[ RadSafe ] [Radsafe] equilibrium factors for thoron?

Nick Tsurikov nick.tsurikov at gmail.com
Wed Sep 9 02:20:48 CDT 2009

Dear colleagues,

I am getting a bit lost...  We all know the equilibrium factors for Rn-222:
0.2 for outdoors and 0.4 for indoors, which work perfectly fine - at least
in a first approximation for assessing the doses.
However, the same equilibrium factor for thoron (Rn-220) is quite a mystery
(maybe just for me...). As there is more equipment now that provides a
direct measurement of radon and thoron concentrations (which is quite
convenient) I am having trouble finding any references to convert the Bq/m3
to dose.  Some rather interesting numbers were quoted to me last week - for
example that the thoron equilibrium factor may be as low as 0.01, on the
other hand other people telling me that it may be up to 0.6-0.7.
It will be very much appreciated if you could point me out in the proper
direction - some regulatory documents, studies, whatever - that I can use in
trying to compare the levels I am measuring with the limits that are in some
places set up for 'equilibrium equivalent concentrations' - EEC.  The
regulators who wrote the document put there the values of 60 Bq for EEC of
Rn-222 and 10 Bq/m3 for EEC of Rn-220, but are not sure themselves what
equilibrium factors were used; and your help will be very much appreciated.

Kind regards
Nick Tsurikov
Western Australia

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