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A custom integrated circuit for detecting alpha particles for application in the monitoring of radon has been designed and tested. The design uses the reverse-biased well to substrate capacitance of a p-n junction in a conventional CMOS process as a sense capacitor for incident alpha particles. A simple CMOS inverter is used as an analog amplifier to detect the small potential change induced by an alpha-particle strike on the sense capacitor. The design was implemented in a 1.2-[[micro]meter] conventional CMOS process with a sense capacitor area of 110 [[[micro]meter].sup.2]. Tests carried out under vacuum conditions using a calibrated 241Am alpha-particle source showed an output voltage swing of [greater than or equal to] 2.0 V for an alpha event. The detector is also shown to have good immunity to noise and high-quantum efficiency for alpha particles. Index Terms - Alpha-particle, amplifier, CMOS, counter, integrated, inverter, monitoring, radon.