Physics: Superhighway for Electrons

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Slicing through the rolling countryside near Palo Alto, and flanked by newly planted oak and eucalyptus trees, the low, two-mile-long structure could easily be mistaken for a new link in California's growing network of freeways. Instead of automobiles, however, it will handle streams of speeding electrons. It is Stanford University's linear accelerator, the newest tool in one of the newest and fastest-growing disciplines of science, high-energy physics. When it achieves full power and goes into operation this fall, the largest atom smasher in the world will give man a closer look at the mysterious subatomic world and its host of newly discovered particles.

Backbone of Stanford's linear accelerator (called SLAC) is a 10,000-ft.-long, 4-in.-diameter copper tube housed in a concrete tunnel and buried 25 ft. underground to protect scientists and any bystanders from its fierce radiation. At one end, an electron beam is generated in much the same manner as the beam inside a home TV picture tube. Injected into a nickel-size hole that runs the length of the copper tube, the beam's electrons are immediately accelerated by 6,000,000-watt microwave pulses generated by 245 klystrons—giant, ultrahigh-frequency radio tubes spaced evenly in the structure above the entire length of the tunnel. Riding the crest of the moving radio wave produced by the klystrons, the electrons move only a few feet before they approach the universal speed limit—the velocity of light.

Beam Switchyard. For the remainder of the two-mile journey, most of the energy imparted to the electrons by the radio wave is in the form of mass. As a result, each electron increases its mass 40,000 times, and has acquired about 20 billion electron volts (BEV) of energy by the time it reaches the far end of the copper tube. There, the extremely powerful stream of charged particles passes through a beam "switchyard," where giant electromagnets direct it into one or another of two target buildings, or split it between both.

Inside the buildings, the electron beam is fired at targets such as metallic sheets or containers of liquid hydrogen. As a high-energy electron approaches the nucleus of an atom in the target, one of two things happens: it veers off in a different direction, or it actually shatters the nucleus—and the reaction often produces new and different particles that exist for only billionths of a second.

Results of the collisions and near collisions are measured in the target buildings by giant spectrometers, or photographed in spark and bubble chambers, which trace the paths of atomic, and subatomic particles. Analysis of the results reveals the mass, charge and energy of particles produced by the interaction of electrons with the target; it gives scientists fresh insight into the structure of the atomic nucleus. It can also identify new and previously unsuspected subatomic particles.