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Neutron-induced Radioactivity

Nuclear physics

 In January of 1934 the French couple Irène Curie and Frédéric Joliot made a revolutionary discovery: they bombarded certain light elements such as boron and aluminum with alpha particles composed of two protons and two neutrons and realized they could create new radioactive elements. Artificial radioactivity had just been born. In Rome, Enrico Fermi, thanks to his recently formulated beta decay theory, immediately intuited the possibility of inducing radioactivity using neutrons as projectiles instead of the alpha particles used by Joliot-Curie. But why neutrons?

Being electrically neutral, neutrons aren’t affected by the coulombian repulsion exerted by the atomic nucleus, as opposed to alpha particles which, being positively charged, are repelled. For this reason, neutrons have a powerful penetrating ability and can more easily reach the nucleus where they’re absorbed to generate new isotopes and elements. Isotopes of an element have the same number of protons and a different number of neutrons; elements are classified based on their number of protons. If a neutron changes into a proton through beta decay then that isotope becomes another element.

The first hurdle Fermi had to overcome was to find a sufficiently strong neutron source: he thought of using the very powerful Radon-Beryllium as a source. Radon was extracted from Radium; this was provided by Professor Giulio Cesare Trabacchi, nicknamed for this reason by Fermi’s team as “Divine Providence”. At the time, Trabacchi was director of the Physics laboratory of the Institute of Public Health which was inside the Institute on Via Panisperna. After the sample was exposed to radiation for a time that varied from a few minutes to a few hours, it was brought into the Geiger counter room to verify the presence or absence of radioactivity. In practical terms, they observed whether this reaction transformed a nucleus into one immediately heavier or lighter.

Deciding to proceed at top speed, Fermi called in Rasetti and all the young men of his group, Edoardo Amaldi, Emilio Segrè and Oscar D’Agostino, “the chemist of Via Panisperna”, to make a systematic examination of the entire periodic table to try and activate as many elements as possible. Fermi and Rasetti did the calculations and measuremnts, Segrè had the job of finding the equipment and substances to irradiate, Amaldi built the Geiger counters and d’Agostino analyzed the byproducts of the bombardment with the most advanced radiochemical techniques. In just a few months they discovered many new radioisotopes.

In May of that same year, they started bombarding heavier elements and observed induced radioactivity in uranium. Complex radiochemcial analyses excluded the possibility that the element might have disintegrated into byproducts that corresponded to elements lighter than uranium down to lead (they theorized that there would be nothing lighter).

Fermi’s interpretation, dictated by logic and the knowledge then available regarding nuclear reactions, was that they had produced two elements never seen before, the so-called transuranic elements, that they baptized ausonium and esperium. These results were then confirmed by other experiments in different laboratories in other cities such as Berlin and Paris and by later research by Fermi and his coworkers. The truth was that the nuclei found by Fermi were not actually transuranic elements but corresponded to elements with a much lower atomic number than Uranium; Fermi had invented nuclear fission.

At the end of the academic year, during a solemn meeting of the Lyncean Academy and in the presence of the King, Orso Mario Corbino held a conference where he emphasized the enormous contribution of Enrico Fermi at the international level to progress in Nuclear Physics thanks to the activation of a large number of elements. Even though Fermi had advised caution, Corbino announced the discovery of new elements, considering them already verified. Every newspaper in Italy and abroad talked about this event and the news spread quickly.

The wooden table and the goldfish pond

The discovery of slow neutrons

 After the summer vacation, Bruno Pontecorvo, the “Puppy” of Via Panisperna, joined the group and assisted Amaldi in the study of optimal conditions for the irradiation of elements. Pontecorvo noted practically immediately that in the dark room where the experiments usually took place, there were some wooden tables that had almost miraculous properties. If the elements were irradiated on those tables instead of those of marble, there was an unexpected increase of radioactivity. Fermi initially wasn’t too happy with the results and harshly criticized them, as did Rasetti, calling Amaldi and Pontecorvo incompetent and incapable of taking clean and replicatable measurements.

