by A.K. Solomon
by A.K. Solomon |
"THE operation of a cyclotron is a complex and varied experience. The vagaries of so complex a machine are unpredictable. The daily operation of the cyclotron and similar atom-smashing machines is a necessity for nuclear physics, since they provide the material for that painstaking accumulation of experimental results required before new discoveries are possible. If the cyclotron is in good working order, running it for a day may be pure routine. But when trouble does develop, a day at the cyclotron may become a long series of unhappy disasters.
Before work is begun, the vacuum must be good enough for the bombarding deuterons to circulate without bumping into too many other particles. If there is a leak in the tank and the gauges indicate that the pressure is too high the leak must be found and sealed before the cyclotron can be turned on. Sometimes experience will indicate where the leak is, but far more often finding it is a tedious, nerve-racking job. Leaks are far too small to be seen or heard, they must be found blindly, by relying on the sensitive vacuum gauges to show when anything done to the tank affects the pressure. The scientist looking for a leak is like a blind man hunting for a black hat in the dark.
If there are no leaks, or after the leaks have been sealed, the water valve is opened. As the water flows through the system, dividing among the many cooling circuits, all the electrical relays that indicate satisfactory water flow come into action. The sound of clicking relays is clearly audible above the hiss of the water. The power used by a cyclotron is so great that most of its parts must be properly cooled; the relays register any failure in the water supply and immediately turn the cyclotron off before it can get too hot. Until all the relays are closed the cyclotron cannot he turned on: as long as they remain closed, the water system needs no further attention.
The cyclotron is powered by three huge generators. As they are started their whine adds another note to the increasing din. Before turning on the magnet and its attendant generator a careful check must be made for any stray pieces of iron around the magnet. There are several glass windows let into the cyclotron tank so that the position of the dees can be observed from outside. Stray iron left around a cyclotron, attracted inexorably by the powerful magnet, has on occasion been snatched through one of these windows. Not only does a shattered window mean a loss of the precious vacuum, but, what is more important, it means that the inside of the tank will be littered with glass fragments. Cleaning a cyclotron is neither an easy nor a pleasant job.
When the filaments in the various radio tubes have been lighted, the cyclotron is ready for operation. Rather, it is ready for a preliminary operation, for it is first necessary to de-gas. Although it has stood idle only overnight, with high-speed vacuum pumps constantly in operation, the metal surfaces in the radio tubes and the cyclotron have picked up gas. As the power is turned on, the surfaces heat up and give off once again the gas they have absorbed in the night. A burst of gas may cause the vacuum relay to release, which turns off the power. After the gas has been pumped away the power is turned on again and more gas driven off, until finally all of it has been driven out of the metal surfaces.
Only when all the gas that isn`t wanted has been eliminated can the gas that is wanted, the heavy hydrogen from which deuterons are made, be let into the cyclotron. The gas valve is opened electrically and the pressure adjusted to its usual value. When the cyclotron filament and its accessory voltage have been turned on, the deuterons are formed in the centre of the tank, waiting their long spiral journey inside the dees. Then the main power is switched on and the deuterons begin their travel as the voltage alternates from dee to dee. But the particles can't get out until the deflector voltage is applied. Then, if the current through the magnet is adjusted to the critical value, a beam may appear. Finally, when the needle begins to move along the face of the beam meter, it is a very welcome sight. As the beam is increased to a maximum by further adjustment, the note of the generator changes its pitch, with the additional load required to spin the particles around.
Even when the beam is at a maximum, the cyclotroneer's task is not complete. The adjustments are critical and the cyclotron after a period of rest is always unsteady. The sparks that often occur inside set off bursts of gas which switch off the high voltage supply through the vacuum relay. The many metal parts heat up, and as they heat, warp. Then the tuning must be adjusted to compensate for the change. All the noises that accompany a cyclotron's operation may be heard through the loud-speaker system that connects it to the distant control desk - any slight change in sound may demand immediate attention. Sometimes even the array of meters on the control desk is not sufficient. When the cyclotron is finally functioning properly the operator can relax his vigilance and run the machine with occasional adjustments in the critical tuning.
Production of a radio-active isotope, like radio-active sodium with its characteristic 14.8 hour half-life, requires preparation of a target suitable for mounting in the cyclotron. The sodium must be held tightly and cooled adequately in vertical position directly in front of the cyclotron beam. If sodium metal is used, great care must be taken, since sodium melts easily, and the beam is strong enough to melt holes even in copper foil. Usually targets made of low melting materials are kept in an extra target chamber, separated from the cyclotron proper by a thin aluminium window transparent to the beam of deuterons. If there is an accident to the target, or if particles of sodium are sprayed off owing to the impact of the swift deuterons, it is only necessary to clean the subsidiary target chamber. Should the contamination enter the cyclotron chamber proper, its removal would be a difficult and delicate job.
The target may consist of a water-cooled plate with several deep grooves in the middle so that the sodium, pressed tightly against the grooves, will adhere to them for support and better cooling. This plate is screwed on to the end of the target chamber with the sodium directly exposed to the deuteron beam and the bombardment is begun."
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