Calibration of Electret-Based Integral Radon Monitors Using NIST Polyethylene-Encapsulated (226)Ra/(222)Rn Emanation (PERE) Standards

The recently developed (222)Rn emanation standards that are based on polyethylene-encapsulated (226)Ra solutions were employed for a first field-measurement application test to demonstrate their efficacy in calibrating passive integral radon monitors. The performance of the capsules was evaluated with respect to the calibration needs of electret ionization chambers (E-PERM(®), Rad Elec Inc.). The encapsulated standards emanate well-characterized and known quantities of (222)Rn, and were used in two different-sized, relatively-small, accumulation vessels (about 3.6 L and 10 L) which also contained the deployed electret monitors under test. Calculated integral (222)Rn activities from the capsules over various accumulation times were compared to the averaged electret responses. Evaluations were made with four encapsulated standards ranging in (226)Ra activity from approximately 15 Bq to 540 Bq (with (222)Rn emanation fractions of 0.888); over accumulation times from 1 d to 33 d; and with four different types of E-PERM detectors that were independently calibrated. The ratio of the electret chamber response E(Rn) to the integral (222)Rn activity I(Rn) was constant (within statistical variations) over the variables of the specific capsule used, the accumulation volume, accumulation time, and detector type. The results clearly demonstrated the practicality and suitability of the encapsulated standards for providing a simple and readily-available calibration for those measurement applications. However, the mean ratio E(Rn)/I(Rn) was approximately 0.91, suggesting a possible systematic bias in the extant E-PERM calibrations. This 9 % systematic difference was verified by an independent test of the E-PERM calibration based on measurements with the NIST radon-in-water standard generator.