«What is less certain, and what we fervently hope, is that man will soon grow sufficiently adult to make good use of the powersthat he acquires over nature».
“The Italian navigator has just landed in the new world.”
It was with these words that the chairman of the National Defense Research Committee was warned over the phone about the operation of the so-called Chicago Pile, the world’s first nuclear reactor. The Italian ‘navigator’ is the Nobel Prize-winning physicist Enrico Fermi. The ‘new world’ is the atomic age in which we still live today.
Yet, what journey took mankind from studying the raw components of matter to the production of energy in nuclear power stations along with the devastating destructive power of the atomic bomb? In this article, we will try to unravel one of the main threads of this intricate journey. To achieve this, we will follow some of the fundamental moments in Enrico Fermi’s scientific history.
First, however, we must take a look at the periodic table of elements. First question: what is an element? It is a substance composed of atoms that are all equal. Well, what is an atom then? An atom is a collection of a nucleus of particles called protons and neutrons, around which other smaller particles, i. e. electrons, orbit. So, what distinguishes one atom from another? The number of protons. The hydrogen atom, the first element in the periodic table, has one proton. Helium, the second element, has two protons. Lithium, the third element, has three protons. And so on with the fourth element and its four protons, the fifth, the sixth, etc. Notice that the periodic table has no ‘gaps’, no missing elements. From hydrogen with its one proton to oganesson and its one hundred and eighteen protons, there is no number missing. Think of any number between one and one hundred and eighteen: the element that has that number of protons exists and has a name. When a meteorite falls from space in the movies and someone analyses it and exclaims “it contains an element that does not exist on Earth”, they are lying through their teeth.
Having said that, let us move on to the function of neutrons. They help stabilise the atomic nucleus. In a way, they are used to cement the nucleus. Without them protons would repel each other electrically. This should not come as a great surprise to us, as we all remember the old saying “charges of opposite sign attract, charges of the same sign repel” – and all protons have a positive charge.
For us, the main thing to bear in mind is this: in order to be stable, an atomic nucleus needs the right balance of protons and neutrons. If the number of protons and neutrons is out of balance, the atom decays, breaking up into smaller nuclei and emitting radiation.
Let us now get started for real, by beginning in Italy, in Rome to be precise, during the 1930s. After distinguishing himself for various works in theoretical physics, still used daily by all physicists today, young Enrico Fermi was at the head of a fruitful and dynamic group of equally young scientists. They were known as ‘the Via Panisperna boys’, named after the street, where the institute of the University of Rome where they worked, was located – thus representing a milestone in the history of science in Italy. In 1934 ‘the boys’ began work on artificial radioactivity. Research by Irène Joliot-Curie (Marie Curie’s daughter, no less) had already shown that bombarding certain atoms with other atomic nuclei could transform the bombarded atom into a radioactive element. Fermi and his team bombarded various elements with neutrons in order to induce artificial radioactivity. Remember what we said earlier: if the number of neutrons and protons is unbalanced, the atom is radioactive. Therefore, if you take an element and start bombarding it with neutrons, the excess of neutrons may lead to a neutron/proton imbalance and thus to its radioactive state.
This is what Fermi, and his team did: by activating a radon-beryllium neutron source, he brought various elements close to the source and measured whether they would become radioactive. Fermi’s idea, however, was that the slower the neutrons travelled, the more easily they would be ‘captured’ by the element that would then become radioactive. Hence, if he could slow down the neutrons, he would increase the level of radioactivity induced in the element he was studying.
And what substances did the ‘Via Panisperna’ boys use to slow down the neutrons? Lead, for example. So, they put a layer of lead between the neutron emitter and the ‘radio-activated’ element, hoping that it would slow the neutrons down. Pity that it didn’t work. In fact, the experiments always gave different results. Even the levels of induced radioactivity in the material changed if the experiment was repeated on a wooden or marble table.
One morning, however, Fermi decided to try something new. He himself said he did not know exactly what led him to that decision. He simply thought to himself “no, I don’t want lead, I want paraffin”. And so, he put paraffin between the neutron source and the target. The Geiger counters immediately measured extreme high levels of radioactivity. The paraffin had worked in slowing down the neutrons and thus induce stronger radioactivity.
But why did lead not work while paraffin did? Because, contrary to what you may think, heavy lead atoms are much less effective at slowing down neutrons than light atoms found in paraffin. To test this hypothesis, Fermi and his boys even dipped the neutron source and target into the fish fountain in the institute’s garden. They wanted to find out whether water, with its light atoms, also worked like paraffin. The result was positive.
