Performance-Specifications-for-Instrumentation-Systems

MS-PC 2015: Performance Specifications for Instrumentation Systems Designed to Measure Radon Gas in Air Page 10 of 20 6.2.3 Electronic Integrating Devices For the purpose of this standard, an electronic radon measuring device that does not provide hourly measurements, or otherwise does not meet the requirements of the definition of a CRM in Section 4, is an Electronic Integrating Device (EID). To comply with this standard, such a device shall only begin displaying average radon concentration measurements after a time period for which the device has demonstrated compliance with the performance requirements of this standard. In particular, EIDs in compliance with this standard must comply with the requirements for MDC, precision and accuracy as described in Section 7. An EID typically utilizes one of the mechanisms described in Subsections 6.1.1 through 6.1.3 to detect alpha particles emitted from radon and/or two of its progeny, 218 Po and 214 Po. The detector and associated electronics record the number of alpha particles detected as counts, and the number of counts over a given period of time is converted to a radon concentration through a calibration factor determined by exposure of the device in a STAR. 6.3 Equilibrating Methods This class of device employs a material such as activated charcoal that adsorbs radon from the air until a state of equilibrium is reached between the quantity of adsorbed radon and the concentration of radon in the surrounding air. Once equilibrium is established, radon may exchange between the charcoal and the air. Some of the adsorbed radon may be replaced by moisture from the air and, thus, the quantity of adsorbed radon may decrease after equilibrium is established. If the concentration of radon in the surrounding air decreases once equilibrium has been established, then radon will desorb from the charcoal to maintain the state of equilibrium. Moisture in the air competes with adsorption sites on the charcoal; therefore, less radon is adsorbed when the air contains more moisture (higher relative humidity). Also, less radon is adsorbed at higher temperatures. Therefore, devices using adsorption of radon by activated carbon are subject to effects of moisture and temperature. Device providers may need to take such effects into consideration to meet the requirements of the standard. This class of device can provide a good representation of the average radon concentration during the exposure period as long as there are no large changes in the radon concentration during the exposure. Depending on the design of the device, its response may be significantly influenced by the radon concentration in the air during the last 12 hours or so of the exposure period. Because of the half-life of radon and the time it takes for equilibrium to be established between the adsorbed radon and the radon concentration in the air, this class of device is typically limited to exposure durations from 2 to 7 days. Calibration of an equilibrating device is accomplished through exposures of representative sets of devices in a STAR f or various time periods and under various controlled and monitored conditions of radon concentration, temperature and humidity. Described below are two types of equilibrating devices that differ by the mechanism used to detect radiation from radon and/or its progeny. This standard does not exclude devices with detection mechanisms that may vary from those described here or devices that use an adsorbing material different from activated charcoal. 6.3.1 Gamma-ray Spectroscopy This type of equilibrating device bases the detection and quantification of the adsorbed radon on gamma-ray spectroscopy, where gamma rays from two short-lived progeny of radon, 214 Pb and 214 Bi, are detected. This is often done using a sodium iodide detector and a multichannel analyzer system. Observed counts from ambient gamma radiation in the energy ranges analyzed must, of course, be subtracted from the gross counts. Depending on the manufacturer or laboratory, the device may be in such forms as metal canisters, paper pouches or trays containing activated charcoal.