Slow-neutron-induced radioactivity

The theory of beta decay played a fundamental role in orienting my subsequent research on neutron-induced radioactivity.
In January 1934, the French spouses Frédéric Joliot (1900-1958) and Irène Curie (1897-1956) made a revolutionary discovery: the creation of new radioactive elements by bombarding some light elements of the periodic table, such as boron and aluminium, with alpha particles (i.e. helium, composed of two protons and two neutrons). They had just discovered artificial radioactivity.

I soon thought of using neutrons instead of alpha particles to induce radioactivity. The reason is simple. Neutrons are unaffected by the nucleus’s Coulombian forces and can reach it more quickly and be absorbed, giving rise to new isotopes or elements.

I obtained the neutron sources necessary for the experiment thanks to Giulio Cesare Trabacchi, who directed the physics laboratory of the Public Health Institute of Rome with headquarters via Panisperna. The sources were sealed glass ampoules containing a small quantity of Radon – a highly radioactive gas that acted as a source of alpha particles – and Beryllium dust. I also made myself a basic Geiger counter.

I achieved my first success by bombarding an aluminium nucleus. The counts revealed that the radiation had created a new radioactive substance. I tried the same thing on Fluoride, and it was a success. At that point, it was necessary to experiment with all the periodic table elements.
Two new companions joined our working group for these experiments: Oscar D’Agostino, nicknamed “the chemist of Via Panisperna”, and  Bruno Pontecorvo, “the Cucciolo”.
Within a few months, we could bomb the heaviest element, Uranium.
Something exciting happened here.
Analysing the results of the bombing of Uranium, I thought I had caused the creation of two new transuranic elements, i.e. with a greater number of atoms than Uranium: Ausonium and Hesperium.
This interpretation went on for many years, so much so that I also spoke about Hesperium and Ausonium in my Nobel Lecture.
In reality, since May 1934, we produced the fission of the nucleus, but we were unaware.

However, in October 1934, we made another fundamental discovery.
We realised that the activation intensity of some substances depended on the surrounding environmet, so much to compromise the reproducibility of the experiments. It was Bruno Pontecorvo who realised that some wooden tables had miraculous properties. The silver sample irradiated for a given time on a wooden table showed an induced activity much more intense than that found by irradiating the same sample, by the same neutron source, and for the same time, but on a marble table.

I then decided to do a somewhat simple experiment. We placed a paraffin plate between the source and the element,  and compared the activation intensity with that obtained without the paraffin plate. We found that paraffin produced a significant increase in activation intensity. With subsequent experiments in the water of the goldfish fountain, I discovered that the same thing happened when the element was surrounded by water.
The presence of paraffin or water produced a slowdown of the neutrons following the elastic collision with the hydrogen nuclei present, which took away part of the energy. Slow neutrons were very effective at producing reactions and were more easily absorbed into the nuclei of heavy elements.

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