AND NOW THE ACCELERATORS
After participating in the Manhattan Project, Enrico Fermi returned to academic activity in Chicago in 1946. His scientific interests focused on elementary particle physics. He therefore contributed to the development of the synchrocyclotron of the University of Chicago. At the time, it was the highest energy accelerator in the world, capable of accelerating protons up to 450 MeV of kinetic energy, and it was called, for short, the Chicago cyclotron.
Cyclotrons consist of a vacuum chamber enclosed between the poles of a magnet, in which the proton beam circulates along a spiral orbit whose radius widens as the energy increases. To obtain high energies, it is necessary to build magnets with the largest possible diameter. The Chicago cyclotron magnet also became known as the “Fermi magnet”. Construction of the Fermi magnet began in 1947, funded by the US Navy and cost a total of $ 2.5 million. The magnet was forged by Bethlehem Steel in Pennsylvania, using some of the largest ingots ever, and reached a weight of approximately 210 tons.
Fermi, in Chicago, focused on subnuclear physics thanks to accelerating machines capable of gradually increasing energy particles. He contributed to the development of the cyclotron, then the most powerful in the world, and in particular to the creation of the great magnet of the machine. Studying the collisions against a hydrogen target of the pions produced by the cyclotron, Fermi in 1952 discovered the first example of a new category of particles of very short life, called “resonances”. This is the Δ ++ particle which will acquire a crucial role in understanding the quark structure of particles and the strong interaction between quarks, called subnuclear “color” interaction.
After participating in the Manhattan Project, Enrico Fermi returned to academic activity in Chicago in 1946. His scientific interests focus on elementary particle physics. He therefore contributed to the development of the synchrocyclotron of that University. At the time it was the highest energy accelerator in the world, capable of accelerating protons up to 450 MeV of kinetic energy, and it was called, for short, the Chicago cyclotron
Particle physics studies, with the exception of cosmic rays, need accelerating machines to create collisions between particles with large energies. The need to reach ever greater energies led in 1930 to the invention of the cyclotron, the first circular accelerator, by the physicist Ernest Orlando Lawrence, who created a version perfected in 1932 at the University of California at Berkeley. For this Lawrence was awarded the Nobel Prize in 1939.
At the end of the war, Fermi accepted a chair of physics at the University of Chicago, which at that time had decided to build a giant synchrocyclotron (i.e., at that time), which was the highest-energy accelerating machine then available (450 MeV). The manufacture of the magnet alone, weighing about 2,000 tons, took two years, from 1947 to 1949, and the synchrocyclotron was finally inaugurated in 1951.
The start of the synchrocyclotron activity provided Fermi with another opportunity to show his innate propensity and exceptional ability to build his own instrumentation. A problem that faced physicists was in fact that of placing the target in the various positions within the synchrocyclotron necessary to extract, among the pions produced by the accelerated proton beam, those of the desired energy. To move the target and not get hit by the radiation generated by the synchrocyclotron, each time it was necessary to turn off the machine, move the target and then turn it back on.
Well, Fermi had the idea of using a cart to transport the target by making it move on the circular edge of the magnet pole. No motor was needed for the cart to move because it exploited precisely the magnetic field of the synchrocyclotron and its effect on the currents that could be passed through the electrical windings to which the cart wheels were connected. The target was a plate of copper or carbon that could rise to intercept the beam, or go down, sending current to a small coil. Fermi did not just design it, he built it himself in the workshop. For years the “Fermi target moving cart”—as everyone called it—worked perfectly. The cart also made it possible to obtain an absolute and precise measurement of the intensity of the beam by measuring, with a thermocouple, the temperature of the target and therefore the energy deposited as a result of ionization and nuclear interactions. As his wife Laura commented, the moving cart was not beautiful to look at, but worrying only about the functioning and not the aesthetics was a characteristic of Fermi.
The cart could move on circular rails placed on the edge of the magnet by maneuvering a series of switches, placed on the outside, which sent current to two coils placed in strategic positions on the axles of the wheels. A third switch regulated the power supply of a third coil to raise or lower the target. The internal target received energy from the proton beam. By measuring the resulting temperature increase, Fermi monitored the intensity of the emitted pion beam.
Diffusion cross section of positively charged pions (crosses) and negatively charged (rectangles) on protons, depending on the energy of the pions
Both cross sections (i.e., the probabilities) of diffusion grow spectacularly with energy. That of the negative pions shows a peak in the characteristic shape of the resonance phenomenon; that of positives is about three times greater and has a similar dependence on energy. Even if the cyclotron did not allow reaching the peak with the positive pions, Fermi’s conclusion was that the diffusion presumably occurred through an intermediate state, a “resonance”, i.e. an excited state of the proton.
