Variability of radon and thoron equilibrium factors in indoor environment of Garhwal Himalaya Mukesh Prasad a, * , Mukesh Rawat a , Anoop Dangwal a , Tushar Kandari a , G.S. Gusain a , Rosaline Mishra b , R.C. Ramola a a Department of Physics, H.N.B. Garhwal University, Badshahi Thaul Campus, Tehri Garhwal, 249 199, India b Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai 400 085, India article info Article history: Received 26 August 2015 Received in revised form 20 October 2015 Accepted 20 October 2015 Available online 30 October 2015 Keywords: Radon Thoron DRPS DTPS EERC EETC abstract The measurements of radon, thoron and their progeny concentrations have been carried out in the dwellings of Uttarkashi and Tehri districts of Garhwal Himalaya, India using LR-115 detector based pin- hole dosimeter and DRPS/DTPS techniques. The equilibrium factors for radon, thoron and their progeny were calculated by using the values measured with these techniques. The average values of equilibrium factor between radon and its progeny have been found to be 0.44, 0.39, 0.39 and 0.28 for rainy, autumn, winter and summer seasons, respectively. For thoron and its progeny, the average values of equilibrium factor have been found to be 0.04, 0.04, 0.04 and 0.03 for rainy, autumn, winter and summer seasons, respectively. The equilibrium factor between radon and its progeny has been found to be dependent on the seasonal changes. However, the equilibrium factor for thoron and progeny has been found to be same for rainy, autumn and winter seasons but slightly different for summer season. The annual average equilibrium factors for radon and thoron have been found to vary from 0.23 to 0.80 with an average of 0.42 and from 0.01 to 0.29 with an average of 0.07, respectively. The detailed discussion of the mea- surement techniques and the explanation for the results obtained is given in the paper. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Radon, thoron and their decay products are present in the in- door environment since their parent nuclei radium and thorium are present in building materials and the soil. It is well known that the inhalation of radon, thoron and their decay products contributes a major part (more than 50%) of the natural background radiation dose to the humans (UNSCEAR, 2008). Further, in the indoor environment, the inhalation doses due to the radon and thoron are predominantly contributed from their decay product concentra- tions in the indoor environment. The estimation of equilibrium factors for radon, thoron and their progeny is very important for assessing the radiation dose received from the inhalation of radon, thoron and their progeny. Therefore, it is very essential to carry out the systematic long terms measurements of the equilibrium factors for radon and its progeny and thoron and its progeny in the dwellings of the general public. In case of radon exposure, the short lived radon progeny imparts radiation dose to lungs mainly and not the gas concentration itself. Radiation exposure due to radon progeny is estimated as the product of potential alpha energy concentration (PAEC) and the exposure time. The ratio of potential alpha energy concentration to the radon concentration can be substituted by the equilibrium factor (F), which is expressed as (Leung et al., 2006): F ¼f0:105 C 1 þ 0:515 C 2 þ 0:380 C 3 g=C o where, C o ,C 1, C 2 and C 3 indicate the activity concentrations (in Bq/ m 3 ) of 222 Rn, 218 Po, 214 Pb and 214 Bi, respectively. In the past, radiation dose to the lungs due to exposure of radon progeny has been estimated by first measuring the radon gas concentration and then applying the equilibrium factor, consid- ering the assumed value (0.4) of equilibrium factor for radon and its progeny (ICRP, 1991; UNSCEAR, 2008). However, the radon progeny and hence the equilibrium factor depends largely on the environmental conditions such as hours and modes of ventilation, humidity, etc (Porstendorfer, 1984; Jilek et al., 2010). The equilib- rium factor has also been found to vary with time and place (Nikezic and Yu, 2005; Ramola et al., 2003; Yu and Nikezic, 2011; Yu et al., 1996). The ventilation conditions of a building and the * Corresponding author. E-mail address: bijalwanmukesh111@gmail.com (M. Prasad). Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad http://dx.doi.org/10.1016/j.jenvrad.2015.10.017 0265-931X/© 2015 Elsevier Ltd. All rights reserved. Journal of Environmental Radioactivity 151 (2016) 238e243