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Section 2: The First Subatomic Particles

The large hadron collider (LHC) is the culmination of a long and illustrious tradition of crunching particles together to figure out their components. Skeptics have likened the process to smashing a delicate Swiss-made watch to find out how it works. Nevertheless, this brute force approach has worked remarkably well.

Inside the LHC tunnel during construction.

Figure 2: Inside the LHC tunnel during construction.

Source: © Wikipedia Commons, GNU License. Author: Juhanson. 19 October 2004. More info

As accelerators have become ever bigger and more powerful during the past century, they have given physicists two advantages. First, the more energetic the accelerated particle, the more deeply it can probe into the structure of matter. Second, the relationship between mass and energy that Albert Einstein formulated in his famous equation indicates that higher-energy collisions can produce more massive particles. With each advance in accelerator technology, therefore, new energy frontiers have delivered dramatic new discoveries and opened up new conceptual frontiers. math icon See the math

A particle accelerator uses an electric field to propel electrically charged particles in a desired direction. An electron accelerated across a potential of one volt acquires a kinetic energy of one electron-volt (eV). In the LHC, an oscillating electric field accelerates two hair-thin beams of protons to 7 trillion electron volts (TeV). Superconducting magnets direct the beams in a circular path with a circumference of 27 kilometers. The two beams of protons race around the ring in opposite directions at 0.999999991 times the speed of light. When two protons collide, they have a center-of-mass energy of 14 TeV. The total energy in the two beams is equivalent to 173 kilograms of Trinitrotoluene (TNT).

The earliest accelerators

We can trace the lineage of the LHC back to an accelerator that was basically a primitive version of the cathode ray tube in an old-fashioned television set. The early experiments with simple accelerators like this led to an increasingly sophisticated understanding of the structure of the atom. In doing so, they provided a blueprint for a method of discovery that generations of Nobel Prize-winning physicists have used ever since.

Thomson used the cathode ray tube in three different experiments.

Figure 3: Thomson used the cathode ray tube in three different experiments.

Source: © Wikimedia Commons, Public Domain. More info

Physicists applied the first accelerators to understanding and then using mysterious forms of radiation that were first detected in the 1890s. English physicist J.J. Thomson used an evacuated glass tube and an anode and cathode to show that the beta rays that emanated from a heated metal filament were actually particles with negative electric charges. Further studies indicated that these electrons had very small masses compared with that of the hydrogen atom. Thomson theorized that an atom resembled a plum pudding, with electrons distributed throughout a uniform, positively charged sphere.

A student of Thomson's, New Zealander Ernest Rutherford, extended the study of atoms by firing alpha rays emitted in certain radioactive decays at thin gold foil. He concluded that the mass of an atom was concentrated in a very small region, which he called the "nucleus," surrounded by a cloud of electrons. Alpha rays turned out to be helium nuclei. Rutherford estimated the diameter of the nucleus to be less than 10-13 meters, compared with the atomic size of about 10-10 meters (1 Ångström). More recent measurements give values for the nucleus that range from about 10-14 meters to 10-15 meters depending on the atomic number.

Modified by Danish physicist Niels Bohr's application of the principles of quantum mechanics that we shall meet in Unit 5, the atomic model led directly to our modern view of the atom: a nucleus consisting of protons and electrically neutral neutrons (discovered in 1932), surrounded by a swarm of electrons, equal in number to the protons. This is a remarkably simple system. By taking different combinations of just three constituents—protons, neutrons, and electrons—we can account for all the elements seen in nature.

An organizing principle

Atomic theory also explained the physics underlying the structure of the periodic table, which Russian chemist Dmitri Mendeleev had first proposed in 1869. The table provided an organizing principle, whose power was shown by the discovery of the noble gases. Long before Rutherford and Bohr explained the underlying structure of the atom, gaps in the periodic table had enabled chemists to predict where new elements might be found. For example, in 1894 Sir William Ramsay and John Strutt, Lord Rayleigh, discovered a new gas in ordinary air. They named it "argon" after the Greek word argos, or "lazy one," because it did not interact readily with other elements. Argon was assigned a place according to its atomic number, where it stuck out like a sore thumb without any obvious neighbors with similar properties. This prompted chemists to search for other nonreactive gases. Within the next five years, they discovered the noble gases helium, krypton, radon, neon, and xenon.

The periodic table of elements.

Figure 4: The periodic table of elements.

Source: © Lawrence Berkeley National Laboratory. More info

The search for new elements continues even today, still based on Bohr's atomic model. Uranium, the heaviest element that naturally occurs on Earth, has an atomic number of 92, meaning that it contains 92 protons and 92 electrons. In 1940, a team at the Lawrence Berkeley Laboratory led by Ed McMillan, produced the first transuranic element. Named, like uranium, after one of the outermost planets, neptunium had an atomic number of 93.

In the subsequent 20 years, physicists using Berkeley's 60-inch cyclotron to create intense beams of slow neutrons created 10 more transuranic elements, with atomic numbers 94 through 103. The elements were mostly named for people and places connected to physics research. Starting in the 1960s, groups in Russia and Germany joined the hunt, creating the next eight transuranic elements. In 2006, a research team working in Dubna, Russia, announced the indirect detection of three nuclei of element 118. This discovery still awaits confirmation and an official name from the International Union of Pure and Applied Chemistry.