Irene Nutini

Light leaves a trace even when there is almost none of it. A single photon, a tiny bundle of energy, strikes a silicon wafer and knocks out a handful of electrons. The trouble is that the handful is too small to notice. You need an amplifier. Usually, the signal is amplified by applying a voltage to metal contacts on the detector's surface.

A ruler for measuring the invisible must be calibrated against something tangible. In particle physics, nuclear models serve as such a ruler, and the decay of technetium-99 provides a tangible benchmark. This artificial isotope lives for 211 thousand years and decays by emitting an electron.