Can Huge New Atom Guns Shoot Out Biggest Secrets? (Jan, 1948)
More about the Bevatron.
Can Huge New Atom Guns Shoot Out Biggest Secrets?
Gigantic ring-shaped machines, with 10-billion-electron-volt wallop, may transform energy into matter.
By Alden P. Armagnac
Drawings by Ray Pioch
SUPPOSE that a bullet could be fired 150,-000 miles, six times the distance around the world. Suppose that it could be given a shove to speed it up every 150 yards. Suppose, too, that it could be so aimed and guided throughout this long, fast flight that it would hit a target no bigger than a mans hat. Now suppose, furthermore, that this bullet were something that no one had ever seen or ever could see.
The experimental physicists have quit supposing such things can be done. They know they can perform the very feat that you have just been asked to imagine. They are as sure of it as you are that two and two make four. They have the blueprints for guns that will impart such tremendous energy to such particlesâ€”and they expect to have at least one of the mammoth machines operating in about three years.
The energy that these machines impart to their projectiles is measured in electron volts. The synchro-cyclotron at the University of California, biggest atom gun in the world today, has a power of 200,000,000 electron volts. Now work has begun in England on the parts for a 1,500,000,000 electron-volt accelerator for the University of Birmingham. Two American machines will be even mightier. Both are designed to reach 10 billion electron volts, 50 times the power of the greatest in use today.
Dr. Ernest O. Lawrence, the inventor of the cyclotron, revealed the plans for one of these machines recently at the Sheffield centennial at Yale. It will be called a bevatron. (Bev signifies “billion electron volts” and tron is a Greek ending that means “the agency for.”) An equally powerful machine, to be called a proton synchrotron because it will accelerate protons by synchronized electrical shoves, is on the drawing boards of the Brookhaven National Laboratory on Long Island.
How Scientists Study Atoms
What these machines will do is comparable to increasing the force of gravity. If you could make a pencil falling from your hand gain momentum fast enough to hit the floor like a sledge hammer, you would have done something similar to the trick the physicist performs with the help of cyclotrons, synchrotrons, and other accelerators. He uses them to speed up atomic projectiles, with which he can then shatter other atoms, and he does it for exactly the same reason that a small boy socks an alarm clock with a hammerâ€”to find out what’s inside it and how it works.
Some of an atom’s “innards,” positively charged particles called protons, account for the difference between chemical elements. Their uncharged partners, the neutrons, account for the instability of some atoms and the stability of others. Hence these strange particles are extremely important in science, politics, and economics. Yet even those who put them to use, in atom bombs and radioactive “tracers,” know no more about nature’s bridles upon them than Columbus knew about America when he sailed westward in 1492.
Why do the physicists want more powerful atom smashers? They want to learn more, for one thing, about the mysterious glue or “binding energy” that holds charged particles like protons together, when by all the old rules they should fly apart. And they want to test their conviction that energy can be turned into matter, just as surely as atom bombs turn matter into energy.
The 10-billion-volt atom smashers projected independently by the University of California and the Brookhaven National Laboratory will work on the same principle. The key feature of each, in which they differ from today’s biggest machines, will be a gigantic ring-shaped magnet. Like the fence that keeps horses within a race track, the ring magnet will restrain atomic particles to a circular orbit, enabling them to be accelerated to fantastic speed and energy within a vacuum tube of practicable length.
The atomic projectiles will be obtained by stripping the electrons from atoms of hydrogen. This can be done by discharging an electric arc in hydrogen gas. Protons, the charged cores of hydrogen atoms, are left.
These protons will be catapulted into a “doughnut” or circular vacuum tube built into the ring magnet. The device that gives the protons a running start will be a 4,000-000-volt electrostatic machine called a Van de Graaff generatorâ€”a formidable atom-smashing tool itselfâ€”or a small cyclotron.
Zooming around the circular racecourse, the protons will gain speed at every revolution, as they get an electrical kick from a cylindrical metal electrode in their path.
Finally, when they have reached full speed, the protons will be deflected and will strike a target. Some of them will knock other particles, such as neutrons, out of the atoms in the target. The dislodged particles will fly into a cloud chamber where they will knock still other particles around, leaving visible tracks on photographic plates, which will show what has happened.
