Automatic Counting of Electrochemically Etched Tracks in Compact Discs. Application to Retrospective Measurements of Rn-222

This work describes an algorithm for automatic counting of alpha tracks in scanned images of electrochemically etched CDs and DVDs. It is demonstrated that the algorithm copes with the specific problems encountered in retrospective $^{222}{\rm Rn}$ measurements with CDs and DVDs; it reliably separates overlapping tracks and ignores artifacts in the image. The algorithm is validated by comparison to manual counting and a very good correlation is found. The measurement range of $^{222}{\rm Rn}$ concentrations in dwellings and soil gas is estimated. It is shown that the wide span of track densities that can be covered by the algorithm combined with the specific advantages of the compact disc method can provide reliable measurements in the whole range of $^{222}{\rm Rn}$ concentrations of practical interest. For retrospective measurements of $^{222}{\rm Rn}$ in dwellings the estimated range is from 10 ${\rm Bq}/{\rm m}^{3}$ to $100~{\rm kBq}/{\rm m}^{3}$ for five years exposure time. For $^{222}{\rm Rn}$ in soil gas the estimated range is from $1~{\rm kBq}/{\rm m}^{3}$ to 10 ${\rm MBq}/{\rm m}^{3}$ for ten days exposure time.