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In March 1934, Fermi succeeded in a feat that had been deemed impossible. Using simple but ingenious equipment, he discovered that some substances, irradiated with neutrons, acquire beta-type radioactive properties. Subsequently, in October of the same year, he discovered that the slowing down of neutrons, through layers of paraffin or water, whose molecules are rich in hydrogen, can greatly increase their ability to induce radioactivity. This discovery will prove to have an exceptional scientific and technological impact.

In 1938, the Nobel Prize in Physics was awarded to Enrico Fermi, for the following reasons:

“For your discovery of new radioactive substances belonging to the entire field of elements and for the discovery, made by you in the course of these studies, of the selective power of slow neutrons.”

It is an all-Italian Nobel Prize that crowned the research carried out by Fermi in Rome over the course of four intense years, which first led to the discovery of the possibility of inducing radioactivity in a large number of elements through neutron bombardment, and then to the discovery that the effects of neutrons are greatly enhanced in some cases by their slowing down.

We will retrace the salient phases of this great success.

At the end of 1933, Fermi arrived at the formulation of his theory of beta decay, which introduces a new type of fundamental interaction and paves the way for the modern theory of weak interactions.

The beta decay theory played a fundamental role in guiding his subsequent research on neutron-induced radioactivity.

According to Fermi’s theory, in the nucleus undergoing beta decay, a neutron (n) is transformed into a proton with a positive electric charge (p), with the simultaneous creation of an electron with a negative electric charge (e–) and a neutrino (ν). This decay is called “negative beta” because an e is emitted.

Fermi’s theory itself predicts that a proton (p) can transform into a neutron (n), with the simultaneous creation of a positively charged electron (e+), called an antielectron, and an antineutrino (ν). In this case, it would be a “positive beta” decay, up to that moment never seen before in nature. This prediction was confirmed shortly thereafter in some experiments by Irène Curie and Frédéric Joliot in Paris.


From the point of view of quantum field theory, ַβ and β+ decays are the inverse of each other, and can develop when the appropriate energetic conditions occur (when the proton is lighter than the neutron).

In January 1934, Irène and Frédéric Joliot–Curie discovered that it is possible to artificially create new radioactive nuclei by bombarding them with alpha particles. Alpha particles are actually (light) helium nuclei. The radioactivity caused by this bombardment is of the β+-type in which positive electrons are emitted.

The 1935 Nobel Prize for Chemistry crowned their discovery.

The experiment scheme is very simple. A leaflet of a non-radioactive substance, for example, aluminum, is subjected to intense irradiation with alpha particles produced by a source of polonium.

The leaflet is then removed from the source and placed in contact with a Geiger–Müller detector, which counts the electrons emitted, revealing the presence of a new radioactive substance that emits positive electrons with an easily measurable decay time.

We are in the presence of a phenomenon of radioactivity induced by the bombardment with alpha particles. The elements that are activated by the alpha particles are however only few and of low atomic number (that is, they contain a low number of protons), due to the Coulomb repulsion between the alpha and the nuclei, both positively charged.

After the discovery of the Joliot–Curie of the possibility of producing induced radioactivity of the β+ type by bombardment with alpha particles, Fermi, guided by his theory of beta decay, conceived the possibility of producing induced radioactivity of the beta type by bombarding with neutrons.

The neutron, as a nuclear projectile, has the obvious advantage of being able to reach the nucleus more easily, unlike alpha particles which undergo a strong Coulomb repulsion. On the other hand, neutron sources are much less intense than alpha-particle sources because neutrons are not emitted directly from nuclei, but are the product of a nuclear reaction (to produce neutrons it is necessary to bombard light nuclei, e.g., of beryllium, with alpha particles).

Fermi managed to counter the difficulty of the low intensity of neutron sources through a careful exploitation of the geometric conditions in the irradiation of the samples and in the detection of any beta emission.

The neutron sources were made available to Fermi by Giulio Cesare Trabacchi, director of the Physics laboratory of the Institute of Public Health in Rome, housed in the building in Via Panisperna. These consisted of a small sealed glass ampoule in which a small amount of radon, a strongly radioactive gas that acts as a source of alpha particles, and some beryllium powder had been inserted. Fermi obtained the first spring on Tuesday, 20 March 1934.

To carry out the experiments, a Geiger–Müller detector is required, capable of counting any emitted electrons. In the previous days, Fermi developed a very simple but perfectly functional one.

It is possible to follow the discovery phase in complete detail by consulting the laboratory notebook, in which Fermi recorded the results of his first experiments on neutrons. This notebook remained hidden among the papers of his collaborator Oscar D’Agostino, at the “Oscar D’Agostino” Technical Institute in Avellino, but it was identified in 2002, and brought to the attention of scholars. It is now on public display for the first time in this exhibition.

