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New Accelerator Probes Structure of Proton

By J. MICHAEL Crichton

HALF-BURIED in a mount of earth behind the Biological Labs lies the newest and most spectacular addition to the University's scientific facilities, the Cambridge Electron Accelerator. Built in conjunction with M.I.T., the accelerator is the largest and most powerful in the world, and is expected to probe deeply into the many unsolved problems of high energy physics.

From the outside, the accelerator complex is unimpressive, almost inconspicuous. The administrative building and surface structures are the same as those of other recent laboratory plain and boxlike, with large glass windows. The accelerator itself, with its grassy ramparts leading to bare concrete walls, appears more like a waterworks than anything else.

Yet the Cambridge accelerator is five times more powerful than any other, able to whip electrons to speeds very near the speed of light. It was built by the Atomic Energy Commission after four years of planning at a cost of 12 million dollars, and operating expenses may exceed four million dollars yearly. When in operation, the accelerator magnets consume 1034 kilowatts--the power required by 100 average American homes. Electrons travel 14,000 miles around the accelerator's ring of magnets in eight milliseconds, and emerge with an energy of six billion electron volts (BEV).

The C.E.A. is actually two accelerating devices--a linear accelerator, which feeds electrons into the rings, and the circular "race track" itself. After injection, the electrons whirl around the circular orbit through a slender evacuated stainless-steel tube, The tube lies sandwiched between the jaws of 48 C-shaped magnets, each 12 feet long and weighing six tons. These magnets provide the transverse force which keeps the electrons in a circular path.

Technicians speak of an electron "beam," but it is incorrect to think off the machine as producing a continuous flow of high-energy electrons. In reality, the electrons spurt into the ring from the linear accelerator in bunches of 100 million at the rate of 60 bunches per second. At 16 places in the ring, there are radio-frequency powered acceleration cavities. Each time the electron bunch passes through a cavity, its energy increases. The electron pulses thus receive discrete "kicks" of energy as they orbit, until they have finally reached the energy level desired for any particular experiment. The machine is capable of pushing electrons up to an energy of six billion electron volts. At that energy level, the electrons are travelling at 99.999,9996 per cent of the speed of light, and their mass has increased 12,000 times.

Most accelerators in the past have been proton accelerators. Protons, nearly 2,000 times as heavy as electrons are substantial projectiles and were among the first of the "atom-smashing" particles. The proton has drawbacks, however. It is surrounded by a strong nuclear force field, and when two protons pass near each other, the interaction is a strong one.

The electron, with its opposing electrical field, does not react nearly so strongly with protons. It can pass near, or even through a proton and be scattered away without violently disturbing the proton itself. For this reason, the electron is a useful probe for examining the internal structure of the proton.

First on the experimental agenda at the Cambridge accelerator is an attempt to ascertain the internal structure of the proton. The experiment may take four years, and will require 1.5 million dollars of equipment. It is for this kind of work that the electron accelerator was expressly designed.

In actual experimentation, the electron beam will be aimed at a protonrich liquid hydrogen target. The electrons will be scattered as they emerge from the target. Scientists hope that close study of the scattering pattern may yield clues to the internal composition of the proton, which is no longer considered an indivisible particle but rather a composite of smaller bodies.

A second important experiment the accelerator will undertake is the study of so-called "strange particles." These are tiny sub-atomic particles which form the nucleus of atoms. More than 30 such particles have already been discovered. A number of their properties and characteristics are known, but understanding of their interrelationship is still poor.

In this second experiment the beam will be used to create, through bombardment of a special target, a stream of high energy photons. The photons, in turn, will be directed into a hydrogen bubble chamber. Interaction of the photons with the hydrogen nuclei will produce strange particles. These particles will leave a track of bubbles in the hydrogen, and in this way can be observed and studied.

Like the electron-proton scattering experiment, the bubble chamber program will require over a million dollars worth of equipment, and several years to complete.

All experiments will take place in huge underground experimental hall. The hall is 100 by 300 feet--the size of a football field--and is located at a tangent to the main accelerator ring. It is in this room that the electron pulses emerge to be directed at various targets.

Like everything else in the accelerator, the equipment in the experimental hall is designed for maximum flexibility. Lead-and-concrete shielding blocks weighing 35 tons each are moved from experiment to experiment by a giant overhead crane. It is expected that several beams will emerge simultaneously from the ring at different places, and thus as many as six experiments may be conducted at once.

To one side of the experimental hall, scientists have constructed one of the most important pieces of apparatus to be used in the C.E.A.'s experimental program: a liquid helium cryostat. This device will cool and liquify gases, principally hydrogen, for beam targets and bubble chambers. The bubble chambers are large jars of liquid hydrogen, and are used as tracking devices. Many short-lived, invisible particles leave a track of bubbles as they travel through the hydrogen. These tracks can be photographed and later studied.

