Abstract: A device for the continuous monitoring in situ of radon concentration in ambient air, and the simultaneous measurement of working level (WL) values of the radon daughters in ambient air. The device comprises an electrical field formed of two hemispherical walls facing each other with a gap therebetween allowing ambient air to diffuse through. The area between the walls is considered a “sensitive volume”. A positively-charged photodiode is disposed in communication with the sensitive volume for the detection of alpha emissions from radon or radon progeny electrically isolated within said electrical field.