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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.)
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