A portable monitoring system of airborne radioactive materials was developed for sequential monitoring of many workrooms. The system consisted of a room dust monitor detectable for the particulates emitting beta rays with its maximum energy higher than 0.15 MeV and a personal computer connected with a multichannel interface. The characteristics of background activity resulted from airborne radon progeny were investigated. The background activity in a well ventilated workroom was relatively low and equilibrated in 3-4 h after the start of sampling. Moreover, it was affected by the change of atmospheric pressure. The minimum detectable activity for 32P with the system was estimated to be 2.3×10-5 Bq cm-3 for 10 min measurement, which was less than 1/10 of the limit of airborne radioactivity concentration. The system is suggested to be more suitable than the conventional methods for the monitoring of airborne radioactive materials in the working environment.
