The Crystal That Catches the Invisible express
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. On one side of the crystal, they applied lithium paint and melted it in with heat — creating a contact that holds voltage well. On the other side, they deposited a thin layer of aluminum over amorphous germanium. The side walls were coated with the same amorphous germanium to prevent current from leaking along the edges.
At liquid nitrogen temperature (77 K), the device passed a current of only a few picoamperes, like a single drop leaking from a full bucket over a year. The full depletion voltage was about 1300 volts. Energy resolution: 1.57 keV for 59.5 keV gamma rays and 2.57 keV for 662 keV. This means the detector can distinguish events with very close energies.
Why does this matter? For future detectors with annular grooves on the crystal. Such a shape allows making large crystals without losing signal quality. The hybrid lithium-aluminum contact is a viable way to fabricate such devices.
A leakage current of a few picoamperes is about one electron per billionth of a second, but in the detector it flows constantly and can be measured.
In the TV series 'Dark Matter,' characters search for particles that barely interact with matter — requiring detectors like the one described here.