Fermi had shown much earlier that neutrons were far more effectively captured by atoms if they were of low energy (so-called "slow" or "thermal" neutrons), because for quantum reasons it made the atoms look like much larger targets to the neutrons.
Fermi havde vist meget tidligere at neutroner blev meget mere effektivt indfanget af atomer, hvis de havde lav energi (såkaldte "langsomme" eller "termiske" neutroner), fordi det af kvantemekaniske årsager fik atomerne til at ligne større mål for neutronerne.