The Continuous Radon Monitor (CRM) is a microprocessor-based radiation monitoring system developed by RDS for the monitoring of the radon activity concentration in outdoor and indoor air. Connected to an external 12 V DC power source, the CRM can operate continuously for 3 months at a 10-minute logging interval; with a 60-minute logging interval the system can gather data for about 1.5 years. For short-term operation the internal battery supports up to 20 hours of continuous monitoring.
Filtered air with radon is continuously sampled by a constant flow-rate air sampler. While the air flows through the Lucas Cell, alpha particles emitted by radon and radon daughters inside the cell produce scintillations of light within the ZnS(Ag) scintillator coating of the Lucas Cell walls. The light pulses are detected by a photomultiplier tube and the voltage pulses are pre-amplified, amplified and logged into the microcontroller memory. The background count is subtracted from the total count, and the net count is multiplied by the count-to-radon-activity-concentration conversion factor to obtain the radon activity concentration that is displayed and stored. The Lowest Limit of Detection (LLD) of the CRM is approximately 5 Bq/m³.
Logged data can be downloaded into a MS Windows Hyperterminal as a text file that can be opened in MS Excel. The net alpha-count-rate to radon-activity- concentration conversion factor is obtained through system calibration inside the South Australian EPA Radiation Protection Branch Radon Calibration Chamber.