Let us take a four-year leap in time. Stockholm Academy of Sciences, the 10th December 1938. Fermi, aged thirty-seven, officially received the Nobel Prize. He received it in a tailcoat, not in the fascist uniform he was supposed to wear. He shook hands with the King of Sweden but did not give him the Fascist salute. He had already decided that he would not return to the regime’s Italy; his wife was Jewish and, besides, research in Italy was fading, unlike the burgeoning opportunities in the United States. So, when the ceremonies were over and after a short trip to Copenhagen, Fermi and his family boarded the ocean liner to the United States. This was where the second chapter of his career as a scientist began. From a speculative study on nature to an extremely applied one. This is where he was able to use his slow neutrons to lead mankind into the atomic age.
Just a few years later, in fact, Fermi succeeded in creating the Chicago Pile, a euphemistic name for something of such great historical importance: it is none other than the world’s first self-powered nuclear chain reactor. It was built in Chicago, under the stands of an abandoned university stadium and without radiation shielding. To get a mental picture of the ‘pile’, we can imagine a kind of big tower made of uranium pellets, graphite blocks and cadmium rods. From the outside it looked like a big Jenga tower. In fact, the cadmium rods, which were used to absorb excess neutrons, were inserted, and removed manually – just like the bricks in the game. The reason for this was that an overabundance of neutrons could have caused the reaction process to speed up too much and thus produce uncontrolled energy.
The idea behind the battery is that, unlike other materials, when uranium is bombarded by neutrons, it not only becomes radioactive and breaks into other lighter atoms, thus releasing a large amount of energy, but it also emits more neutrons. Therefore, for every neutron launched, several others are produced, which in turn are capable of colliding with other uranium atoms, breaking them, causing them to produce energy and other neutrons, and so on. The uranium, therefore, causes a chain reaction that feeds on itself and can become impossible to stop. This is the mechanism behind the nuclear power plant: triggering a reaction that, feeding itself, produces more energy than is spent to activate the reaction itself. Notably, in the Chicago Pile, uranium was used to generate the energy and activate the chain reaction mechanism; graphite blocks were used to slow down the neutrons to make the process more efficient, and cadmium rods could be inserted to absorb the neutrons – thereby slow down the reaction.
The Chicago Pile represented the first major success of the so-called ‘Manhattan Project’, the secret government plan for US nuclear research. The controversial programme, which involved the collaboration of a huge number of extraordinary scientists, both American and European expatriates fighting the Nazi-fascism that plagued their continent, were responsible for the creation of atomic bombs.
In fact, the only fundamental difference between the Chicago Pile and a nuclear bomb is the speed with which the chain reaction spreads. More precisely, there are different types of uranium atoms depending on the number of neutrons they possess. Uranium-238 has ninety-two protons and one hundred and forty-six neutrons (92+146=238). It accounts for over 99% of uranium in nature and does not play a major role in the self-propelled chain reaction of nuclear reactors. It is uranium-235 that is the true key element, as it can release the extra neutrons that will then trigger the chain reaction.
The fundamental difference between a nuclear power plant and an atomic bomb is in quantitative terms: a nuclear power plant contains 3-5% uranium-235, whereas in an atomic bomb the percentage increases to around 90%, triggering an extremely rapid chain reaction and releasing all the energy in a very short space of time – with the well-known destructive consequences.
Fermi, the Italian navigator who landed in the New World, was the main creator of the Chicago Pile and was called the architect of the atomic age. In the years following the Pile, he supervised other reactors capable of establishing the self-feeding regime of the nuclear chain reaction. He also contributed to the development of the nuclear bomb. Together with other outstanding scientists, he witnessed the detonation test of the first nuclear bomb in the New Mexico desert.
Years later, Fermi, as well as other scientists who had taken part in the ‘Manhattan Project’, began to grow concerned about a world with nuclear weapons and their destructive capability. To this day we are left with his warning, as a brilliant, eclectic scientist and, at the same time, a simple person who was nonetheless the leading character of a crucial turning point in the scientific evolution of mankind that still has us questioning its consequences:
[The] history of science and technology has constantly taught us that scientific advances in basic understanding have sooner or later led to technical and industrial applications that have revolutionized our way of life. It seems to me improbable that this effort to get at the structure of matter should be an exception to the rule — what is less certain, and what we fervently hope, is that man will soon grow sufficiently adult to make good use of the powers that he acquires over nature.