Available online at www.sciencedirect.com Radiation Measurements 38 (2004) 5–17 www.elsevier.com/locate/radmeas Calibration factors for LR-115 (type-II) based radon thoron discriminating dosimeter K.P. Eappen ∗ , Y.S. Mayya Environmental Assessment Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India Abstract Twin chamber solid state nuclear track detector based dosimeters are used for estimating radon ( 222 Rn), thoron ( 220 Rn) gases and their progeny concentrations in air. These dosimeters employ two numbers of LR-115 (type II) detectors inside each of the two chambers for gas concentration estimations and a third detector is placed externally for progeny measurements. The tracks formed on the detector are etched under specic conditions and counted by spark counting techniques. A theoretical model has been developed to characterize the response of these detectors to alpha particles of dierent energies incident at dierent angles on the detector surfaces. This involves the estimation of the total time required for track formation that incorporates both bulk etch rates and the track etch rates. Using this information, the region of inuence for the given geometry of the dosimeter is determined. The detectable tracks from this region of inuence, computed by the model, are used to obtain the calibration factors (CFs). Results yielded the CF for each of the species in the bare mode exposure as 0:02 tr cm -2 per Bq d m -3 while those for 222 Rn and 220 Rn in the cup dosimeters were estimated as 0.023 and 0:018 tr cm -2 per Bq d m -3 , respectively. The CFs for the cup dosimeters were also determined experimentally in the laboratory by exposing the dosimeters to known concentrations of 222 Rn, 220 Rn and their progeny concentrations. These compared fairly well with the theoretically compounded values. This methodology will be useful in designing cup dosimeters with better eciencies. c 2003 Elsevier Ltd. All rights reserved. Keywords: Radon; Thoron; SSNTD; LR-115 1. Introduction It is well known that the radiations emitted from the pri- mordial radionuclides originating from the Earth’s crust are the major contributors to the total background exposures to the human populations. These include external gamma ex- posures and inhalation exposures, approximately in equal measure, the latter being due to radon, thoron and their pro- genies in the indoor environment. An assessment of these exposures in dierent parts of the country provides the basic input for obtaining national averages of background popu- lation exposures and their distributions across the country. In this context, a countrywide survey of the external gamma exposures was carried out by Nambi et al. (1986), which enabled the generation of gamma exposure map of India. A ∗ Corresponding author. Fax: +91-22-25505313. E-mail address: eappen@magnum.barc.ernet.in (K.P. Eappen). similar prole of the terrestrial radioactivity was constructed by Sankaran et al. (1986). For obtaining a picture of total radiation exposures, these proles need to be supplemented with those of the inhalation exposures. In view of this, coun- trywide radon mapping programmes were sponsored in In- dia by the Department of Atomic Energy, which included the participation of several Universities and National Insti- tutions. This programme consisted of two parts: (i) devel- opment and standardization of passive monitoring devices (based on solid state nuclear track detectors (SSNTDs)) for the simultaneous estimations of radon, thoron and their pro- genies; (ii) large-scale deployment of these dosimeters and generation of countrywide radon, thoron base line data. In this paper, we focus upon the rst aspect namely, the exper- imental and the theoretical details of the development, stan- dardization and calibration of the radon, thoron dosimetric system. The second part, involving the details of the mea- surements and the results obtained will be published in a separate study. 1350-4487/$-see front matter c 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2003.09.003