Speaker
Description
Radioactive noble gases and tritium are emitted from nuclear reactors when electricity is produced, accidents occur and as the result of nuclear weapons testing or use and hence are of great interest to the nuclear forensics communities. In addition, some radioactive noble gases serve critical medical functions, as is the case of Xe-133 in the diagnostic imaging for certain lung cancers. The accepted method by which radioactive noble gases have been standardized in terms of primary activity concentration is through the use of a length-compensated proportional counter (LCPC). The National Research Council (NRC) of Canada is developing a new primary method for radioactive noble gas counting using an LCPC. The NRC system consists of three stainless tubes, vertically mounted and serially connected, each with an inner diameter of 25 mm and lengths of (152, 254 and 381) mm. A stainless steel wire with a diameter of 0.0254 mm is located at the centre of each tube forming the three proportional counters. Activity concentration of the contained gas is determined from the differences in count rates for each pair of counters, an approach that eliminates end effects since the tubes are nominally identical. Two counters are in principle sufficient, and the third provides a measure of redundancy to permit the detection of potential malfunctions.
Operationally, the radioactive gas under study, Xe-133, is mixed with a counting gas (i.e., P10) and homogenously expanded in the three proportional counters, which are initially evacuated. As the counting is carried out, accurate knowledge of the absolute chamber volumes, pressures and temperatures is needed to accurately determine the activity concentration of the radioactive gas sample. A programmable high-voltage power supply is used to generate the biases fed to the proportional counters. The acquisition of the signals from the counters is done via three independent charge-sensitive preamplifiers, immediately followed by digitization and pulse shaping on parallel channels.
In terms of design features, a circulation fan and mixing chamber are incorporated into the gas handling system and permit the counting measurements to be performed in either a dynamic gas flow or a static state. Different methods for sample introduction have been incorporated into the system (e.g., digital gas regulators, quick-connect manifold, casse-ampoule) and a ballast tank has been included to mitigate emergency evacuation occurrences. A turbo pump that vents to the outside of the building is used for routine evacuation of the gas handling system.
The system has been tested and its overall performance evaluated with Xe-133. The determined activity concentrations obtained from the three pairs of counters were consistent to within 0.3 %. The mean absolute value of the activity concentration was found to be in line with expectations based on Xe-133 supplier-provided information (pressurized bottle volume and total activity).
