An instrument for measuring the unattached fraction of radon progeny with etched track detectors.

To measure the unattached fraction of radon progeny, a new type of portable, low cost, reusable integrating monitor with etched track detector (CR-39) was developed. This monitor simultaneously gives the average equilibrium-equivalent concentrations of unattached (222)Rn progeny (EEC(Rn)(u)), attached (222)Rn progeny (EEC(Rn)(a)) and also (220)Rn progeny (EEC(Tn)). It was calculated, according to the detection efficiencies which were derived by the Monte Carlo method, that the lower detection limits for EEC(Rn)(u), EEC(Rn)(a) and EEC(Tn) are 0.03  Bq m(-3), 0.12  Bq m(-3) and 0.07 Bq m(-3), respectively for 24 h integrating sampling at a flow rate of 2.92  l min(-1). The validity of the new monitor was confirmed through a series of intercomparison experiments.