On 20 March 1934, Fermi was in possession of a convenient source of neutrons and a Geiger–Müller counter. During some preliminary tests, Fermi ascertained that the meter still recorded about ten barrels per minute, as an effect of the very weak natural radioactivity coming from the walls, and of the inevitable cosmic rays that rain on the meter anyway. Fermi then attempted to activate a platinum champion, with no result.


However, success was immediately achieved with an aluminum sample. Fermi brought a hollow aluminum cylinder into the room where the source was. It was subjected to irradiation by the source being placed inside the cylinder, in order to have the maximum activation effect by exploiting all of the solid angles, i.e., the fact that the sample enveloped the source like a sleeve.

After irradiation, Fermi quickly brought the cylindrical sample back to the room where the meter was located and placed the meter inside it, in order to reveal all the emission contained in the cylinder cavity, thus exploiting again the simple and ingenious geometry of the apparatus.

The counts undoubtedly revealed that a new radioactive substance has been created. In fact, the counts were much higher than the fund: 82 counts every five minutes, against the 50 of the fund, as reported on page 19 of the laboratory notebook. After about half an hour the counts are statistically reduced to the fund value. The average life of the new radioactive substance is therefore approximately ten minutes. The notebook allows us to estimate that the discovery was made in the afternoon of 20 March.

It can be stated that these simple operations earned practically half the Nobel Prize!

Subsequently, Fermi ascertained that also fluorine, in a sample of calcium fluoride, is activated with an average lifetime of about ten seconds (see page 29 of the notebook). The results on aluminum and fluorine were immediately made known to the scientific community with a short communication dated 25 March 1934, and published in “La Ricerca Scientifica”, the official journal of the National Research Council. Ernest Rutherford congratulated him personally.

After discovering the radioactivity induced in aluminum and fluorine, Fermi dedicated himself to a systematic exploration of the entire periodic table of the elements, subsequently benefiting from the collaboration, in chronological order, of Oscar D’Agostino, then of Edoardo Amaldi and Emilio Segrè, and, finally, of Franco Rasetti.

The results achieved were published in a series of articles in “La Ricerca Scientifica”, in “Il Nuovo Cimento”, and then, in a concluding article, signed by the entire research group and published in the Proceedings of the Royal Society.

The work was intense. At the arrival of the summer break of 1934, about 40 elements out of about 60 tested had been activated.

The results on the bombardment of uranium by neutrons constitute a case of extreme interest. Fermi was led to a physical interpretation of the results that would imply the production of two “transuranic” elements, i.e., with atomic numbers greater than that of uranium, the last element of Mendeleev’s table, which were baptized as ausonium (Z = 93) and esperio (Z = 94).

These results are confirmed by Irène Curie in Paris and by Otto Hahn and Lise Meitner in Berlin.

Unfortunately, this is a misinterpretation destined to last until the discovery of nuclear fission by Otto Hahn and Fritz Strassmann in 1938. We can speak of a real “mockery of the transuranics”.

In May 1934, Fermi studied the effect of neutron bombardment on uranium (Z = 92), which is the most extreme element of the periodic table of elements. It was found that the newly found radioactive nuclei, also called radionuclides, are produced with appreciable average lifetimes (as long as 100 minutes).

Fermi’s interpretation is very simple and natural. First, it was ascertained with accurate chemical analyses that the radionuclides produced cannot be known elements close to uranium in the periodic table—with a Z of 91 down to lead (Z = 82). The only possible interpretation is then that the elements with Z = 93 (ausonium) and Z = 94 (esperio) had been produced, according to a simple mechanism of two successive beta decays: The neutron of the bombardment is absorbed by the nucleus of the uranium, with emission of a gamma ray, producing an isotope. The new isotope of uranium gives rise to a beta decay with the known transformation of a neutron into a proton, transforming to the element with Z = 93, and a subsequent beta decay then leads to the element with Z = 94.

This interpretation persisted for many years, so much so that Fermi mentions ausonium (Ao) and esperio (Hs) in his Nobel Lecture.

Unfortunately, the discovery of nuclear fission by Hahn and Strassmann in December 1938 led to a drastic physical reinterpretation of these results.

In reality, Fermi had caused since May 1934 the splitting of the nucleus, without realizing it. The new radionuclides observed are not transuranic elements, but simply active beta products of the fission of the uranium nucleus. The mechanism envisioned by Fermi for the production of transuranics actually exists in nature, but could not be observed in Fermi’s experiments, due to the low intensity of the neutron sources used. The first “real” transuranics would only be discovered starting in 1940.

This is a double joke. Fermi performed fission without knowing it, believing he had discovered transuranics, while instead they were products of fission. The true transuranics produced in irradiation had not yet been revealed.

Fermi was forced to modify the drafts of his Stockholm report since the discovery of fission occurred after his Nobel Lecture, but before the related publication of the Proceedings.

