Hunting for the rarest decay is like searching for a single counterfeit coin in a pile of real ones. You not only have to find it, but also prove it's not just a glint on the metal. In the LEGEND experiment, they're looking for the neutrinoless double beta decay of germanium-76—an event that would occur if the neutrino is its own antiparticle. But detectors made of ultra-pure germanium also catch ordinary particles coming from outside. Events on the crystal surface are especially troublesome: they leave a signal similar to the one sought, but with a different pulse rise time.

To learn how to tell them apart, they built the CAGE test stand — a vacuum chamber with a movable particle source. Inside sits a collimator — a tube that emits a narrow beam of alpha particles, gamma rays, or electrons. Three motors move the source, allowing any point on the detector surface to be bombarded from various angles. This way, you can take a "passport" of each region: how the pulse shape changes depending on where the particle hits.

Scanning with gamma rays at 59.5 keV showed that events near the detector edge have a longer pulse rise time than those at the center. It's like an echo in a room: sound bounces off walls differently than in the middle. This signature will help reject spurious events in LEGEND data and boost the chances of spotting that very rare decay.