Enrico Fermi, Abduction, and Slow Neutrons

In his acceptance speech for the Nobel Prize in Physics in 1938, Enrico Fermi spoke briefly and thoughtfully about the theoretical and experimental work which had earned him this honor.  His talk, ‘Artificial Radioactivity Produced by Neutron Bombardment,’ is a little gem of scientific writing, which showcases not only descriptions of the results of the groundbreaking work in atomic and nuclear physics he had engaged in, but scientific explanation as well.

I provide here a little extract to show Fermi demonstrating abduction–inference to the best explanation–in his recounting of the phenomena of ‘slow neutrons’:

The intensity of the activation as a function of the distance from the neutron source shows in some cases anomalies apparently dependent on the objects that surround the source. A careful investigation of these effects led to the unexpected result that surrounding both source and body to be activated with masses of paraffin, increases in some cases the intensity of activation by a very large factor (up to 100). A similar effect is produced by water, and in general by substances containing a large concentration of hydrogen. Substances not containing hydrogen show sometimes similar features, though extremely less pronounced.

The interpretation of these results was the following: The neutron and the proton having approximately the same mass, any elastic impact of a fast neutron against a proton initially at rest, gives rise to a partition of the available kinetic energy between neutron and proton; it can be shown that a neutron having an initial energy of 10^6 volts after about 20 impacts against hydrogen atoms has its energy already reduced  to a value close to that corresponding to thermal agitation. It follows that, when neutrons of high energy are shot by a source inside a large mass of paraffin or water, they very rapidly lose most of their energy and are transformed into ‘slow neutrons.’ Both theory and experiment show that certain types of neutron reactions…occur with a much larger cross section for slow neutrons than fast neutrons , thus accounting for the larger intensities of activation, observed when irradiation is performed inside a large mass of paraffin or water.

This explanation of experimental data–or ‘interpretation’ as Fermi terms it–is, I think, a particularly elegant one. It is concise both in its form and content; it does justice to the observations with very few claims on our credulity; it integrates the new into the old with a minimum of effort. It is dazzling too–as many explanations of that heady time in atomic and nuclear physics were–in the seeming sleight of hand it performs: it takes the broad, chunky, mundane details of macroscopic phenomena and reduces them to the minute interactions of invisible particles. It pulls off that trick that is so distinctive of so many memorable scientific explanations: such sparse data, such elaborate theory.

I first read of the phenomena that Fermi describes in my eleventh-grade physics textbook; it is only recently that I have read of them in Fermi’s own words. The explanations seemed elegant then, but their style is even more acute in Fermi’s formulation.

Quotes from: Emilio Segrè, Enrico Fermi, Physicist, Appendix 2, University of Chicago Press, 1970, pp. 217-218.