Radon calibration chamber assembly, detector accuracy, and pertinent operational findings.

Reliable radon laboratory operations motivated the construction of three radon gas calibration chambers. One chamber is used for background calibrations while the other two cover a radon gas concentration range of 110 to >5,550 Bq m(-3) (3-150 pCi L(-1)). The chambers entered commercial service, for tertiary radon calibration chambers, after review and listing by the National Radon Safety Board. The chambers were used since 2003 to calibrate over 500 activated charcoal (AC) detectors, half open face, and the remainder a diffusion barrier type, and >150 continuous radon (CR) monitors. Nearly all the CR devices were Sun Nuclear model 1027. Analysis of the calibration results show the overall AC measurement uncertainties, expressed as standard deviations of the relative percent differences, R%D, were <10% and were about the same as the Sun Nuclear CR devices. The diffusion barrier AC detectors displayed a smaller uncertainty than the CR monitor group. All but one CR monitor satisfied the U.S. EPA Radon Proficiency Program requirement of +/-25%. In view of the similar R%D between CR and AC detectors, good agreement was found for all simultaneous field radon measurements with CR and single AC devices. The U.S. EPA appears to have adopted a position that CR devices are superior to AC detectors. Neither the present results nor U.S. EPA's published detector uncertainties support this presumed position.