A shield could increase the activity
In Rome in 1934, Enrico Fermi and his collaborators were using neutrons to make ordinary materials radioactive. The procedure was straightforward in outline: expose a sample, take it away from the source, and count its radiation as it faded. Different materials and different decay times pointed to different nuclear products. The neutron, discovered only two years earlier, was already giving experimenters access to many nuclei.
Then the surroundings of the experiment began to matter. Hydrogen-rich materials such as paraffin and water could greatly increase the induced activity of some targets. Adding material between a source and a sample sounds like a way to weaken a beam. Here it sometimes strengthened the effect being measured. The explanation was that the added material changed the neutrons before they reached the target.
Hydrogen gave the neutrons a way to lose speed
A neutron and a hydrogen nucleus have almost the same mass. In a collision between them, the neutron can give a substantial share of its kinetic energy to the hydrogen nucleus. Repeated collisions bring a fast neutron down toward the energy associated with the random thermal motion of the surrounding material. A substance used to slow neutrons this way is called a moderator.
The process is easier to picture than a block of wax simply swallowing a ray. Some neutrons scatter again and again inside it. Their directions wander as their energies fall. Other neutrons escape or are captured before finishing this process. By the time they reach a sample, they form a different mix of speeds and directions from the original source. Changing the moderator therefore changes the particles available to react.
Heavy nuclei can also scatter neutrons, but one collision generally removes less of a neutron’s energy than a collision with hydrogen. The mass comparison is the reason hydrogen-rich materials were so effective. Temperature also matters after neutrons have slowed: thermal motion supplies a characteristic energy scale. Fermi’s lecture describes experiments that investigated this dependence instead of treating all slow neutrons as a single fixed energy.
Sources: [1]
Slower did not mean less likely to be captured
The word cross section names an effective area that describes the likelihood of a reaction. It need not equal the visible geometric area of a nucleus, and it can change sharply with neutron energy. For many low-energy capture reactions, slower neutrons have a larger capture cross section. A target exposed to the moderated population can therefore undergo more captures, even though each incident neutron carries less kinetic energy.
A capture may leave a nucleus in an excited state. It can release radiation as it settles, and the resulting isotope can later decay. That later emission is the activity counted after the sample has been moved away. The measured result combines the neutron energies, the capture probabilities, the irradiation time and the product’s decay time. It is not a universal rule that more wax always gives more activity. A useful experiment compares the same target under controlled conditions and follows its decay afterward.
The time after irradiation matters as well. A product that decays quickly can give a strong initial count and then fade before a delayed measurement. A long-lived product may remain but emit only a small fraction of its nuclei during the counting interval. Taking a series of readings at known times helps separate these cases. This is why the irradiation and the later decay curve belong to the same experiment: neither the first count nor the neutron intensity alone identifies the product.
A good measurement could outlast a wrong identification
The neutron-irradiation work also included uranium. Some induced activities were first interpreted as elements beyond uranium in the periodic table. In his 1938 Nobel lecture, Fermi still discussed that interpretation. The discovery of barium among uranium’s products by Hahn and Strassmann, followed by the interpretation of nuclear fission, forced a reassessment. An added footnote in the published lecture explicitly says that some supposed transuranic products could instead come from uranium splitting.
This historical correction changes what we should take from the prize citation. The existence of induced activity and the strong effect of slow neutrons were experimental achievements. The identity assigned to every uranium product was another question. Moderation became a central tool in nuclear experiments because its mechanism could be tested across targets and temperatures. The unexpectedly active sample showed how strongly a reaction could depend on the energy of the neutron reaching it.
Sources: [1]