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Chicago Pile-1 was the experimental nuclear reactor that achieved a controlled self-sustaining fission chain reaction on 2 December 1942, in a structure beneath the University of Chicago's Stagg Field stands. The project was led by Enrico Fermi.

Uranium supplied fissile material, graphite moderated neutrons, and neutron-absorbing control rods adjusted the chain reaction. Gradually changing the absorber configuration let the team test approach to criticality. The scientific milestone was controlled neutron multiplication sufficient to sustain the reaction; the pile was an experimental assembly, not a power-producing station.^[ref]

Enrico Fermi developed neutron bombardment as a way to create and investigate radioactive isotopes. Unlike a charged projectile, a neutron does not face the same repulsive potential barrier when approaching a positive atomic nucleus. Induced radioactivity supplies a delayed readout of reactions that occurred during irradiation. His Nobel lecture describes how comparing target materials, projectile conditions and the subsequent decay products connected experimental activation with the structure and transformations of atomic nuclei.^[ref]

A physicist.^[ref]

Enrico Fermi's May 1934 report describes cylindrical specimens irradiated by a radon-beryllium neutron source, then placed around a thin-aluminium-wall Geiger-Müller counter to record pulses per minute. The irradiation and counting were successive operations.^[ref]

The October paper by Enrico Fermi, Edoardo Amaldi, Bruno Pontecorvo, Franco Rasetti and Emilio Segrè reports that interposed paraffin wax increased silver activation; surrounding blocks enhanced it further, depending on geometry. Water gave a comparable effect. A preparation without beryllium gave no effect, arguing against gamma rays alone. Oxygen-containing material without hydrogen did not give a comparable enhancement, and not all activated elements responded alike. Repeated collisions with hydrogen were proposed to slow the neutrons.^[ref]

The University of Pisa's account of Bruno Pontecorvo's work with Edoardo Amaldi describes stronger silver activation on wood than on marble. This is a retrospective account; the October paper above directly documents the deliberate paraffin wax and water comparisons.^[ref]

To interpret this readout, compare an irradiated specimen with an unirradiated specimen or empty-counter background, using the same counting geometry. Compare irradiation conditions at matched irradiation durations, delays and counting windows. These are requirements of the measurement model, rather than a claim that every such control is documented in the short October letter. Otherwise ordinary radioactive decay during transfer can imitate a change in activation. A persistent, decaying excess after irradiation supports an unstable product; neither a larger count nor a fitted half-life uniquely identifies an isotope.

In his 1938 Nobel lecture, Enrico Fermi distinguished single and superposed exponential decay curves and described chemical separation to identify their carriers. His slowing interpretation connected elastic collisions with enhanced capture. The lecture also described neutron capture resonances, so its account cannot support a universal rule that slower always means more reactive.^[ref]

The 1934 assignments of some uranium activities to elements beyond uranium were mistaken interpretations, not a discovery of nuclear fission. The published lecture's later footnote explicitly calls for reexamining the transuranic assignments after evidence of splitting. Distinguish that correction from the lecture as delivered.^[ref]

Awarded to Enrico Fermi for neutron-induced radioactivity and reactions brought about by slow neutrons. The neutron article follows the activation experiments and their interpretation.^[ref]