= Fermi slow-neutron activation experiment
{c}
<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.<Radioactivity Induced by Neutron Bombardment by Enrico Fermi>[Radioactivity Induced by Neutron Bombardment, Nature 133, 757 (1934); specimen irradiation and subsequent counter readout]{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>.<Azione di sostanze idrogenate sulla radioattività provocata da neutroni. I by Enrico Fermi and others>[Azione di sostanze idrogenate sulla radioattività provocata da neutroni. I, Ricerca Scientifica 5(2), 282–283 (1934); English translation no. 105b, pp. 761–762, observations (a)–(c) and interpretation]{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.<The Scientist by the Bruno Pontecorvo Department Centre>[The scientist; September–October 1934 silver activation and wooden-table passage]{ref}
\Image[/fermi-silver-activation-apparatus.png]
{title=Silver activation as a delayed measurement of the response to neutron irradiation}
{description=Original schematic separating irradiation, later counter readout, a historical comparison and the measurement model. The arrows show stages of inference, not a scale drawing of the apparatus.}
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.<Nobel Lectures, Physics 1922–1941/Artificial Radioactivity Produced by Neutron Bombardment>[Nobel Lectures, Physics 1922–1941; Artificial radioactivity produced by neutron bombardment, lecture delivered 12 December 1938; pp. 415, 417–418, 420–421]{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.<Nobel Lectures, Physics 1922–1941/Artificial Radioactivity Produced by Neutron Bombardment>[Nobel Lectures, Physics 1922–1941; Published lecture, p. 417 footnote correcting the transuranic interpretation; this note refers to subsequent evidence]{ref}
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