An international marine-atmospheric Rn-222 measurement intercomparison in Bermuda .1. NIST calibration and methodology for standardized sample additions

As part of an international Rn-222 measurement intercomparison conducted at Bermuda in October 1991, NIST provided standardized sample additions of known, but undisclosed (''blind'') Rn-222 concentrations that could be related to U.S. national standards. The standardized sample additions were obtained with a calibrated Ra-226 source and a specially-designed manifold used to obtain well-known dilution factors from simultaneous flow-rate measurements. The additions were introduced over sampling periods of several hours (typically 4h) into a common streamline on a sampling tower used by the participating laboratories for their measurements. The standardized Rn-222 activity concentrations for the intercomparison ranged from approximately 2.5 Bq . m(-3) to 35 Bq . m(-3) (of which the lower end of this range approached concentration levels for ambient Bermudian air) and had overall uncertainties, approximating a 3 standard deviation uncertainty interval, of about 6% to 13%. This paper describes the calibration and methodology for the standardized sample additions.