[en] A radon exhalation monitor, RADEX, was developed at the Environmental Measurements Laboratory. RADEX is an active radon monitor that continuously samples radon gas emanating from the soil. A hemispherical accumulator is placed on the soil and radon gas is drawn into the RADEX counter at a low, controlled flow rate. As ²²²Rn decays, the resulting radon daughters are focused by an electrostatic field, deposited directly onto a semiconductor detector, and counted. RADEX's ability to detect rapid changes in radon exhalation is made possible by analyzing RaA alphas with fast decay times. The pulses produced by the radon daughters are integrated hourly and are recorded. This system can operate unattended for 1 week, at which time a desiccant column must be changed. Thus, RADEX allows one to observe and to record continuously the radon flux from the soil. 8 references, 10 figures, 1 table