So, Fermi decided to work personally in the lab and inserted a block of paraffin between the neutron source and the target: he found that the radioactivity increased for no apparent reason, something that had his coworkers’ jaws dropping. At lunch that same day, Fermi decided to go home to meditate on what happened and, in the afternoon, he returned with the answer to the mystery: paraffin contains hydrogen. Upon impact with hydrogen neutrons lose energy, cooling down; the slow neutrons have a greater probability of being absorbed into the atomic nucleus. This explained why wood (which contains hydrogen) was more efficient than marble. Physicist Hans Bethe noted years later that in the USA nobody would have noticed the effect since they didn’t use marble tables. Since a simple piece of paraffin increased the activity so drastically the whole group ran out to the goldfish pond in the courtyard of the Institute to see if Fermi’s hypothesis was correct about the hydrogen in the water: here, too, they measured high levels of activity.

That evening the whole group met at Amaldi’s home to write the results in the form of an article. Fermi dictated, Segrè wrote it down and everybody else put in their two cents. Excitement was high and at times uncontrollable. The next day, given the unusual uproar, Amaldi’s maid asked Ginestra, Amaldi’s wife, if somebody hadn’t had a bit too much to drink the night before. Corbino immediately realized the potential the discovery could have on various chemical, medical and biological applications and pushed to have the patent registered in the name of all the Boys of Via Panisperna, Trabacchi, Mr. “Divine Providence” included. Fermi never stopped thanking him.

At any rate, there were still several pieces missing from the puzzle that led from the discovery of the role of slow neutrons in induced radioactivity to recognition of the phenomenon of nuclear fission and its systemiatic use: the split into two large pieces, energy production and the production of other fundamental neutrons to self-sustain the reaction. To understand all that would take a few years more.

Chain reaction is possible

The fission of uranium

To understand what happens when uranium is impacted by slow neutrons, two German physicists in Berlin, Otto Hahn and Fritz Strassmann, carefully analyzed the composition of the irradiated uranium sample. They concluded that the resulting element must be barium, an element with about half the mass of uranium. This information turned everything that was known about nuclear physics upside down because it was thought that decay occurred in small “splinters”; splitting the nucleus of an atom in two simply wasn’t imaginable.

Hahn was in contact with Lisa Meitner, an Austrian theoretical physicist of Jewish origins who had fled Nazi Germany for Sweden. He asked her if there were physical explanations for their results. Fermi had gone to Stockholm in that moment to receive the Nobel prize and then had embarked for the United States of America and wasn’t reachable.

Lisa Meitner reflected on the liquid drop model proposed recently by Danish physicist Neils Bohr according to which the uranium nucleus behaved like a drop of liquid, held together by the strong nuclear force and in tension by coulombian repulsion of the protons. Lisa Meitner felt that absorption of a neutron set the drop vibrating; if a “neck” forms, coulombian repulsion stretches it until the drop splits in two parts with an atomic weight of about half each of the atomic weight of uranium: these correspond to the fragments of nuclear fission. The two fragments (nuclei) from the fission have a smaller total mass than the uranium nucleus had at the start. With this difference of mass, Lisa Meitner, using Einstein’s famous formula from the theory of relativity E=mc², calculated the energy liberated during fission. She obtained a result of 200 million electronvolts for every split nucleus. With this calculation Lisa Meitner placed the foundation for experimental development of nuclear fission, for its future use in war and in peace in nuclear plants. A few days after the discovery, Hahn informed Niels Bohr who was leaving for a conference in the United States of America. When Bohr heard of Meitner’s results he exclaimed enthusiastically: «What idiots we’ve all been! It’s fantastic! It must be exactly like that!».

At that point it was immediately clear that if, during the fission of uranium, neutrons were liberated, these could further cause the fission of other nuclei and thus initiate a potentially self-sustaining chain reaction. Bohr and Fermi took it upon themselves to furnish immediate proof, placing uranium in an ionization chamber and blasting it with slow neutrons: the chamber showed strong traces of ionization in uranium’s radioactive decay, much more intense than the traces of alpha particles. These traces correspond to the fact that the mass of the urianum nucleus fragments is much greater than that of the alpha particles. Fermi hadn’t seen anything like this in Rome because, in the experiments at Via Panisperna, the uranium samples were wrapped in aluminum foil which blocked the fission fragments and for this reason his results had been interpreted as transuranic elements.

In just a few months, Fermi, together with Hungarian physicist Leo Szilàrd, demonstrated that in every fission two neutrons were released and so chain reactions were possible: controlled nuclear energy—not to mention the atomic bomb—was now within reach.