Diffusion length of Rn-222 in home-stored CDs/DVDs — influence on Rn-222 and Rn-220 measurements

The CDs/DVDs stored indoors could serve as retrospective
²²²
Rn (radon) and
²²⁰
Rn (thoron) detectors. In the method the disks are used as track-etch detectors of alpha-particles. The signal used for radon measurement is formed by
²²²
Rn which diffuses inside the disk volume. The signal used for thoron measurements is estimated after a correction for the contribution of radon is made. This correction and the radon-related signal both depend on the diffusion length of
²²²
Rn in the disk material (L
_D
). The work studies the influence of L
_D
on the results of
²²²
Rn and
²²⁰
Rn measurement by CDs/DVDs. For this purpose, L
_D
is determined for used disks naturally exposed to radon in the dwellings from which they were collected. This is done a posteriori using the alpha-tracks accumulated in the disks. The observed values range from 45.4 ± 2.0 μm to 74.3 ± 1.8 μm with an average of 54.2 ± 5.4 μm. Examples of the analysis of 62 disks collected from dwellings demonstrate the influence of L
_D
on the results for thoron. In some cases using an average L
_D
(instead of the actual) can lead to false detection or oversight of thoron. It is recommended that for such disks the diffusion length of radon during the exposure is estimated a posteriori.