A ratio of three between the two cross sections indicated that the resonance had “isospin” equal to 3/2. The total cross sections for positive and negative pions on the proton, known today to be a function of the moment of the pions, are shown in the following figure. Fermi had already glimpsed the peak corresponding to the first resonance. Over the years, many more have been discovered. Fermi calculated the cross sections in his own way, assuming the simplest hypothesis: the reactions proceed through a single resonance with spin J = 3/2, isospin I = 3/2 and orbital angular momentum L = 1. He didn’t have enough data to prove it, but that’s actually the case.
Many other resonances will be discovered by measuring the cross sections of mesons on nucleons. Nucleons are no more fundamental than all of these older “brothers”. They are stable only because they are the lightest of all. In the 1950s and thereafter, the number of known hadrons (mesons and baryons) would increase dramatically. So much so that there were those who proposed that the Nobel Prize should be given to those who did not discover new ones, rather than to those who did.
A first theoretical clarification came in 1961 with the proposed classification of hadrons to form multiplets of a well-defined internal symmetry, known as SU (3) to experts. The proposal was formulated independently by Y. Ne’eman and M. Gell-Mann. The simplest multiplet, however, was a trio, but its components would have electric charges 1/3 and 2/3 of the charge of the proton and spin 1/2. They were hypothesized independently by M. Gell-Mann and G. Zweig in 1964: They are quarks.
The “hadrons”, including the proton, the neutron and also the pion, are the particles capable of interacting through strong nuclear interactions and, depending on their spin, are divided into “mesons” (with a whole spin, therefore bosons) and “Baryons” (with a half-integer spin, hence fermions). The hadrons all possess an internal quark structure. The “leptons” on the other hand, including the electron and the neutrino, cannot interact in the same way and are not made of quarks. Leptons, like quarks, are spin-1/2 fermions.
There were many experiments dedicated to finding quarks, and they all have failed. Today we know that quarks cannot exist freely, they only live in hadrons, but initially few believed in their existence. They were finally discovered in 1969 at the SLAC electron accelerator in Stanford by observing their indirect effects on the scattering of accelerated electrons on hydrogen targets.
But there was a problem, precisely with the Δ ++. In fact, it had to be made up of three identical quarks, all of type up, all with spins in the same direction, in a state that is forbidden to fermions by the Pauli exclusion principle. In 1965, several theoretical researchers proposed that quarks had an extra “charge”, which was imaginatively called “color” (but which has nothing to do with the colors we see). If the three quarks that make up Δ ++ have different colors, the problem is solved. And indeed it is so: The color charge is the one that originates the force that binds the quarks inside the hadrons, just as the electric charge originates the force that holds together electrons and nuclei in atoms. Today we know the theory as quantum chromodynamics.
Δ ++ was the latest discovery by Enrico Fermi, who died in 1954, well before quarks and color were hypothesized.
The largest US subnuclear physics laboratory was founded in 1967 about 60 km from Chicago and was called the National Accelerator Laboratory (NAL). Starting in 1971, the Fermi magnet became the instrument around which a series of experiments were built that probed the internal structure of the proton with beams of muons. In 1974, the US government decided to change the name of the largest laboratory in the country in honor of Enrico Fermi, calling it Fermi National Accelerator Laboratory (FNAL). Speaking at the dedication ceremony, Laura Fermi spoke of her husband’s affection for the cyclotron magnet. “When I saw him here,” he said, “it was like seeing an old friend again.” At the FNAL, the Fermi magnet was used in a series of six experiments over a period of 20 years, up to 1991.
In the summer of 2006, the magnet was taken out of the experimental room and placed outdoors, separated into its elements and painted with the color of the laboratory and with a yellow circle wanted 20 years earlier by the founding director of the laboratory, Robert Wilson. It rests intact as a witness to scientific feats, but is ready for a reincarnation if a future experiment requires such a large magnet.
On 29 January 1954, at the time of his retirement as president of the American Physical Society, Fermi attempted to predict how the maximum energy achieved by the accelerators would increase over the next 40 years. It began with a first cyclotron in 1930 and culminated in 1994 with what his pupil Nobel laureate J. Cronin called the Globatron, an accelerator that surrounds the Earth along the equator.
In this case, Fermi dreamed of something apparently impossible but necessary for the progress of knowledge, with the vision of genius. He was wrong about the size and cost of the Globatron, but his prediction for 1994 was surprisingly accurate, at a much smaller size and cost, thanks to a number of technological advances.
In 1954, M. Stanley Livingston prepared a graph showing the maximum energy reached as a function of the year. He observed that an order of magnitude was gained in energy every six years. Since then, the Livingston diagram has been continuously updated. The adjacent figure is the diagram for proton accelerators (there is a similar one for electron accelerators). Growth has slowed since the 1990s.
Please note. A high-energy accelerator provides particles with energy rather than acceleration. For example, an electron with an energy of 1 MeV already has a speed equal to 94% of that of light; at one GeV, the speed has increased by just under 6%, but the energy is a 1,000 times greater.
Historically, when a type of accelerator reached its practical limits, new inventions and discoveries made it possible to start a new path of growth. The process has lasted for nearly a century. It is estimated that around 20,000 accelerators are currently in operation worldwide. Of these, only a small minority are used for research, while the vast majority work in hospitals and industry.
