CALIBRATION OF ELECTRET-BASED INTEGRAL RADON MONITORS USING NIST POLYETHYLENE-ENCAPSULATED RA-226 RN-222 EMANATION (PERE) STANDARDS/

The recently developed Rn-222 emanation standards that are based on po lyethylene-encapsulated Ra-226 solutions were employed for a first fie ld-measurement application test to demonstrate their efficacy in calib rating passive integral radon monitors. The performance of the capsule s was evaluated with respect to the calibration needs of electret ioni zation chambers (E-PERM(R), Rad Elec Inc.). The encapsulated standards emanate well-characterized and known quantities of Rn-222, and were u sed in two different-sized, relatively-small, accumulation vessels (ab out 3.6 L and 10 L) which also contained the deployed electret monitor s under test. Calculated integral Rn-222 activities from the capsules over various accumulation times were compared to the averaged electret responses. Evaluations were made with four encapsulated standards ran ging in Ra-226 activity from approximately 15 Bq to 540 Bq (with Rn-22 2 emanation fractions of 0.888); over accumulation times from 1 d to 3 3 d; and with four different types of E-PERM detectors that were indep endently calibrated. The ratio of the electret chamber response E(Rn) to the integral Rn-222 activity I-Rn was constant (within statistical variations) over the variables of the specific capsule used, the accum ulation volume, accumulation time, and detector type. The results clea rly 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 wa s approximately 0.91, suggesting a possible systematic bias in the ext ant E-PERM calibrations. This 9 % systematic difference was verified b y an independent test of the E-PERM calibration based on measurements with the NIST radon-in-water standard generator.