A new CR-39 nuclear track passive thoron measuring device

220Rn monitoring in dwellings and in workplaces is seldom performed, both because usually the associated risk is considered negligible and for the difficulties of properly measuring it. As a consequence the state of art of thoron measurements by using solid-state nuclear track detectors shows a significant lag compared with radon. However the demand of determining the thoron contribution to natural dose assessment is worldwide increasing, and in the presence of some building materials as tuff, should be mandatory. To tackle this problem, the ENEA Radon Measuring Service has developed a dose evaluation procedure based on the measurement of the 220Rn concentration at a standard distance from the wall, by using a new (patent pending) CR-39 alpha track detector thoron passive device (ATD-TnPD), coupled to our own radon passive device (ATD-RnPD). The two devices are geometrically similar and their responses were extensively evaluated to optimise the uncertainty of the thoron measurement. This paper depicts the prime features of the new device, and the introduced changes aimed at reducing the device's equilibrium time constant from 20.8 min to 3.1 min. Since no reliable thoron calibration facility has resulted available up to now, we had to state the ATD-TnPD's sensitivity parameter by performing the calibration step by our own. A closed circuit thoron exposure facility was set up to provide thoron exposures at 3 different levels. The exposed detectors were chemically etched at 7 increasing etching times to study the relationship between sensitivity and etching conditions, so to optimise the detector response and the measurement uncertainty.