How Design Was Chosen
Choice of a design using a ring magnet follows considerationâ€”and discardâ€”of two possible alternatives, a “linear accelerator” and a cyclotron.
All three types are alike in using successive boosts from electrodes to speed up particles flying through a vacuum chamber. They differ in the shape of the path they impart to the particles. In a linear accelerator this path is a beeline, and no magnet is needed. A cyclotron employs a magnet to bend the path into a relatively compact spiral; its magnet must have pole pieces at least as large as the spiral in area, so that the particles always travel between them. A synchrotron, the class to which the new billion-volt machines belong, employs a magnet to bend the particles’ path into a circle instead of a spiral. The circular track is so much narrower than the spiral that a ring-shaped magnet, which suffices to enclose it, has only a fraction of the bulk of the magnet required for a cyclotron of comparable size and power.
Some time ago, Dr. Luis Alvarez, of the University of California, proposed to accelerate protons to unprecedented energy in a linear accelerator. The hitch was that, to get protons up to speeds equivalent to billions of volts, the tube would have to be at least half a mile long. In contrast, the experts calculate that a ring-shaped racecourse 160 feet in diameter, the size of the projected machines, will yield the aimed-at figure of 10,000,000,000 electron volts.
The ring-magnet design also hurdles the practical limit of power attainable by cyclotrons, which today have reached staggering dimensions and complexity. The magnet of the 4,000-ton California synchrocyclotron, largest of existing atom smashers, weighs as much as 20 ordinary locomotives. Even Dr. Lawrence, who has been building such monsters for years, confesses he is sometimes bewildered by the maze of dials and switches on this machine’s control panels. When they are properly juggled, the synchro-cyclotron emits a superpenetrating neutron beam that retains half its power after passing through nearly a foot of concrete or lead, and operators need 10 feet of concrete for protection. Workmen with steel-cleated shoes find their feet misbehaving when the powerful magnet is turned on. Yet this machine, whose particles whirl within a vacuum chamber of 184-inch diameter, will be a toy compared to the projected atomic race track of 160-foot diameter.
An attempt to “scale up” the design of a cyclotron to reach 10 billion electron volts would require a magnet, with the colossal weight of 3,000,000 tonsâ€”an amount of metal that all the iron and steel plants in the United States would require nearly two weeks to produce. With only a slight twinkle in their eyes, Brookhaven scientists suggest that such a burden might sink Long Island.
Magnet to Weigh More than Cruiser
Actual plans call for something more within reason, a ring magnet about three times as heavy as the synchro-cyclotron’s magnet. It will still be a sizable chunk of iron. Its estimated weight, 12,000 to 13,000 tons, exceeds that of a heavy cruiser!
As the speed of atomic particles approaches the speed of light, they tend to lag behind regularly timed electrical impulses, because they become heavier with increasing velocity. Fortunately a new principle shows how to overcome this tendency by giving pulsations of an alternating-current electrical circuit a correspondingly decreasing frequency, so that they will always be correctly timed to make the particles continue gaining speed. Called “the principle of phase stability,” the idea was incorporated in the California synchro-cyclotron, and will also help the new machines â€¢ to develop their enormous power.
How nearly the rival California and Brookhaven groups have arrived at the same design for a 10-billion-electron-volt machine, even to almost identical dimensions, indicates the soundness of its theory. Preliminary engineering designs shown to the writer differ only in such details as arrangements for access to the “doughnut,” a vacuum tube of ceramic or other material about a foot by four feet in cross section, which will be placed between the magnet poles.
William M. Brobeck, the bevatron designer, plans a magnet with four circular quadrants. Between each pair of these will be a straightaway section where the vacuum tube will be accessible for pumping. The design favored by Dr. M. Stanley Livingston of the Brookhaven National Laboratory, in which a perfectly circular magnet can be used, uses a magnet of C-shaped cross section that does not completely enclose the tube. Vacuum pumps and experimental apparatus can therefore be connected to the tube all the way around its circumference.
Listening to one of these mighty machines in action, from a vantage point behind a sufficient thickness of protective shielding, you would hear an eerie symphony of sound. First the whine of great generators coming up to speed will rise to a high-pitched crescendo. Then, as their entire output is virtually short-circuited through the magnet coils, the pitch of the sound will drop abruptly like that of a plane in a power dive. At that moment, as much power will surge from the generators as it takes to run a 35,000-ton battleship at top speed. Such a discharge of electricity, required to keep the protons on their course, will take place at intervals of a minute, or less. The mechanical shock will be a brutal one, requiring special reinforcement in the anchorage of the magnet and generators.
