In giant accelerators, particles zip through detectors in billionths of a second, and telling one from another requires a stopwatch of incredible precision. The new µRWELL-PICOSEC detector acts as such a stopwatch: it captures the moment a particle passes with a precision of 48 picoseconds: that's 48 trillionths of a second.
The secret is that the detector combines two phenomena. When a charged particle enters a transparent material, it produces a faint flash of light, like a plane breaking the sound barrier leaves a shock wave behind. This light hits a thin layer of cesium iodide, which knocks out electrons. The electrons then accelerate in a gas chamber with a micro-perforated foil, and the signal is amplified thousands of times. Thus, a weak flash becomes a clear electrical pulse.
In tests, the 10 by 10 centimeter detector was bombarded with 150 GeV muons — particles similar to electrons but 200 times heavier. By measuring time at two separate pads of the detector, engineers obtained resolutions of 48 and 52 picoseconds under different settings. Such precision allows distinguishing particles traveling at nearly the same speed — for instance, in experiments searching for new particles or in positron emission tomography, where knowing exactly where a particle annihilated is crucial.
For now, the signal was read out with a standard oscilloscope from a single channel, but in the future the detector can be equipped with multi-channel electronics to track particles across the entire area. Then the stopwatch would also become a map, and physicists would get a tool that simultaneously sees where and when a particle passed.
In 48 picoseconds, light travels only about one and a half centimeters — less than the width of a pinky finger.