Light in a particle detector behaves like a voice in the mountains. One shout, and the echo repeats several times, growing fainter with each reflection. So it is in a silicon detector: a real flash from a particle gives rise to false ones that confuse measurements.
Silicon photomultipliers are arrays of tiny cells, each capable of detecting a single photon. But after a cell fires, trapped charge carriers remain inside. They are released with a delay and cause a parasitic signal — an afterpulse. It's as if a light bulb, after blinking, flashed again on its own a moment later, but fainter.
The authors built a special detector where each cell is read out individually. Such a device allows seeing each false flash separately, even after strong irradiation. They developed three independent analysis methods: one based on charge measurement, the other two on decomposing the signal into components and precisely determining the times of false flashes. All methods were verified by computer simulations.
Three samples were measured: one fresh and two irradiated with neutrons in a nuclear reactor. It turned out that the probability of a false flash does not exceed 6%, and its decay time is less than 10 nanoseconds. Crucially, irradiation barely changes these parameters. The detector remains reliable even after harsh radiation exposure.
This is important for future experiments at accelerators and in space, where detectors operate for years under particle bombardment. If false echoes are predictable and rare, they can be subtracted from the data, and real signals can be seen without interference.
The decay time of a false flash is less than 10 nanoseconds. In that time, light travels only 3 meters.