Dongming Mei

An ordinary piece of germanium the size of your palm can spot a single dark matter particle flying through it. But for that, the crystal must be nearly perfect: free of electrical noise, with a uniform internal field, and contacts that don't spark at high voltage. In the new work, they assembled the KL01 detector.

Growing an ultra-pure germanium crystal is a bit like baking a finicky soufflé: a slight overheating or an extra speck of impurity, and the whole ingot is only good for remelting. Such crystals are the heart of detectors that catch the rarest events, from dark matter to double beta decay.

In an ultra-pure germanium crystal cooled to near absolute zero, a single electron can trigger a whole avalanche of charges, like a small pebble dislodged from a slope that sweeps down a stream of snow. Such internal signal amplification is a dream for detectors of dark matter or rare decays.

To catch a whisper in a noisy room, you need a highly sensitive microphone that barely makes noise itself. In the physics of rare events, that role is played by detectors made of ultra-pure germanium — a semiconductor crystal that turns a passing particle into a tiny electrical pulse.