Improved Calibration and Operation of a Stable-Gas Quantification Manifold as Part of a Radioxenon Collection System

摘要

Radioxenon detection as part of the Nuclear Explosion Monitoring program involves measuring the ratio of the radioactive Xe isotopes ((135)Xe, (133)Xe, (131m)Xe and (133m)Xe) to the stable (non-radioactive) isotoptic abundance. Pacific Northwest National Laboratory currently uses a thermal conductivity detector (TCD) to measure the stable xenon gas composition immediately after the nuclear measurement is complete on their automated radionuclide samplers. When combined with absolute pressure, temperature and volume, the total volume of stable xenon (cubic centimeters of xenon at standard temperature and pressure) collected can be calculated and the quantity of radioxenon per volume of air is derived. A commercial TCD is a reliable device for measuring percent composition of binary mixtures provided the pressure of the sample is at or near atmospheric pressure; under these conditions the thermal conductivity of the sample is independent of pressure. However, at low sample pressures a pressure-dependent term begins to influence the thermal conductivity of the gas sample and needs to be accounted for if reliable results are to be obtained. To establish a means of correcting for the pressure effect, several blended gas mixtures (xenon in nitrogen) were analyzed over a wide range of pressures (10-760 Torr) on a test manifold fitted with a commercial TCD. These data were then analyzed and an automated calibration algorithm established and used to correct for the pressure effect in the raw TCD readings. The currently implemented routine uses the absolute pressure measurement to 'look-up' the necessary calibration coefficients, which are used for TCD pressure normalization. It is estimated that this technique allows the use of TCD instruments to measure binary mixtures reliably down to a pressure of 10 Torr, much lower than is currently achievable with these devices.