Performance-Specifications-for-Instrumentation-Systems

MS-PC 2015: Performance Specifications for Instrumentation Systems Designed to Measure Radon Gas in Air Page 8 of 20 devices with detection mechanisms that may vary from those described here, provided that the device meets all of the requirements for a CRM. Measurements made during short time periods, such as 1 hour, are not independently sufficient for determining if there is need for remedial action, or mitigation, of a residence or building (ANSI/AARST MAMF 2012). Such measurements of short duration may be used to supplement measurements of longer duration by helping to judge whether or not there was some unusual occurrence (for example, a severe weather disturbance, tampering, instrument malfunction, etc.) during the measurement period that would invalidate the overall measurement. To ensure that 1-hour measurements provide results that are usable for such tasks, additional requirements for CRMs used to produce hourly measurements are that such devices must have an MDC of no greater than 148 Bq/m 3 (4 pCi/L) for a 1-hour measurement and have a calibration factor of no less than 2 cph per 37 Bq/m 3 (0.054 cph per Bq/m 3 or 2 cph per pCi/L). For results with less uncertainty, the calibration factor should be larger. Continuous radon monitors may be affected by the temperature and relative humidity of the surrounding air. For example, some devices require that a desiccant be used so that the air coming into the monitor is very dry. 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. 6.1.1 Scintillation Cells This type of CRM uses a combination of a scintillation cell and a photomultiplier tube as the detection mechanism. Depending on the design of the scintillation cell, air containing radon is continuously pumped through the scintillation cell, or radon passively diffuses into the cell. The interior surface of the cell is coated with zinc sulfide (ZnS). Alpha particles emitted by radon and its short-lived progeny, 218 Po and 214 Po, strike the ZnS coating and cause it to scintillate; i.e., emit weak flashes of light. The photomultiplier tube detects the flashes of light, converts them to electronic signals, and amplifies those signals so that they can be detected as electrical pulses. The pulses are further processed by an electronics circuit and are counted using a scaler. The number of counts detected over a specific period of time is then converted to an average radon concentration through a calibration factor. The calibration is achieved by exposing the CRM to a reference radon concentration in a STAR. 6.1.2 Ion Chambers This type of CRM uses an ion chamber, as described below, to detect emissions of alpha particles from radon and in some cases also from two short-lived progeny of radon, 218 Po and 214 Po. The ion chamber typically consists of a metal cage or cylinder defining the sensing volume with an electrode through the center. An electrical potential is applied between the cage or cylinder and the center electrode. When alpha particles travel through the air in the sensing volume, they strip electrons from molecules of gas and thus create negatively charged electrons and positively charged ions. The electrons are collected on the positively charged electrode, and the positively charged ions are collected on the negatively charged electrode. An electronics circuit detects the resulting electrical signals, shapes and amplifies the pulses, and counts them using a scaler. Depending on the design of the monitor, radon passively diffuses into the sensing volume, or a pump or blower is used to bring air containing radon into the sensing volume. 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. 6.1.3 Solid-state Detectors This type of CRM uses a solid-state detector to detect alpha particles primarily from 218 Po and 214 Po, and to a lesser extent from radon itself. Alpha particles striking the surface of the detector create electrical pulses. An electronics circuit detects the pulses, shapes them and counts them using a scaler. Depending on the design of the monitor, radon passively diffuses into the sensing volume, or a pump or blower is used to bring air containing radon into the sensing volume. Depending on the type of detector, it may be possible to perform