| 摘要 |
An apparatus and method for measuring the density of a low density medium such as air by measuring the absorption of radioactivity decay particles such as alpha particles in the air. The apparatus includes a source of alpha particles from which the alpha particles are directed to the air sample to be measured. An alpha particle detector is spaced from the source by a path distance which traverses the air sample. The detector is located at an operating point which corresponds to the path distance where one half of the emitted alpha particles are detected. A control system employs a double sideband, suppressed carrier waveform generator which supplies a double sideband, suppressed carrier as a reference signal and supplies a double sideband, suppressed carrier waveform that is modulated by the detection signal. A difference signal between the reference signal and the modulated detection signal is used to operate a feedback controlled servomechanism that moves the source of alpha particles toward or away from the detector depending upon the density of the medium. Distances moved by the alpha particles source are related to the density of the medium.
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