Development of an absolute gas-counting capability for low to medium activities

•Length-compensated internal proportional counting is discussed.•We described the development of low-background unequal-length proportional counters.•We described the development of unequal-length OFHC proportional counters.•A gas mixing and transfer system is described.•An uncertainty analysis, following the ISO GUM guidelines, is presented.

Pacific Northwest National Laboratory (PNNL) is developing a capability to measure the absolute activity concentration of gaseous radionuclides using length-compensated proportional-counting. This capability will enable the validation of low-level calibration standards for use in PNNL's new shallow underground laboratory. Two sets of unequal length proportional counters have been fabricated; one set has been fabricated using ultra-low background (ULB) electroformed copper and a second set fabricated from Oxygen-Free High-Conductivity Copper (OFHC).