What are accelerators used for?
For many years, accelerators were typically research tools. They have been used to make things radioactive, study the effects of radiation on materials, and even to explore our subatomic universe. Lately, however, accelerators have found good use in medicine and industry. In medicine, accelerators are used directly to treat diseases like cancer by irradiating tumors or lesions. They are also used to produce many of the radioactive elements that nuclear medicine departments in your local hospital use for diagnosis and treatment. In industry, accelerators are used for a host of purposes, including food sterilization and isotope production.
Exactly what is an accelerator?
A nuclear particle accelerator is a device designed to produce a stream of ions that are directed along some path. This is typically achieved by first generating the ions, then causing them to pass through a large electrical potential difference in order to increase their energy even further. From there, a series of magnets are used to direct the high-energy beam towards a “target”. The beam interacts with the target material, producing radiations that are then made available for some predetermined use.
So an accelerator consists of an ion source and a magnet?
Actually, accelerators contains a few more parts than that. Most accelerators have a source of high voltage, a vacuum system, the beam path with its magnets, a target, and some shielding, as well as the source of ions.
Are all ion sources the same?
No. For example in devices that accelerate positively-charged ions, a high frequency electrical field causes sufficient forces on the electrons of an electrically neutral gas so that they break their binding forces and produce positive ions. This type of device is called a “radio-frequency stripping” source. Other sources produce different types of ions.
Are the high voltage supplies used to apply the potential difference for the positive ions all pretty much the same?
No, these differ as well. The way in which the high voltage connection between the ion source and the target is configured defines the variety of modern accelerator types.
Why do you need a vacuum?
There is a beam pipe that contains the accelerated ions. This pipe must be evacuated in order for the ions to efficiently reach the target.
Air molecules present in the beam pipe will collide with the accelerated ions, causing them to slow down and lose their ability to produce the desired reaction in the target.
So what are the magnets used for?
These are used to “focus” and steer the beam of ions along the desired path.
And the target?
This is actually where the important reactions occur. In some applications, the effect of the accelerated beam on some object might be studied. In other applications, the beam might be directed onto some type of material in order to enhance or modify its physical properties. In still other applications, the beam interacts with the target material and produces different kinds of radiations that can then be used for research, medical or industrial purposes.
Have we covered all of the parts that are common to most accelerators?
Almost. Some accelerators have what is known as a “beam dump” at the end of its path that is used to remove any remaining energy from the beam and to dissipate the heat that is generated during the interactions. And don’t forget that we said some sort of shield is necessary in order to protect operating personnel and the general public from the radiological and physical hazards associated with an operating accelerator.
You said previously that the configuration of the high voltage connection between the ion source and the target defines the accelerator type. What types are there?
One of the most common is the Linear Accelerator, colloquially referred to as “linacs”. These accelerate the ions along a straight path. A Cockroft-Walton accelerator design, and the Van de Graaff generator are examples of low-energy linacs. You may have seen a Van de Graaff generator at your local science museum. It is a tall metal object that, when a person places his or her hand on its surface while operating, it causes the hair on his or her head to stand on end. The effect is quite amazing for females with slightly long hair.
Is there such a thing as a high energy linac?
Yes. These often incorporate pulses of microwave radiation in a wave guide. These lead to a traveling wavefront that carries the ions along much like a surfer rides a water wave. In this case, the longer the “ride” on the wavefront, the higher the energy of the ion beam. The best example of a high energy linac is the Stanford Linear Accelerator (SLAC), which is located at Stanford University in California. On this device, the wave guide is over two miles long! Ion beams with very, very high energy are created in the SLAC.
Are there other configurations?
Yes. Another common one is the cyclotron. These devices have the ion beam traveling in a circular path.
How does a cyclotron work?
There are two evacuated beam paths in the shape of the letter “D”. Each of these is placed between the poles of a large electromagnet, which is used to confine the beam to the circular path, and to help it “accelerate” as the beam passes between the gap between the two D’s. In this case, relatively small potential differences, applied with each pass of the ion beam, result in beam energies that can be quite high. The best example of a cyclotron is the one located at the Fermilab in Batavia, Illinois. This circular accelerator has a circumference of many miles. In it, both protons and anti-protons are accelerated in the same ring, but traveling in opposite directions. They are then brought together in a head-on collision, producing a number of useful effects.
What kinds of ionizing radiation are associated with accelerators?
Actually, there are two types. These are “prompt” radiations and “induced radiations”.
What are prompt radiations?
These are the radiations present only when the accelerator beam is turned on. Much like the light from a lamp, the radiations disappear when the switch is turned off.
What kinds of radiations are they?
Well, depending upon the accelerator type, x-rays, neutrons and even subatomic particles can be produced. Although each can be detected perpendicular to the beam line, by far the majority are found in the forward direction.
What do you mean by induced radiation?
Induced radiation comes from materials in the vicinity of the accelerator that have been made radioactive due to the presence of neutrons. The most predominant elements that emit induced radiation are radioactive oxygen, nitrogen, carbon and argon from the air within the facility. If concrete is used as a shield, then radioactive sodium is also readily detectable. In fact, it is not uncommon, particularly for older accelerators, to even see radioactive cobalt and other elements commonly associated with metal parts.
Which is the greatest source of radiation safety concern? Prompt or induced radiations?
Generally speaking, the induced radiation field that is present immediately after the accelerator is shut down presents the greatest radiological hazard. Fortunately, however, the majority of the radionuclides produced have very short half-lives. So if the operators just wait a bit before entering the accelerator area, the field intensity drops quite a bit. However, as I said before, we can’t ignore the build-up of induced radioactivity in the surrounding metal parts.