The experiments of 1934 – March 27th

In 1934, Fermi and his collaborators became the world’s experts on using neutrons as projectiles to hit samples of different substances and make them radioactive.

The Joliot-Curies discover artificial radioactivity.

It all started in Paris in January, with the discovery of artificial radioactivity by French physicist Irène Curie, daughter of Marie Curie, and her husband Frédéric Joliot. The couple bombarded an aluminium leaf with alpha particles emitted by a natural radioactive substance. The metal foil became an emitting source of positrons, particles with the same mass as electrons but an opposite electrical charge. Aluminum had become, that is, radioactive.

Laura Capon,  Enrico Fermi’s wife, tells the discovery in her autobiography “Atoms in the Family”, Chicago University Press, 1954

“In January, 1934, the French physicists  Frédéric Joliot and his wife Irène Curie announced that they had discovered artificial radioactivity. they had been bombarding aluminum with fast alpha particles. They noticed that the product of disintegration was not stable but that within a few minutes, it emitted small particles (positrons), thus behaving like a radioactive substance. Not only aluminum but also a limited number of light-weight elements transformed into radioactive substances under alpha bombardment. on heavier elements alpha particles had no effect.

Alpha particles are positively charged helium nuclei. their efficiency as nuclear projectiles is limited by their positive charge, which acts as a double obstacle: on the one hand, the attraction exerted on them by the negatively charged electrons surrounding the nuclei slows them down so rapidly that they are stopped. Their chances of encountering a nucleus on the short path they travel are exceedingly low. On the other hand, if an alpha particle manages to come in contact with the nucleus, the impact of the collision is greatly reduced because both target and projectile are positively charged, and they repel each other with a force that is enormous when the distance between them becomes very small. the number of electrons and the positive charge of the nucleus are larger in heavier elements, a fact explaining why bombardment of heavy elements with alpha particles produces no result.”

Neutrons as projectiles

Upon learning of the Joliot-Curie discovery, Fermi immediately decided to use neutrons as projectiles instead of alpha particles. The neutron, having zero electrical charge, is neither slowed down by the electrons surrounding the atoms nor repelled by the atomic nucleus. It penetrates more easily and can thus produce various reactions within the nucleus itself: nuclear reactions.

Fermi immediately sensed neutrons could be used more effectively as projectiles to hit the nucleus and induce nuclear reactions. How had no one until then thought of using the neutron as a projectile to probe the nuclear structure?

The reason was the extreme technical difficulty of producing neutron sources, as Laura Capon further explains:

“Having learned of the Joliot couple, Fermi decided to try using neutrons to produce artificial radioactivity. Neutrons have no electric charge and are therefore not attracted by electrons, nor repelled by nuclei: they therefore travel a much longer path in matter than alpha particles, while retaining higher speed and energy; they, therefore, have a greater probability of encountering a nucleus and hitting it with full force. Against these indisputable advantages, neutrons present a major drawback: unlike alpha particles, they are not emitted spontaneously by radioactive substances, and to obtain them it is necessary to bombard some elements with alpha particles; in this way only one electron is obtained for every one hundred thousand alpha particles used. This very low efficiency raised doubts about the wisdom of using electrons. There was nothing left to do but try, and Fermi decided to become an experimental physicist.”

 

The laboratory notebook

In 2002, physics historians Nadia Robotti and Francesco Guerra rediscovered the laboratory notebook where Enrico Fermi noted down day by day the work he did alone in those early months of 1934. In our Museum, it is possible to see an anastatic copy of this notebook, named “Quaderno di Irpino” after the place where the two historians found it: the Library of the Technical Institute for Surveyors “Oscar D’Agostino,” in Avellino.

The accurate historical reconstruction differs from the recollections that the various Via Panisperna researchers have subsequently narrated. In this case, the protagonists’ memory turns out to be fallacious. It does not account for the modus operandi that Enrico Fermi adopted then, which is well described in his laboratory notebook.

The experiment. Irradiating the elements of the periodic table with neutrons.

In March 1934, Fermi plunged into experimental work alone, doggedly devoting himself to setting up the necessary devices, particularly the Geiger-Müller counters, which were to be used to record the passage of charged particles or photons.

Once the experimental apparatus was set up, Fermi began to irradiate the periodic table elements with neutrons. He did not start by bombarding the lightest ones, as Amaldi, Segrè and Rasetti later erroneously recounted. Still, on the contrary, he began with platinum (atomic number of 78) and gradually worked his way up the periodic table toward the nuclei with lower atomic numbers.

Fermi’s choice to begin by irradiating a heavy element with neutrons was not accidental. It was a course of action that derived logically from his theory of beta decay, which he had formulated just a few months earlier. In light of Fermi’s theory, the heavy elements in the periodic table were those most likely to undergo beta decay.

Fermi’s theory predicted that in a nucleus subject to beta decay, a neutron transforms into a proton with an electron emission.

The number of neutrons is greater in the heavier elements of the periodic table. Indeed, the excess of neutrons is always greater the more the atomic number, the number of protons, increases. By 1934, it was already clear that a congruent number of neutrons was necessary to keep more protons bound together in the narrow nuclear space and overcome the repulsive coulomb force exerted on each other. Based on the Heisenberg-Majorana theory of the nucleus, neutrons and protons interacted with intense, attractive exchange forces on a nuclear scale that could overcome the coulomb repulsion between protons.

It was clear then that for Fermi, using an element with a high atomic number, and therefore with a large excess of neutrons over light elements, as a target increased the probability that beta decay would occur.

This explains why Fermi began to irradiate platinum with neutrons, a heavy element he had available in his laboratory.

When Fermi bombarded aluminum and fluorine with neutrons, the Geiger-Müller counter finally registered some counts, indicating that the target nuclei had thus ‘activated.’ It was March 27, 1934.

 

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