Tests of CDs/DVDs as passive radon and thoron detectors for mines and caves

Abstract

Radon (²²²Rn) exposure is evident health hazard in a substantial number of underground mines. Exposure to thoron (²²⁰Rn) is usually considered to be of secondary importance, but radiation data regarding ²²⁰Rn is very scarce compared to that for ²²²Rn. In planning area and personal monitoring in underground places (mines and caves) ²²⁰Rn issue should be addressed at least for two reasons: (1) to revise the possibility for increased exposure related to ²²⁰Rn; (2) To ensure that ²²²Rn results are not biased due to the sensitivity of detectors to ²²⁰Rn, as it was recently found for many commercial ²²²Rn detectors. In this report we test the applicability of CDs/DVDs as novel passive detectors for differential measurements of ²²²Rn and ²²⁰Rn in underground mines and caves. This is achieved by etching and counting alpha tracks at two different depths beneath the disk surface and analysis of the signals. To test the performance of the method and to check whether there is significant presence of thoron, 13 CDs were exposed in the underground galleries of a metal mine for 4 months. Results revealed that both ²²²Rn and ²²⁰Rn are present at the monitored places and in some cases the levels of ²²⁰Rn are higher than those of ²²²Rn. The CDs proved to be appropriate for measurements in the underground environment and stayed undamaged during the exposure. Additionally, dedicated laboratory experiments were made to test an approach to expand the range of ²²⁰Rn concentrations measurable at high ²²²Rn concentrations. The method proved its applicability for differential measurements of ²²²Rn and ²²⁰Rn over a wide range of concentrations. Radon concentrations well below the regulatory levels for workers are measurable even within 1 month exposure.