Proper Quality Assurance Means for Radon Monitoring Devices - A Convenient and Accurate Method for Routine Calibration Traceable to NIST, USA

Transcription

1 Proper Quality ssurance Means for Radon Monitoring Devices - Convenient and ccurate Method for Routine Calibration Traceable to NIST, US Walter Bürkin Genitron Instruments GmbH, Frankfurt, Germany wbuerkin@genitron.de bstract. convenient method for routine calibration of radon devices traceable to the US National Institute of Standards (NIST) is presented. s an appropriate Q measure and with a minimum of auxiliary equipment the user himself can perform periodic calibration checks in order to maintain accuracy and reliability of applied radon measurement devices on a high level. 1. Introduction Performing reliable measurements of radon in homes and workplaces which are in compliance with given regulations require the application of measuring devices disposing of stable and proven accuracy. To ensure this accuracy on a permanent basis periodic verification of the applied calibration constants is required. Consecutively a radon calibration method is described which serves for periodic testing of active radon devices in order to trace their calibration back to a radon standard provided by the United States National Institute of Standards and Technology (NIST). The method consists in a relatively simple procedure which can be carried out by the user himself without a lot of sophisticated laboratory instrumentation. 1. Hardware Requirements Following equipment is required: (1) NIST Standard Reference Material stored in an Emanation Bulb (see FIG. 1) radon source provided by NIST, type SRM 4968 consisting of a small polyethylene-capsulated radium-226 solution with known (certified) activity (~ 400 Bq each) and emanation of radon-222 gas (~ 90 %) [1]. This standard reference material serves as primary reference for calibrating the radon device (reference unit) traceable to NIST, US. glass bulb is used for storing the capsule and for undisturbed radon accumulation maintaining inside the vessel a 100 % humidity atmosphere. FIG. 1 (2) Emanation & Calibration Container (see FIG. 2) gas tight container made of stainless steal serves for retaining the reference monitor to be calibrated. Two leak tested valves allow the exchange of gas e.g. injecting the required radon concentration into the container. FIG. 2 1

2 (3) Gas tight electronic pump (see FIG. 3) gas proof electronic pump is required to transfer the accumulated radon gas from the glass bulb into the container without gas loss and dilution respectively. FIG. 3 (4) Radon Monitor (see FIG. 4) s reference for calibrating other active and passive radon measurement detectors a reliable radon monitor with accurate calibration is needed. Here a device of type lphagurd which disposes of a long-time stable calibration factor and of a highly sophisticated inbuilt Q-System is applied [2]. FIG Calibration Procedure One or more radon devices are exposed within the calibration container. fter reducing and determining the radon background the radon which has been released from the NIST standard material and accumulated within the glass bulb is transferred into the vessel 1) (see FIG. 5). FIG. 5. Setup for transferring the radon activity released by a standard source to the inner of the container in order to calibrate an active radon measurement device traceable to NIST. 1) Recommendation: To avoid the need of additional mathematical corrections the period of undisturbed built-up process of radon should last 3 weeks minimum. 2

3 4. Evaluation of Measurement For data evaluation following steps have to be carried out: (1) Data of the calibration experiment have to be read out and displayed using DataEXPERT software (see FIG. 6). (2) The background concentration has to be defined by averaging the radon concentration over the period of low-level measurement (see FIG. 6). Background / Mean radon concentration FIG. 6. Data series of radon concentration covering background and calibration measurement. 3

4 (3) synthetic straight line is fitted in the data curve representing the radon decay function (see FIG. 7). FIG. 7. Radon data of calibration experiment with fit in of decay function (4) The initial radon concentration (C Rn m ) at the time the radon was separated from the standard source and injected into the container can now be defined. This is obtained by extrapolation using previously created decay function (see FIG. 8). C Rn m FIG. 8. Radon data of calibration experiment with zoom in and determination of initial radon concentration C Rn m 4

5 5. Calculation of Deviation In order to obtain the deviation of the calibration factor of the tested device the measured initial radon concentration (C Rn m ) has to be referred to the actually released activity of the standard material. This has to be done in three steps (1) Calculation of actually released activity from the NIST material S ( SRM ) = SRM f (1) S(SRM) : Emanated activity by NIST SRM [Bq] SRM : Certified activity of NIST SRM [Bq] f : Emanation fraction (2) Converting measured radon concentration to activity Rn Cal ( C C ) ( V nv ) = 0 (2) Rn m Rn Con lgu Rn Cal : Measured net activity [Bq] C Rn m : Initial radon concentration [Bq/m³] at the time point the radon gas was injected into the container C Rn 0 : Measured background radon concentration [Bq/m³] V Con : Volume of container [m³] n : Number of measurement devices exposed within the container V lgu : Volume of one measurement device [m³] (here lphagurd : V=0,00255m³) (3) Defining Deviation in ctivity = : Deviation [%] Rn Cal S ( SRM ) S ( SRM ) 100% (3) The realized result of above calculations complies with the variation in calibration of the tested radon device to the NIST standard reference material SRM References 1. Collé R., Hutchinson J.M.R., Unterweger M.P., The NIST Primary Radon-222 Measurement System, in J. Res. Nat. Inst. Stds. Tech. 95, (1990) 2. Genrich V., lphagurd PQ2000 / MC50, Multiparameter Radon Monitor, Characterization of its physical properties under normal climatic and severe environmental conditions, (Genitron Instruments 1994) 5