After the great success of the research that culminated in the summer of 1934 with the activation of about 40 substances out of about 60 tested and after the summer resumption, some serious difficulties arose, both of a theoretical and experimental type.

Fermi has difficulty understanding how the process, called the (n, gamma) reaction, is possible, in which a neutron is absorbed into the nucleus resulting in the emission of a gamma ray, and the formation of a new radioactive beta nucleus.

In fact, the neutrons produced by the sources used should have had the energy of some MeV, and therefore the absorption in the nucleus should have taken place in a very short time, too short to allow the electromagnetic emission of a gamma ray according to existing theories.

It also happens that, in some cases, the intensity of activation of some substances depends on the surrounding environment, resulting in a compromise of the reproducibility of the experiments. According to Amaldi’s testimony, it was Bruno Pontecorvo, who joined the neutron research after the summer break, who realized that some wooden tables have “miraculous” properties. A sample of silver, irradiated for a determined time on a wooden table, shows much more intense induced activity than that the same sample put on a marble table and irradiated by the same neutron source and the same time.


Guided by his “phenomenal intuition”, Fermi manages to concretize the solution of the theoretical problems and the reproducibility problems of the experiments, arriving at the discovery he considered “the most important” of his life: the effects of slow neutrons.

The discovery took place on Saturday, 20 October, and was documented by precise laboratory notes.

Guided by his “phenomenal intuition” (the definition is Fermi’s himself), he managed to understand how to solve, in one fell swoop, the theoretical difficulties on reactions (n, gamma) and those due to the apparent imperfect reproducibility of the activation experiments.

The new experiment was extremely simple. All other conditions being equal, the activation intensity of a silver sample is compared when a four-centimeter-thick paraffin plate is interposed between the source and the sample, with that obtained without paraffin.

Surprisingly, the presence of paraffin produces a large increase in the intensity of activation. With subsequent experiments, carried out on Saturday and Sunday, 20–21 October, Fermi found that similar effects also occur when the source is surrounded by a large mass of water, in particular, that of the “goldfish fountain” in the garden of the Physics Institute in via Panisperna.

On the following Monday the 22nd, Fermi communicated these results to his collaborators, together with the physical interpretation. The presence of paraffin, or water, produces a slowing down of the neutrons following the elastic collision with the hydrogen nuclei present, which takes away part of the energy. Slow neutrons are very effective in producing (n, gamma) reactions and are more easily absorbed in the nuclei of heavy elements.

The discovery was announced in a short letter in “La Ricerca Scientifica”, dated 22 October 1934. Subsequently, all research resumed with this new experimental method.

The discovery was of great practical interest, so much so that a patent was required. In fact, the possibility was envisaged of artificially producing significant quantities of specific active beta nuclides, to be used concretely for medical treatments, or as tracers in the study of metabolism and chemical reactions.

The discovery of the effects of slow neutrons allows important practical developments. Among other things, the new artificial radionuclides, produced in significant quantities through slowdown techniques, constitute a high-level technological accomplishment.

Orso Mario Corbino, the Director of the Institute, with deep industrial experience, suggested patenting the method of slowing down neutrons. The patent was immediately obtained on 26 October, 1934, and then extended to other European countries, and to the United States and Canada.

The patent holders are Enrico Fermi and then, in alphabetical order, Edoardo Amaldi, Oscar D’Agostino, Bruno Pontecorvo, Franco Rasetti, Emilio Segrè, Giulio Cesare Trabacchi.

The effects of slow neutrons will prove to be of great importance for all practical applications of the exploitation of nuclear fission energy, in particular for the construction of nuclear reactors.

After the end of WWII, the patent holders were forced into a lengthy judicial proceeding against the United States government, to have their rights recognized and received an appropriate compensation, which in an initial phase was valued at around ten-million US dollars. The transfer of Bruno Pontecorvo to the Soviet Union, which took place in secret in September 1950, risked threatening the outcome of the procedure. However, in 1953, an agreement was reached in which the inventors were awarded a modest compensation of 300,000 dollars.

After the end of the war, the patent holders are forced to a lengthy judicial proceeding against the United States Administration, to have their rights recognized, in the form of an appropriate compensation, which in an initial phase is valued at around 10 million. dollars. The transfer of Bruno Pontecorvo to the Soviet Union, which took place in secret in September 1950, risks threatening the outcome of the procedure. However, in 1953 an agreement was reached in which the inventors were awarded a modest compensation of 300,000 dollars.

After the discovery of the effects of slowing neutrons, Fermi and his collaborators continued the experiments on the activation of the elements of the periodic table using slow neutrons.

Later, he focused on the theoretical and experimental study of neutron scattering, absorption and slowing, with a series of studies that formed the basis of all subsequent applications of slow neutrons in nuclear physics.

In December 1938, the Nobel Prize in Physics was awarded to Enrico Fermi in recognition of his discoveries on neutron-induced radioactivity and on the effects of slow neutrons.