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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