A radon detector employs an electrically charged pressed, porous metal filter that allows radon gas diffusion, while blocking ambient light, so that it readily traps both attached and unattached Po-214 and Po-218 ions, that may be present in gas passing through the filter, the filter being charged positively relative to an unbiased PN junction of a photodiode detector within a detection chamber. As a consequence, radon daughter products are prevented from corrupting the radon measurement. Since no voltage differential is applied across its PN junction, the photodiode detector operates in a photovoltaic mode, which avoids the problem of Schottky noise, producing low amplitude current pulses, which are amplified, passband filtered, and thresholded to provide well-defined pulses that are counted over a given measurement interval and converted to radon concentration in terms of picocuries per liter.