cvitae.org - From Lightning Bolts to Synchrotrons: The Evolution of the Particle Accelerator

In the years that followed, it soon became clear that the alpha particles provided by nature were inconvenient as probes into the subnuclear world. For one, the stream of alpha particles was too thin; fewer than 1 million particles could be directed at a 1-square-centimeter sample per second. Considering that nuclei occupy only 1/500,000th of the target area for a thin ﬁlm that’s about 10,000 atoms thick, this is a tenuous stream indeed! This made data-gathering prohibitively time consuming if one was looking for a rare event. A second problem with the alpha particles provided by nature was that their energy was less than was needed to probe deeper into the atom. The deeper one wants to probe, the more energy is required (see Fig. 3).

Rutherford expressed these concerns at a meeting of the British Royal Society on November 30th, 1927. He stated that it had long been his ambition to have “a copious supply of atoms and electrons [with] an energy far transcending that of the alpha and beta particles from radioactive bodies”[21]. This statement fueled a race toward ever higher energies, and that was just beginning.

Guided by experiments carried out with high-energy particles from accelerators, physicists have been able piece together a consistent picture of the subnuclear world known as the ‘Standard Model.’ So far, all experimental results can be explained by this model. However, the ‘Standard Model’ has so many ad-hoc components that it can hardly be the fundamental theory that explains everything. The hope today is that more powerful accelerators can produce particles that lend insight into the truly fundamental laws of nature. One important mystery that could be solved within this decade is the search for the particle that is responsible for mass. The most promising theory explaining the origin of mass assumes the existence of the ‘Higgs’ boson. If it exists, this particle has a calculated energy that could be reached with the Large Hadron Collider, an accelerator currently under construction in Europe. [9]

From Lightning Bolts to Synchrotrons

A simple particle accelerator is easy to come by. Just take a battery and place the two leads into a vacuum chamber. If the battery is charged, there will be a an excess of charge on one terminal compared to the other (hence the small spark when one brings the two leads close). Now place an electron near the negative terminal inside the gap. The electron is negatively charged, so it will rush away from the negative and toward the positive terminal. As it accelerates, it gains energy of motion, called kinetic energy. For small things like electrons or protons, this energy gain is typically measured in electron-volts (eV). If the battery is of type AAA (1.5V), the electron gains an energy of 1.5eV as it crosses between the terminals.

An energy of 1.5eV is about enough to knock an electron out of its orbit around an atom. If you’re trying to investigate the nucleus, however, 1.5eV won’t suﬃce; the bond between protons and neutrons is far too strong. Higher energies are needed. The natural thing to do, then, is to increase the voltage. Unfortunately, there aren’t any 1-million volt batteries, so physicists had to come up with clever alternatives.

One early attempt was to harness the enormous voltages—about 300,000V—of lightning storms. German physicists tried to do this in the late 20’s, but had little success setting up a useful electric ﬁeld, and abandoned the approach after one physicist was killed while adjusting the apparatus. Another idea was to use static electricity. Static electricity builds up and discharges when one touches a door after shuﬄing on a carpet on a dry day. Shoe soles are typically made from a material (such as rubber) that holds negative charge slightly more than the carpet. This negative charge then builds up on ones body until it jumps oﬀ onto some grounded object such as the door handle. In the Van de Graaﬀ generator (invented in 1929 by the American physicist Robert Van de Graaﬀ), this same charging process occurs on a large scale. A silicon or silk conveyor belt replaces ones shoes and a large metal shell ones body. The Van de Graaf generator proved quite eﬀective. It is still used today and can reach voltages of up to 20 million Volts—as powerful as some of the most powerful lightning!

The ﬁrst accelerator that actually cracked an atomic nucleus was one designed by John Cockcroft and Ernest Walton, both in Rutherford’s research group at Cambridge University in England. The machine used a transformer and a voltage multiplier circuit to produce a voltage diﬀerence of 700,000 Volts. With this machine, the researchers accelerated protons (hydrogen atoms stripped of their electron) to an energy of 400,000 eV—enough to disintegrate the target lithium atoms into two helium atoms (see Fig. 2). [2]

Far higher energies were needed for further research (see Fig. 3). However, there appeared to be a limit to the voltage one could produce across a gap. In 1929, American physicist Ernest Orlando Lawrence asked himself whether the same gap could be used over and over. The gap voltage could then be far smaller, but the particle would attain very high energies after a number of passes across the gap.

According to the laws of electrodynamics, the path of a charged particle is bent when it moves through a magnetic ﬁeld. Relying on this eﬀect, Lawrence built a hollow disk, sandwiched

Figure 2: The ﬁrst nuclear reaction achieved with artiﬁcially accelerated particles: Cockcroft and Walton bombarded lithium atoms with accelerated protons to produced alpha particles.

between the South and North magnet halves, in which charged particles circulated through the magnetic ﬁeld. Twice during each revolution, the particles passed through a voltage gap that pushed them to higher velocities (energies), as shown in Fig. 4. As the particles accelerated, they slowly spiraled outward because the force needed to keep them in orbit increased faster than the bending force due to the magnetic ﬁeld. Luckily, it turned out that at velocities far lower than the speed of light, the time change needed to complete a longer perimeter is exactly canceled by the time gain from traveling faster, so that the time for each round-trip stayed constant. This convenient fact allowed the switching of the accelerating voltage to be done at a constant frequency.

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