Peter,
I'm not sure if this helps, but I hope
so. In the UK we have three different sized areas for removable
contamination measurements. These are:
100 cm2 for skin and clothing.
300 cm2 for inanimate objects e.g items of plant an
equipment (pumps, valves, pipework etc.)
and 1000 cm2 for floors and walls.
These area and the associated derived limits for
contamination (i.e. 3.7 Bq/cm2 for most betas [corresponding to approx. 200
dpm/cm2] and 037 Bq/cm2 for most alphas) were first published in the UK in
1979. The National Radiological Protection Board (similar to the NCRP)
published a document entitled Derived limits for surface contamination, ref.
NRPB-DL 2. This documents summarised how the above values were
calculated and how they related to possible doses/ uptakes by workers in the
nuclear industry. In the document it states that "advice on monitoring
of skin contamination is given by the ICRP [ICRP publication 26 1977], which
recommends the averaging of measurements over an area of 100cm2." This
may seem to be the origin of the area of 100 cm2, but it would seem that it
was not intended to apply to anything other than skin contamination, although
without a copy of ICRP 26 I cannot confirm this. (is there anyone out
there who can?) Unfortunately the values of 300cm2 and 1000cm2 were
already custom and practice within the UK nuclear industry in 1979 and there
is no explanation included on how these values were derived.
It is interesting to not however that the clearance level in
the UK for most beta emitting isotopes is 3.7Bq/cm2 which is very close to the
value of 200dpm/cm2 used in the US and that the value of 100cm2 is also common
to both countries.
What about other countries? (e.g. Europe, Canada
etc.)
----- Original Message -----
Sent: Friday, February 01, 2002 7:50
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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