In the basement of a laboratory in Jyväskylä, a trap holds a single atom aloft, like a magician balancing a ball on a fingertip. The tin-113 atom is unstable: sooner or later it captures one of its own electrons and becomes indium-113, emitting an elusive particle — the neutrino. How much energy is released in the process was previously known only approximately.
Physicists measured this energy using a device that compares atomic masses by making them spin in a magnetic field. It turned out that tin-113 has two decay paths that release very little energy, as if a stone rolled down a gentle slope rather than falling off a cliff. One path is especially intriguing: in it, almost all the energy goes into the neutrino, not into other particles.
Knowing the decay energy precisely is like having an accurate map before a hike. Future experiments can use this decay to catch neutrinos and, perhaps, finally weigh them. For now, tin-113 remains in its trap, patiently awaiting its transformation.
The energy released in one of tin-113's decays is less than that of a dental X-ray — about 15 thousand electronvolts.