Alpha particle detecting devices are disclosed that have a chamber that can hold a fluid in a tensioned metastable state. The chamber is tuned with a suitable fluid and tension such that alpha emitting materials such as radon and one or more of its decay products can be detected. The devices can be portable and can be placed in areas, such as rooms in dwellings or laboratories and used to measure radon in these areas, in situ and in real time. The disclosed detectors can detect radon at and below 4 pCi/L in air; also, at and below 4,000 pCi/L or 300 pCi/L in water. When the fluid is tensioned the presence of radon can be determined by the formation of bubbles which give off detectable signals including a shock wave, light-beam cutoff, or a light burst, any of which can be measured to derive information on radon and progeny radioactivity levels in air or water.