Performance-Specifications-for-Instrumentation-Systems

MS-PC 2015: Performance Specifications for Instrumentation Systems Designed to Measure Radon Gas in Air Page 9 of 20 alpha spectroscopy; i.e., measure the energy of the alpha particle and thus identify the specific nuclide from which the alpha particle was emitted. The number of counts detected over a specific period of time is converted to an average radon concentration through a calibration factor that is determined by exposing the instrument to a reference radon concentration in a STAR. 6.2 Integrating Methods This class of device stores a signal that continuously updates in such forms as an increasing number of counts or latent tracks, or a decreasing electrical charge, during the exposure period. Some devices are later processed or read out to produce a response that is directly related to the integral of the radon concentration over the exposure time, enabling the calculation of the average concentration during the measurement period. Electronic devices in this class may produce measurements in real time or store data to be read out at a later time. Depending on the specific design, an integrating device may be used for a measurement duration as short as 2 days or as long as 1 year. Some devices in this class may be affected by the temperature and relative humidity of the surrounding air. If effects of temperature and humidity on the response of the device are significant, then some method must be used to eliminate or compensate for the effects. Described below are three types of integrating 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. 6.2.1 Electret Ion Chambers This type of device uses an ion chamber made of an electrically conductive plastic with an electret as the detecting mechanism. The surface voltage of the positively charged electret is measured before and after the exposure to radon. During the exposure, radon passively diffuses into the ion chamber and subsequently decays. The decay of radon and its short-lived progeny ionizes the air inside the chamber. Electrons are attracted to the electret and discharge it. From the surface voltage of the electret measured before and after the exposure, and the length of the exposure, the average radon concentration during the exposure can be calculated using calibration factors determined through exposures of devices in a STAR. Ambient gamma rays also ionize air inside the chamber; therefore, the effect of ambient gamma radiation must be taken into account. Electrets of different sensitivities and chambers of different sizes can be used in combination to measure a range of radon concentration over time periods ranging from 2 days to 1 year. 6.2.2 Alpha-track Detectors This type of device utilizes a piece of plastic, typically of either allyl diglycol carbonate or cellulose nitrate, inside a container typically made of electrically conducting plastic. Radon diffuses passively into the container, where it subsequently decays. Alpha particles emitted from radon and two of its short-lived progeny, 218 Po and 214 Po, strike the plastic detector and create damaged volumes or “latent tracks.” The plastic is etched in a caustic solution, which produces tracks that are visible with the use of a microscope, because the latent tracks are more soluble than the surrounding undamaged material in such a solution. The plastic is scanned and the track density is determined in terms of tracks/mm 2 . A calibration factor, determined through exposures of devices in a STAR, is used to convert the track density to a value of integrated concentration in the unit of Bq-h/m 3 or pCi-days/liter. The average radon concentration during the exposure is determined by dividing the integrated concentration by the length of time of the exposure.