Abstract: A low-cost radon detector using a solid state diode having a depletion layer disposed close enough to a test medium that alpha particles deposit their energy in the depletion layer. The signal produced by the diode is amplified and pulses are discriminated for levels over a threshold typical of pulse levels produced by radioactive decay of radon daughters on the surface of the sensor. More than one amplifier and threshold detector are provided, and the sensor can have plural solid state diodes. Pulses occurring exclusively on one channel are counted and pulses occurring in coincidence on more than one channel are ignored as instances of transient disturbance. A conductive enclosure around the sensor element can be biased to force radon daughter ions to plate out the sensor, whereupon decay of the daughters produces pulses falling closely into pulse height ranges well above the noise threshold. Pulse counts and time lapse are monitored both long term and short term.