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A new generation of silicon detectors can distinguish events with energy below one electronvolt.
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Light hits a silicon wafer, and a single electron is born inside it. An ordinary instrument wouldn't notice — the signal is too faint. The new detector not only hears it but also determines its energy with an accuracy of 589 millielectronvolts. That's like distinguishing the footsteps of an ant at arm's length.
Inside the detector, a particle knocks an electron out of a silicon atom. This takes 0.92 electronvolts — a value measured with an uncertainty of 0.02 electronvolts. The result matched across two different setups, so it can be trusted. The detector itself works like a highly sensitive thermometer: the birth of a charge slightly warms a tiny pad, and the instrument registers the temperature change.
To catch a single electron, the detector must be cooled to near absolute zero — otherwise thermal noise will drown out the signal.
Such detectors will be useful in the search for dark matter — the mysterious substance that doesn't emit light but attracts ordinary matter. If a dark matter particle collides with silicon, it will knock out an electron, and the instrument will record the event. The lower the noise, the weaker the signal you can spot.