Flywheels on the generator shafts will keep them turning despite the braking effect of this jolt. Provision will also be made to feed the stored energy of the magnet back into the flywheels, and thus harness the back surge of power from the magnet coils, by using the generators as motors during this part of the cycle. This must be done because so much energy is put into the magnetic field that no cooling system could handle it. It must be instantaneously withdrawn, or the conductors would vanish into pools of molten copper.
Streaking through the vacuum tube, the flying particles will circle it a couple of million times, receiving a boost from the electrode within the tube on each time around. Then the great magnet will “let go” of them and they will fly outward; or, alternately, its pull may be momentarily increased to deflect them inward.
That’s when the nuclear bullets will hit the target, a wedge-shaped piece of metal inserted through the wall of the “doughnut” . What Happens then will be front-page news.
For the first time, scientists will command a beam comparable in power to the cosmic rays that strike the earth’s outer atmosphere. These rays are known to create mysterious particles called mesons, larger than electrons and- smaller than protons, whose role in atomic structure is little understood. With the superpower atom smashers, physicists aspire to manufacture mesons themselves, and bare their secrets. It should take only a billion electron volts or so.
And at their full power of ten billion volts, the men at the controls may fulfill their dream of turning energy into matter. Collision of the speeding particles with the target, they hope, will produce more protons than existed before. Then they will actually be turning kilowatt hours into something as tangible as hydrogen or helium!
By progressing from the transmutation of atoms to the transmutation, and even creation, of subatomic particles, the scientific adventurers hope to discover the underlying laws of the atom’s nucleus.
Neutrons and protons are very curious animals. Werner Heisenberg, the physicist who headed Germany’s atomic-bomb project, suggested several years ago that when a neutron collided with a proton, they might exchange identities. In other words, the neutron might become a proton, and the proton a neutron, in about 1/1,000,000,000,000,000,000,000 of a second. Million-electron-volt particles of the California synchro-cyclotron proved Heisenberg was correct.
Billion-electron-volt beams of the beva-tron and proton synchrotron may reveal still stranger things. It is entirely possible that a reaction in which little atoms unite to form a big one may yield energy on a grander scale than does the fission reaction of the atom bomb, in which a big atom splits into little ones.
Much more remains to be learned about the core of the atom before such techniques can be used in bombs or power plants. Recent advances in technical skill have far outdistanced progress in learning the basic facts about the atom’s nucleus. Now the theorists, in their turn, hope to catch up.
Engineers estimate that it will cost about $15,000,000 to build a 10-billion-electron-volt accelerator. The need for the knowledge it should yield is so urgent that scientists are confident that the funds will be forthcoming.
Today’s Line-Up of Atom Smashers
Linear accelerator. A straight vacuum tube, with accelerating electrodes at intervals, in which all types of charged particles travel along a straightaway path.
Van de Graaff generator. A static machine using moving belts of silk or paper to build up a potential of millions of volts. It may contain a built-in tube of the linear-accelerator type, as when used as an injector for feeding particles into other accelerators.
Cyclotron. An accelerator for protons, deuter-ons, or alpha particles, in which they follow a spiral path from a source, at the center, to a target at the outer edge of a vacuum chamber between the pole pieces of an electromagnet. Particles are accelerated by one or two D-shaped electrodes.
Synchro-Cyclotron. A form of cyclotron in which the frequency of pulses to electrodes is not constant, but varies so as to keep in step with the motion of particles of extremely high energy. This is necessary because of the particles’ change in mass due to relativity.
Betatron. An accelerator for electrons, in which they follow a circular orbit in a doughnut tube between the pole pieces of an electromagnet and are accelerated by the transformer action of the flux within the orbit. On being released and striking a target, the speeding electrons give rise to a beam of X-rays.
Synchrotron. An accelerator for electrons or protons, in which they follow a circular orbit within a doughnut tube, and are accelerated by an electrode. The tube may be enclosed in a ring-type magnet. On being released and striking a target, these particles produce all types of nuclear radiation.