An aluminum sphere the size of a large globe is filled with ordinary nitrogen, the same gas that makes up most of the air. When a neutron flies into the sphere, it strikes a nitrogen nucleus, and a charged particle is knocked out. This particle leaves a trail of free electrons, which get amplified near tiny steel balls inside the sphere. In this way, a neutron, which is invisible by itself, turns into an electrical signal.

Previously, helium-3 was used for such measurements, but it is rare and expensive. Nitrogen costs pennies and is safe. The problem was that no one had checked how accurately such a sphere could measure neutron energy. In Athens, at an accelerator, neutrons were produced with seven different energies, from 0.75 to 2.75 MeV, and the sphere's signal was measured for each energy. It turned out that the signal grows exactly in proportion to the neutron energy.

Now such a sphere can be used as a spectrometer: it not only tells you that a neutron was present, but also reports its exact energy. This is needed for monitoring radiation conditions at accelerators, for medicine, and for calibrating other detectors. The sphere hears every strike and responds with a precise note, like a tuning fork set to the neutron's voice.