We were taught in the US Navy nuke program that
the regulation was alpha or beta-gamma contamination/1cm^2. And if you look
in the regs that's what it says.
However, the 100 cm^2 became a convention based
on the princple background of K40 is around 5 to 50 uuCi/100 cm^2
. A count of 450 uuCi/100cm^2 was 10 to 100 times normal background or
100 cpm above background, with a DT-304 frisker probe at 1/2 inch above the
surface, which is where we were told to set our GM detectors to alarm
at. This allowed for random fluctuation in background without
continuous alarms. This comes from NAVSEA 0989-015 printed in 1975 Rev 4 and
first appeared in 1957 in the Navy's RadCon Manual NAVSHIPS 0153.
The motion of the smear as stated by others is
based on two fingers pressed together on a smear makes a 1" wide mark. Now
if that 1" is dragged over 16" then approx. 100cm^2 has been covered.
The 'S' shape was to ensure the area was "averaged", which is not done is a
straight line is done.
If you look at 49CFR 173.443 a wipe of up to
300cm^2 is permissible.
Dan Mackney
Dirrector of Radiochemistry
Wate Stream
Technology
----- Original Message -----
Sent: Friday, February 01, 2002 2:50
PM
Subject: Removable Contamination
Surveys
Can anyone give me some insight (and possible
source documents) into the reasons for the 100cm^2 standard (or
suggestion?) used for removable contamination surveys? Why is it
100cm^2 and not 1,000cm^2, or something smaller or larger? Is there
anything wrong with increasing the swiped area to increasing the
likelihood of detection?
Specifically, assume a removable contamination
limit is set at 200 dpm/100cm^2 for a laboratory and a researcher wishes
to use a survey meter to count the swipes. According to the
manufacturer's formula and values (for efficiency of the radionuclide,
etc), the meter has an MDA of 400 dpm. Is there anything wrong with
performing the survey over 200 cm^2 so that the detection limit would meet
the removable contamination limit?
Thanks,
Pete Jenkins
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