The stage is bathed in icy light, and on it stand silicon spectators. Each one is a wafer with thousands of tiny sensors that can detect individual particles of light. But such sensors only work in extreme cold, at temperatures around 165 K (which is minus 108 degrees Celsius). Previously, they were tested one by one, manually, and it was impossible to tell how they would behave together in a real detector.

Now a special stage has been built for them. It has a window that transmits ultraviolet light and a movable spotlight that can aim at each sensor with precision down to tenths of a millimeter. This light is not ordinary: it is the same kind that flashes inside dark matter detectors when a particle passes through. But ultraviolet light is easily absorbed by moisture in the air, so the window is constantly flushed with dry nitrogen, like wiping your glasses before an important conversation. And behind the scenes, the cold is maintained with shivering stability: the temperature fluctuates by only 1.2 mK over ten minutes.

Ten spectators from the same batch took the stage, and three things were measured for each. First, how many times the sensor fires in complete darkness, when there is no light. Second, what electric charge is produced by a single photon. Third, what is the probability that a photon will trigger a signal at all. At 165 K, this probability was 16.3%. It was also found that for every degree of warming, the voltage must be increased to keep the sensor working.

Previously, no one could simultaneously cool a hundred such spectators and direct a beam of ultraviolet at each one with millimeter precision. Now there is a conveyor belt: load the wafer, set the coordinates, and the stage will automatically put the spectator through all the tests. Already today, this setup is helping to prepare detectors for the nEXO experiment, which requires thousands of these sensors. In the future, it can be used to study how ultraviolet gradually degrades detectors and to build a model of spurious signals that hinder the identification of rare events. The icy light continues to fall on the silicon, and now it has an audience ready for the big premiere.