Overall, our research indicates
that the surface efficiency used for alpha radiation (0.25) is not a bad
starting point, but that under some situations encountered in
decommissioning projects, even an alpha surface efficiency of 0.1 may be
overly optimistic.   

Finally, one question that usually comes up in this discussion is: at
what point does your surface contamination become a volume contamination
problem?

Sincerely,
Eric Abelquist
ORISE
abelquie@orau.gov
423 576 3740

-----Original Message-----
From: Redmond, Randy R. (RXQ) [mailto:RXQ@ornl.gov]
Sent: Wednesday, November 04, 1998 6:38 AM
To: Multiple recipients of list
Subject: Data on Porous Surfaces


I am trying to find some data or, preferably, reports or articles, which
deal with the expected quantity of rad contamination which could be
trapped in the pores of a porous surface and remain undetected while an
alpha scan shows the surface to be at or below some release criteria.
Surely, someone has done an experimental or theoretical study of this -
I am just after a very rough estimate of what we might have hidden in
the pores of a closed cell foam rubber.  Data for large cell porous
ceramics or weathered wood would probably be quite relevant.


Randy Redmond
Lockheed Martin Energy Systems
P.O. Box 2009
Bldg.  9769,  MS 8081
Oak Ridge, TN  37831-8081
Email:  rxq@cosmail3.ctd.ornl.gov
Phone:  423-574-5640
Fax:  423-576-6047

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