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. But the bigger the crystal, the harder it is to keep it 'quiet': its electrical capacitance grows, and so does the noise.

A new idea is to replace the flat contact with a ring-shaped groove cut around the crystal. This shape should keep capacitance low even in large crystals, just like in small point-contact detectors. But cutting a groove in germanium is no easy task: the material is brittle, the surface must be perfectly restored after machining, and then coated with thin layers of amorphous germanium and aluminum. Until now, no one had managed to get such a detector to work.

The team at the University of South Dakota has built two compact prototypes of these detectors for the first time and tested them at liquid nitrogen temperature. Both samples held voltage steadily, and when about 340 volts were applied, they began to 'feel' particles — matching calculations. The detectors picked up gamma rays from americium-241 (found in household smoke detectors) and cesium-137 (a product of nuclear reactions). This proved that the whole technological chain (drilling, groove cutting, polishing, deposition, and assembly) works.

These aren't full-sized detectors yet, just a proof of principle. The next step is to learn how to apply a lithium contact to the uneven groove surface. If that works, such crystals could form the basis of future experiments searching for neutrinoless double beta decay — a process that could explain why the Universe contains matter and not just light.