Continuous thoron gas measurement using single scintillation cell e Correction for 212 Pb deposition C.G. Sumesh a, * , P. Ashokkumar b , A. Vinod Kumar b , M.P. Ratheesh b , P.M. Ravi a , R.M. Tripathi a , A.K. Mitra c a Health Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India b Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India c Astrophysical Sciences Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India highlights  Continuous thoron gas measurement using scintillation cell.  Deposition of 212 Pb atoms on the surface of the cell is theoretically estimated.  Automatic correction for previously deposited 212 Pb atoms.  Method is validated using standard thoron monitors. article info Article history: Received 10 January 2014 Received in revised form 29 March 2014 Accepted 9 May 2014 Available online 23 May 2014 Keywords: Lucas cell Delay period Alpha activity RAD7 abstract Scintillation cells are used in continuous mode for thoron estimation using gross alpha counts. Such methods give rise to overestimation resulting from buildup of background counts due to the deposition of Lead ( 212 Pb). Here, the background counts in the cell for a specific counting period owing to previously deposited 212 Pb has been estimated using a model to get a time dependent correction factor. This correction for 212 Pb deposition has been utilized to estimate the current thoron concentration of the sample. The concentration obtained by this method is validated using the scintillation cell by grab sampling method and a continuous thoron monitor, RAD7. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Scintillation cells are most commonly used all over the world for the estimation of radon ( 222 Rn) and thoron ( 220 Rn) concentrations. The cell was originally devised by Vandilla and Taysum (1955). It has since been modified by others (Lucas, 1957; Raghavayya, 1977; Quindos-Poncela et al., 2003). Several workers have reported grab sampling of radon and thoron gas using scintillation cell (Hutter, 1995; Tokonami et al., 2002; Eappen et al., 2007). Air is sampled through a filter into the scintillation cell. The concentration is evaluated from the measured disintegration rates and calibration factor. The calibration factor is obtained from the theoretical buildup of thoron decay products due to a unit pure thoron source. The principle of detection is counting of photons resulting from the interaction of alpha particles produced by the decay of thoron and progeny with the ZnS (Ag) phosphor. A photomultiplier tube (PMT) and associated electronic module converts the photon events into respective concentrations. In a ZnS (Ag) based scintillation cell, all the alpha particles produced in the decay chain of thoron gas is counted. The decay series of thoron is shown in Table 1 . The progenies of thoron gas are particulate and will plate out (deposit) on the walls of the scintil- lation cell. 212 Pb is an immediate progeny of thoron through 216 Po (T 1/2 ¼ 0.15 s) decay chain. Although, 212 Pb is a beta emitter, its progeny 212 Bi (T 1/2 ¼ 60.6 min) is an alpha emitter, thereby making 212 Pb deposition as a source of background counts in the cell. Half life of 212 Pb being 10.64 h, the impact of 212 Pb deposition will persist for nearly 74 h (7 half life). Eappen et al. (2007) reported a theoretical model for continuous measurement of thoron gas using scintillation cell. In this method, * Corresponding author. Tel.: þ91 22 2559 2281; fax: þ91 22 2550 5313. E-mail addresses: sumeshcg@barc.gov.in, sumeshgopalc@gmail.com (C.G. Sumesh). Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas http://dx.doi.org/10.1016/j.radmeas.2014.05.007 1350-4487/© 2014 Elsevier Ltd. All rights reserved. Radiation Measurements 67 (2014) 1e4