Development of a continuous radon concentration monitoring system in underground soil

A continuous radon (Rn-222) concentration monitoring system in underground soil was developed and tested. The system consists of 19 mm diameter, 1100 mm long detector and a microprocessor based data logger. A small volume chamber is installed at the tip of the detector part. In the chamber, a thin ZnS(Ag) scintillator film and a photomultiplier tube (PMT) detect alpha particles from radon and its daughters. When the system is in measurement, the detector part is buried into underground soil. The energy resolution of approximately 70% full width half maximum (FWHM) was obtained for 5.5 MeV alpha particles from Am-241. Both rise time and fall time of the system were measured to be approximately 1 to 2 hours. Time variations of the radon concentration in a different depth of the underground soil were successively obtained using four sets of the developed system. With these results, it is confirmed that the developed system is useful for continuous measurements of radon concentration in underground soil.