ROOM 13

Discovering the atomic nucleus

Japanese physicist Hideki Yukawa published an article in 1935 in which he theorized that the force that holds two nucleons together (protons or neutrons) within the nucleus is caused by the emission of a mediating particle in the interaction between a nucleon and its absorption by another nucleon. An estimate of the mass of this particle placed it between the mass of the electron and the mass of a nucleon so this mediating particle in nuclear interactions was called the meson.

Hitting the nucleus with enough energy might stimulate the creation of these particles which could then be used to explore the nucleus. Fermi encouraged the building of a cyclotron capable of accelerating relatively heavy protons to reach an energy of 60 million electronvolts  sufficient to produce mesons. It would be 5 years before Fermi’s dream of a large accelerator would become reality and in 1951, most of Fermi’s experimentation would center around an “upgraded” cyclotron.

The accelerator grew from a radius of 2.34 meters to its final dimension of 3.23 meters, with a magnet that weighed more than 2,400 tons and required close collaboration between General Electric, Westinghouse and the Office of Naval Research of the US Navy.  Developing the necessary energy demanded the creation of new and complex technology from the electrical engineers building the machine, to which Fermi made an important contribution. By 1951 the Chicago cyclotron was up and running with an energy demand that was exceptional for the time: it was one of the most powerful accelerators in the world. 

In 1951, physicists held that the PI meson or pion, was responsible for the strong force and interacted in interesting ways with the protons and neutrons inside the nucleus. The Chicago cyclotron was one of the few machines powerful enough to create pions with enough energy to penetrate the nucleus. Fermi’s experiments created a ray of pions, accelerating protons in the cyclotron to extremely high energy levels then shooting them at a target. The protons stimulated the target nucleus into emitting pions that were then used to penetrate other nuclei. The pions impacted these nuclei and their interaction revealed interesting fatcs about protons and neutrons and their relationship with pions.

Analysis of the results of these experiments showed that protons were rather complicated particles: bombarding them stimulated them into decaying into various subproducts. It was discovered that these particles had an internal structure that could be further explored. It was an important discovery that showed the way towards the future of particle physics, developments that would overturn our ideas of nuclear particles.