Because liquid hydrogen is dangerously explosive, the cryogenics room is explosion-proof and equipped with a fast ventilating system. The roof off the experimental hall is also designed for rapid ventilation to minimize the hazards of spilled hydrogen.

Standing about on the floor of the hall are a number of large bending magnets, created to deflect the beam toward targets, as it comes from the accelerator itself. Adjacent to the hall is a huge machine shop, enabling technicians to turn out special fittings on the spot.

Everywhere there is color. The huge concrete shielding blocks are green, purple, and gray. Pipes are blue, nitrogen tanks a vivid yellow, heavily insulated cables are red, and black. Overhead ventilation ducts are fire red. The personnel of the C.E.A. are oblivious to the gaudy, nursery school quality of their surroundings. "It's better than just having everything gray," said a technician, "but I don't suppose it really matters much. You get used to the color, whatever it is."

Scattered about the experimental hall are sober reminders of the potential danger in the work being conducted here. Three-pronged "Radiation" signs are all about; flashing lights warn of special dangers. Above the door which leads into the accelerator tunnel is an illuminated, blood-red notice, "Hazard to Life Machine On."

"It's no joke," one scientist commented. "Inside the ring, there is tremendous leakage of energy in the form of high-frequency radiation. A person caught in there when the machine is running would receive a fatal does in a small fraction of a second." Eight feet of concrete and lead are used to shield the experimental hall personnel from the ring's radiation.

Elaborate safeguards have been devised to insure that no one is ever caught inside the heavily shielded circular tunnel when the accelerator is in operation. Before the machine is started up, a crew walks the length of the tunnel, checking to see that all personnel are out of the danger area, and locking the doors to the ring as they go. When they finish, a gong sounds at five second intervals. Later, red lights flash warning, and a still more insistent gong begins to sound. When the flashers and gong cease, anyone caught inside has one minute to hit any of a dozen crash buttons located inside the tunnel. Pushing the button immediately shuts down the machine.

Scientists are not disturbed by the dangers inherent in their work. "Some scientists have to be careful with explosives, or poisons. We have to be careful with radiation. It's not really very different," said one. Technicians at the accelerator need not wear dosage clips, but no job is without its peculiarities. At the C.E.A., nobody wears a wristwatch-the powerful magnets in the ring will quickly ruin a watch.

The staff of Harvard and M.I.T. people have been careful to emphasize the joint nature of the project, even down to the smallest details. In the conference rooms, for instance, Harvard and M.I.T. alumni chairs are carefully alternated. Every sign, every press release includes the name of both institutions. More importantly, the executive committee of the C.E.A., the policy-making body, is composed of five members (three scientists, two administrators) from each of the two universities.

The third party in the undertaking the Atomic Energy Commission, provided the funds. Proponents of federal aid to education point to the C.E.A. as an ideal case of government supported research with no strings attached. Although over 12 million dollars have already been spent on the project, none of the work is classified, and the federal agency has chosen to take an unobtrusive role. Were it not for an occasional "U.S. Government Property" stencilled on equipment, the causal observer might never guess the government was involved at all.

When planning for the accelerator begain in 1954, the 11 million dollar anticipated outlay by the AEC was unusually large. It is worth nothing, however, that last spring, just as Cambridge accelerator started operations, the government approved an expenditure of 114 million to build the two-mile Linear Accelerator near Stanford University in Palo Alto, California.

This accelerator will overcome the most serious objection to a circular machine: the tremendous quantity of energy that is radiated as the electron revolves around the track. For a fixed orbit radius, the radiation losses increase with the fourth power of the particle energy. Eventually, the point is reached where most of the accelerating energy is lost immediately as radiation.

For example, most of the energy radiated in the C.E.A. occurs while the particles are increasing in energy from 5 to 6 bev. At 6 Bev, the electrons are losing 4.5 mev per turn. But should the energy of the particles be increased by only one-sixth, to 7 Bev, this energy loss would nearly double. As a result, power requirements for the accelerator would double-and in some systems quadruple.

The Stanford Accelerator will avoid this problem by building a straight line course two miles long, along down which the electrons will travel. The major problem here will be to construct a level track for such a great distance. If accomplished, it will be an engineering feat without parallel. The Stanford machine will be considerably more powerful than the C.E.A.: it is designed to operate at 25 Bev, and eventually reach as much as 45 Bev. But, it will concentrate on the same problems the C.E.A. is currently attacking

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