New Accelerator Probes Structure of Proton

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.