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FIELD TESTS OF A RADON PROGENY SAMPLER FOR THE DETERMINATION OF EFFECTIVE DOSE

S.B. Solomon
Australian Radiation Protection and Nuclear Safety Agency
Lower Plenty Road, Yallambie
Victoria, Australia, 3085
Tel. +613 94332211, Fax. +613 9432 1835, Stephen.Solomon@health.gov.au

The International Commission on Radiological Protection (ICRP) recommends the use of a single
conversion factor, derived from epidemiological studies of exposure to uranium miners, for the
determination of the effective dose from inhalation of radon progeny. Dosimetric models of radon
progeny inhalation predict that the dose conversion factors (DCF) are dependent upon the form of
the radon progeny activity size distribution. The measurement of these activity size distributions is
difficult and an alternative approach has been proposed. The so-called Effective Dosimeter uses a
two-screen sampler with a collection efficiency matched to the particle size behaviour of the radon
progeny DCF, as determined from the ICRP Human Respiratory Tract Model. For this present
work an Effective Dosimeter was constructed as the second stage of a six-stage wire screen
diffusion battery. This diffusion battery was operated at a continuous sampling rate of 0.8 lpm,
with in-situ counting of the alpha particle activity from the progeny deposited on the filters
providing an estimate of the radon progeny potential alpha energy concentration (PAEC). This
hybrid system allowed two methods for the determination of the radon progeny DCF. The activity
size distributions, measured using the diffusion battery, were combined with the values of the DCF
as a function of particle size to obtain a size-weighted DCF. The DCF values were obtained from
the ICRP respiratory tract model, as implemented in the computer code RADEP. The second
determination of DCF was obtained directly from the fraction collected by the Effective
Dosimeter. The hybrid diffusion battery was used to measure radon progeny in the Fairy Cave,
Buchan, Victoria at 20-minute intervals over 30 hour period. This cave had radon concentrations
exceeding 2000 Bq m -3 , with low aerosol concentration and ventilation rate. The measurements
were analysed to determine the radon progeny PAEC, the activity size distribution, the size-weighted
DCF and the Effective Dosimeter collected fraction. The Effective Dosimeter DCFs
were determined from the collected fraction using firstly a simple linear function and then using a
more complex polynomial function to correct for residual errors. For the linear factor alone, the
calculated Effective Dosimeter DCFs were on average 11% lower than the equivalent size-weighted
DCF values. The agreement using the polynomial function was improved markedly, with
a with a linear regression of the DCF yielding a fitted ratio of 0.965, with a R value of 0.99.
Key words: Radon, radon progeny, activity size distributions, unattached fractions, ICRP
Respiratory Tract Model, dose conversion factors, show caves, Effective Dosimeter