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Dose from Rn decay products collected on a filter

A few days (weeks?) ago someone was asking about dose rate from an air filter collecting radon progeny.  Perhaps someone else gave the answer directly the interested person. 
A back-of-an-envelope calculation gives the following for a Rn concentration of 37 Bq/m3 (1 pCi/L) at equilibrium 0.7, 0.5, 0.3 for Po 218, Pb 214, Bi 214 respectively, a sampling rate of 60 L/min (1 L/s) and a gamma constant of 2.25 e-6 Sv/h.Bq (ORNL/RSIC-45/R1 Report (1982) :
Bi 214 is the only significant gamma-emitting radon decay product.  The equilibrium activity for Bi 214 (sampling time more than 5 hours) on the filter is about 6 e4 Bq, giving a steady state dose rate at one meter of
 6 e4 Bq x 2.25 e-6 Sv/h.Bq = about 1.4 e-7 Sv/h or 0.14 microSv/h  for 37 Bq/L of radon at stated equilibrium factors.
Philippe Duport
International Centre for Low Dose Radiation Research
University of Ottawa
555 King Edward Ave.
Ottawa, ON, Canada, K1N 6N5
Tel: (613) 562 5800, ext. 1270