[ RadSafe ] [Radsafe] equilibrium factors for thoron?

Kai Kaletsch eic at shaw.ca
Wed Sep 9 10:55:34 CDT 2009

Hi Nick,

I think the equilibrium factor for thoron can vary widely, depending on the 

First, lets look at a room model. To get an equilibrium factor of 0.4 for 
Rn-222, you need a mechanical (ventilation and plateout) radon progeny 
removal rate  equivalent to 1.7 air changes per hour. See 

With a similar removal rate, the equilibrium factor for Rn-220 is about 
0.04. (It can be approximated by the ratio of the decay constant of Pb-212, 
0.063/h, and the mechanical Pb-212 removal rate i.e. 1.7/h, in this 
example.) In a situation with more air changes than in a house, i.e. most 
industrial facilities, the equilibrium factor would be even lower.

However, in a different physical situation, you can get equilibrium factors 
greater than 1. Suppose you are 10 minutes downwind of a Rn-220 source. Most 
of the Rn-220 will have decayed and you are left with mostly Pb-212 and your 
equilibrium factor is 1.5. (The original Rn-220 concentration has been 
reduced by 10 half lives (i.e. a factor of 1/1024), while the activity of 
the resulting Pb-212, compared to the original Rn-220 activity, is given by 
the ratios of the half lives of Rn-220 and Pb-212 (1 minute / 11 hours = 
1/660) and the equilibrium factor is 1024/660 = 1.5)

On the other hand, if you are 10 minutes downwind of a Rn-222 source, the 
equilibrium factor is 0.2 see 
http://members.shaw.ca/eic/Tools/Mine_Tunnel.htm . So, in these 2 different 
scenarios, the Rn-222 progeny equilibrium varied between 0.4 and 0.2, while 
the Rn-220 progeny varied between 0.04 and 1.5, which is a much greater 

Bq/m3 EEC, is a (very confusing) unit of potential alpha energy 
concentration and has very little to do with the unit of Bq/m3, which is a 
unit of activity concentration. So, if a regulator sets a limit of X Bq/m3 
EEC, he is limiting the potential alpha energy concentration in air and, 
therefore, the dose to anyone breathing this air. This is a reasonable thing 
to do and it does not involve any assumption about equilibrium factors.

BTW, I thought Australia was using µJ/m3 as the unit for the potential alpha 
energy concentration in air, rather than Bq/m3 EEC. Can you confirm this? In 
North America, we use WL and I thought Bq/m3 EEC was used mainly by the 
former eastern block countries. (We offer an on-line radiation data 
management system for U mines and we should get the units used by different 
countries right!)

Hope this helps.

Kai Kaletsch
Environmental Instruments Canada Inc.
#202 135 Robin Cr.
Saskatoon, Saskatchewan
Canada S7L 6M3

----- Original Message ----- 
From: "Nick Tsurikov" <nick.tsurikov at gmail.com>
To: <radsafe at radlab.nl>
Sent: Wednesday, September 09, 2009 1:20 AM
Subject: [ RadSafe ] [Radsafe] equilibrium factors for thoron?

> 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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