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<p>In this work, a full characterization of the new user-friendly version of the Atmospheric Radon MONitor (ARMON), used to measure very low activity concentrations of the radioactive radon gas in the outdoor atmosphere, is carried out. The ARMON is based on the electrostatic collection of <span class="inline-formula">²¹⁸</span>Po<span class="inline-formula">⁺</span> particles on a semiconductor detector surface. A main advantage of this instrument is that it offers high-resolution alpha-energy spectra, which will allow us to separate radon progeny (<span class="inline-formula">²¹⁰</span>Po, <span class="inline-formula">²¹⁸</span>Po, and <span class="inline-formula">²¹⁴</span>Po). The monitor feature may also allow measurements of thoron (<span class="inline-formula">²²⁰</span>Rn) by collection of <span class="inline-formula">²¹⁶</span>Po<span class="inline-formula">⁺</span>, although the instrument is not calibrated for this gas.</p> <p>In the paper, the physical principle; the hardware configuration; and the software development of the automatic and remotely controlled ARMON, conceived and constructed within the MAR<span class="inline-formula">²</span>EA and the traceRadon projects, are described. The monitor efficiency and its linearity over a wide span of radon concentration activities have been evaluated and tested here using theoretical and experimental approaches. Finally, a complete budget analysis of the total uncertainty of the monitor was also achieved.</p> <p>Results from the application of a simplified theoretical approach show a detection efficiency for <span class="inline-formula">²¹⁸</span>Po<span class="inline-formula">⁺</span> of about 0.0075 (Bq m<span class="inline-formula">⁻³</span>)<span class="inline-formula">⁻¹</span> s<span class="inline-formula">⁻¹</span>. The experimental approach, consisting of exposing the ARMON at controlled radon concentrations between a few hundreds to a few thousands of becquerels per cubic metre (Bq m<span class="inline-formula">⁻³</span>), gives a detection efficiency for <span class="inline-formula">²¹⁸</span>Po<span class="inline-formula">⁺</span> of 0.0057 <span class="inline-formula">±</span> 0.0002 (Bq m<span class="inline-formula">⁻³</span>) s<span class="inline-formula">⁻¹</span>. This last value and its independence from the radon levels were also confirmed thanks to a new calibration method which allows us, using low-emanation sources, to obtain controlled radon levels of a few tens of becquerels per cubic metre (Bq m<span class="inline-formula">⁻³</span>).</p> <p>The total uncertainty of the ARMON detection efficiency obtained for hourly radon concentrations above 5 Bq m<span class="inline-formula">⁻³</span> was lower than 10 % (<span class="inline-formula"><i>k</i>=</span> 1). The characteristic limits of the ARMON – being those dependent on the presence of thoron in the sampled air – were also calculated. A detection limit of 0.132 Bq m<span class="inline-formula">⁻³</span> was estimated in the absence of thoron. At a typical thoron concentration at atmospheric sites of 0.017 min<span class="inline-formula">⁻¹</span>, the detection limit was calculated to be 0.3 Bq m<span class="inline-formula">⁻³</span>, but this can be reduced if using a delay volume, obtaining a decision threshold of 0.0045 Bq m<span class="inline-formula">⁻³</span>. Current results may allow us to confirm that the ARMON is suitable to measure low-level radon activity concentrations (1–100 Bq m<span class="inline-formula">⁻³</span>) and to be used as a transfer standard to calibrate secondary atmospheric radon monitors.</p>