33.3 Accelerators create matter from energy

Learning objectives

By the end of this section, you will be able to:

• State the principle of a cyclotron.
• Explain the principle of a synchrotron.
• Describe the voltage needed by an accelerator between accelerating tubes.
• State Fermilab's accelerator principle.

The information presented in this section supports the following AP® learning objectives and science practices:

• 1.C.4.1 The student is able to articulate the reasons that the theory of conservation of mass was replaced by the theory of conservation of mass–energy. (S.P. 6.3)
• 4.C.4.1 The student is able to apply mathematical routines to describe the relationship between mass and energy and apply this concept across domains of scale. (S.P. 2.2, 2.3, 7.2)
• 5.B.11.1 The student is able to apply conservation of mass and conservation of energy concepts to a natural phenomenon and use the equation E= mc ² to make a related calculation. (S.P. 2.2, 7.2)

Before looking at all the particles we now know about, let us examine some of the machines that created them. The fundamental process in creating previously unknown particles is to accelerate known particles, such as protons or electrons, and direct a beam of them toward a target. Collisions with target nuclei provide a wealth of information, such as information obtained by Rutherford using energetic helium nuclei from natural $\alpha$ radiation. But if the energy of the incoming particles is large enough, new matter is sometimes created in the collision. The more energy input or $\text{Δ}E$ , the more matter $m$ can be created, since $m=\text{Δ}E/c^2$ . Limitations are placed on what can occur by known conservation laws, such as conservation of mass-energy, momentum, and charge. Even more interesting are the unknown limitations provided by nature. Some expected reactions do occur, while others do not, and still other unexpected reactions may appear. New laws are revealed, and the vast majority of what we know about particle physics has come from accelerator laboratories. It is the particle physicist's favorite indoor sport, which is partly inspired by theory.

Early accelerators

An early accelerator is a relatively simple, large-scale version of the electron gun. The Van de Graaff    (named after the Dutch physicist), which you have likely seen in physics demonstrations, is a small version of the ones used for nuclear research since their invention for that purpose in 1932. For more, see [link] . These machines are electrostatic, creating potentials as great as 50 MV, and are used to accelerate a variety of nuclei for a range of experiments. Energies produced by Van de Graaffs are insufficient to produce new particles, but they have been instrumental in exploring several aspects of the nucleus. Another, equally famous, early accelerator is the cyclotron    , invented in 1930 by the American physicist, E. O. Lawrence (1901–1958). For a visual representation with more detail, see [link] . Cyclotrons use fixed-frequency alternating electric fields to accelerate particles. The particles spiral outward in a magnetic field, making increasingly larger radius orbits during acceleration. This clever arrangement allows the successive addition of electric potential energy and so greater particle energies are possible than in a Van de Graaff. Lawrence was involved in many early discoveries and in the promotion of physics programs in American universities. He was awarded the 1939 Nobel Prize in Physics for the cyclotron and nuclear activations, and he has an element and two major laboratories named for him.