Radiation Protection Dosimetry Advance Access originally published online on August 9, 2005
Radiation Protection Dosimetry 2006 118(1):61-69; doi:10.1093/rpd/nci333
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Calibration of an electrostatic chamber for thoron measurements in exhaled breath
1 Internal Dosimetry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
2 Environmental Assessment Division Health, Safety and Environment Group, Bhabha Atomic Research Centre, Mumbai 400 085, India
* Corresponding author: mayyays{at}magnum.barc.ernet.in
Received May 5, 2005, amended July 9, 2005, accepted July 9, 2005
An electrostatic chamber system used for measuring thoron in the exhaled breath of thorium workers is calibrated using a standard thoron source and a scintillation cell counting system. A formulation based on both a streamlined flow model and a well-mixed flow model in the chamber has been made to estimate thoron concentration in breath from the observed alpha counts. The experimental measurements of the ratio of the outlet and the inlet thoron concentrations across the chamber strongly validate the well-mixed flow model for thoron, as against the streamlined flow model. The progeny collection efficiency on the electrode has been found to be 70%. The study points out a major error by a factor of 
25 in the calibration factor reported in an earlier work. From the present study, the minimum detectable limit of the activity of 224Ra, freely emanating thoron at mouth works out to be 0.36 Bq as against 0.014 Bq reported earlier for the same system.