Development of a multi-purpose radon calibration chamber

Full text: A multi-purpose radon chamber was built at the Regional Center for Nuclear Sciences, Brazilian Nuclear Energy Commission (CRCN/CNEN). Besides its detector calibration capability, the chamber can also be used for determining radon exhalation rates from several materials. The 0.84 m3 chamber, built in PMMA and PVC, has two compartments which can be completely isolated one from the other so that two parallel experiments can be performed simultaneously. The cubic shaped larger compartment has three openings which are used for moving equipment in and out of the chamber or for service, when required. The other compartment, with PVC walls, is a 0.3 m diameter, 0.3 m high cylinder (0.02 m3 volume) with moving top (lid) and bottom plates. Radon is generated by a dry 226Ra flow-through source (Pylon Electronics, Brantford, Ont., Canada) with a nominal strength of 100 kBq (±4%). The decay of 226Ra inside the source generates radon at a continuous rate (12.6 Bq min-1). In order to maintain a constant radon concentration inside the chamber, radon laden air is pumped continuously through the chamber (in open circuit) and vented to the exterior after proper dilution. Air is supplied by a small air pump which is capable of providing a variable output according to the source manufacturer recommendations. At steady state operation, air passing through the source at a flow rate of 1 L min-1 will exit with a radon concentration of 12.6 Bq L-1. The radon equilibrium concentration in the chamber can be set at the desired value by varying the air flow rate through the radium source. Radon concentrations ranging from 1.26 to 25.20 Bq L-1 can be attained for air flow rates varying between 10 and 0.5 L min-1, respectively. This radon concentration can be monitored by sampling air (grab or continuous sampling) through one of the several sampling ports existing in the chamber walls. Measurements can be performed with active radon detectors (scintillation cells or surface barrier based detectors). Preliminary tests were carried out to test for leakage. This was done by introducing a given radon concentration in the chamber, closing the input and output ports and collecting grab samples with a scintillation cell during five days for measuring radon decay as a function of time. The calculated half-life for radon decay in the chamber was 3.72±0.44 days (comparable to the 3.82 days (± 0.05%) physical half-life) showing that leakage from the chamber can be neglected. (author)