The Nobel Prize in Physics honors the most significant discoveries each year. Fermi developed a way to bombard matter with neutrons and create large amounts of artificial radioisotopes. He showed that “thermal” (slow) neutrons, after being slowed down by water or paraffin, have a much higher probability of being captured by nuclei. This insight became essential for later work on nuclear fission and for reactor design, where the choice of moderator is crucial. Today’s medical isotopes and neutron analytical techniques all trace their origins to this breakthrough, which is why Fermi was awarded.

Fermi clarified experimentally that the nuclear reaction probability (cross-section) obeys the 1/v law, i.e., it scales inversely with neutron velocity. By slowing neutrons down to ~0.025 eV with paraffin, he demonstrated orders-of-magnitude increases in the capture cross-sections of elements such as silver and indium, and confirmed the formation of new isotopes through subsequent β-decay. This work provided the technical underpinning for Hahn and Strassmann’s discovery of fission in late 1938 and for the Manhattan Project’s first reactor, Chicago Pile-1, in 1942. Moreover, the idea of producing short-lived nuclides on demand by neutron capture evolved into today’s tracer techniques used throughout medicine, engineering, and environmental science.

Using Bi-Po neutron sources delivering fluxes of several × 10^11 n cm⁻² s⁻¹, Fermi’s team systematically measured (n,γ) sequences followed by β⁻ decay. Spectrum-cut experiments with Cd shielding exploited the giant resonance of cadmium to disentangle true 1/v capture from resonance absorption. Coupling multi-plate Geiger counters with wet-chemical separations, they identified nuclides with half-lives from seconds to days and reported evidence for previously unknown Z > 92 elements, foreshadowing the actinide concept. The resulting datasets were fed into Wigner’s reactor-physics calculations and validated the effective cross-section formalism within diffusion theory. Fermi’s methodology thus inaugurated the empirical calibration of reactor design parameters such as η, k∞, and the mean free path, influencing both thermal and fast reactor technology.