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In underground laboratories where dark matter or neutrinoless double beta decay is sought, detectors are surrounded by silence. But even in this silence, radioactive dust quietly settles on their walls. One source is radon, an invisible gas that seeps from rocks. Its short-lived progeny stick to surfaces and spoil measurements.

To see this invisible film, physicists assembled a setup with nine silicon sensors operating in vacuum. Plexiglas plates were exposed in a chamber with high radon concentration, and then they measured how many alpha particles escape from the surface. The instrument's sensitivity is such that it detects one decay per square centimeter per day, like finding a single grain of sand on a football field.

It turned out that deposition depends on three things. The longer the plastic stays in the radon atmosphere, the more deposits, but the dependence is not linear: first a rise, then a slowdown. The electric charge of the surface enhances deposition — negatively charged plastic collects more radioactive atoms. Humidity can both help and hinder: in dry air, light ions stick faster, while at high humidity they cluster into droplets and settle differently.

These results are not just curiosity. Knowing how radon behaves on different materials, engineers can choose coatings that accumulate less radioactive dirt. Then in the next generations of detectors, the background noise will be quieter, and weak signals from new particles will be easier to hear.