Influence of the type of CD case on the track density distribution in CDs exposed to thoron

A novel approach for retrospective thoron (Rn-220) measurements in buildings was recently proposed. It employs CDs/DVDs as detectors, in which the alpha-tracks formed by thoron's progeny Po-212 are analyzed. Compact disks and DVDs that had been stored in their cases are suitable, because the case provides a fixed and reproducible geometry of the Po-212 source. Since the measurement and calibration procedures involve analysis of different pieces of the disk, it is important to test the homogeneity of the track density over the disk area. This report presents results of such a study, in which disks in different containers were exposed to thoron. In almost all disks, the track density was inhomogeneous, increasing significantly near the holes in the container through which thoron could enter. However, as demonstrated, in disks regularly used and randomly rotated in their containers, the track density is expected to homogenize. It is concluded that the homogeneity of the tracks should be tested in disks exposed to thoron in homes and should be estimated or compensated in calibration exposures.