Cyclotrons. Nearly 400 cyclotrons around the world produce isotopes for medical applications, such as PET. The first cyclotron was built by Ernest Lawrence and M. Stanley Livingston in 1931. It was only 10 cm in diameter but worked as ones today. The vacuum chamber is placed between the pole pieces of a magnet which forces the particles to move on arcs of circles. At each turn, the particles pass through an electric field, which varies in phase with them, accelerating them.
Cockroft–Walton electrostatic accelerator. Invented by John Cockroft and Ernest Walton in 1932. It is still used to accelerate atomic nuclei to a few MeVs.
Van de Graaff electrostatic accelerator. It was invented at Princeton University in the 1930s by the American physicist Robert J. Van de Graaff. The accelerator generates a high electrostatic potential by charging a large metal sphere by means of a rotating belt. It is commercially produced and still used to accelerate ions up to tens of MeV (in the tandem form) for research and industrial applications.
Betatron. Born in 1940 from an idea of D. Kerst of the University of Illinois to modify the design of the cyclotron to achieve greater energies. Betatrons are used for research and for cancer treatment.
Linear accelerators. The microwave technologies developed during WWII for radar were used by physicists to create variable high-frequency electric fields, to keep them in phase with the accelerated beam, in a series of cylindrical cavities of decreasing lengths.
The first LINAC (LInear ACcelerator) was built in Berkeley by L. Alvarez in 1946. The largest LINAC was built in Stanford in 1966. It is 3-km long and accelerates electrons or positrons up to 50 GeV. Nowadays, many hospitals use a LINAC for cancer therapy.
Electrosynchrotrons. A modest pre-acceleration enables injecting the beam of electrons when they are already moving with a speed close to that of light. The electric field in the accelerator cavities must reverse in phase with the transitions of the beam. If the speed of the particles does not vary with acceleration, the frequency of the field can be kept constant, with great simplification of the project. Most of the electrosynchrotrons in operation today, more than 50, are used for the particular “light” (X-rays) that the electrons emit due to the centripetal acceleration of their circular motion. Synchrotron light beams are used, among other fields, for materials and surface science, chemistry and molecular biology.
Protosynchrotrons have used to make fundamental contributions to subnuclear physics in large laboratories in various regions of the world.
In Lawrence’s cyclotron, the radius of the magnet increases linearly with the momentum of the particles; furthermore, the distance between the poles must also be increased to allow sufficient space for the radiofrequency electric fields.
In 1945, the invention of phase stability was made independently by E. McMillan in the US and V. Vexler in the USSR. The particles can be accelerated in synchrony using a variable frequency because their speed increases as the magnetic field necessary to keep them on a circular orbit with a fixed radius increases (and no longer increasing as in the case of the cyclotron).
In 1952, strong focusing was invented, again independently, by N. Chrisopholis and by E. Courant, S. Livingston and H. Snyder. The invention made it possible to greatly reduce the size of the beams and consequently the volumes of the magnetic field and therefore costs.
Most of the elementary particles of the subnuclear world that appear in the Standard Model do not exist in nature, so we had to create them in the laboratory.
To create one of mass m you need an energy equal to mc2. But in the collision with a stationary target, only a small part of the energy of the beam can be used. In fact, the total moment must be preserved, and therefore, after the collision, most of the energy is kinetic energy of the final state. If, on the other hand, two beams proceeding in opposite directions collide, one of particles, one of the corresponding antiparticles, and all the energy is available for the creation of new states. The price is that the density of target particles in a beam is much less than that in a fixed target of material.
In 1960, Bruno Touschek in Frascati invented the accumulation ring, AdA, in which electrons and positrons are accumulated and accelerated in opposite directions. Since then, electron–positron collision rings, or colliders, of increasing size have been built around the world, contributing enormously to particle physics. The largest, the LEP (Large Electron Positron) at CERN, was 27-km long and reached an energy of 209 GeV. It operated between 1989 and 2000 when it was decommissioned to use the tunnel to build the LHC there.
The first proton–proton collider was the ISR (Intersecting Storage Rings), built at CERN in 1971. An antiproton–proton collision ring was proposed in 1976 by C. Rubbia, D. Cline and P. McIntyre. The Super Proton Synchrotron (SPS) of CERN was transformed from a protosynchrotron into a collider thanks to the invention of S. Van der Meer of the stochastic cooling of the beams; it makes possible greatly increasing the density of particles in the colliding beams. The SPS collider led to the discovery of the W and Z bosons in 1983.
The Large Hadron Collider (LHC) at CERN is a proton–proton collision ring, which started operation in 2011. The total energy reached 13 TeV in 2015, equivalent to 85 PeV for collisions on a stationary target, and will be brought in the future to 14 TeV. The LHC already led to the discovery of the Higgs boson in 2012 and promises, with its characteristics of very high energy and density of particles in colliding beams, new fundamental discoveries.