What’s an “Atom Smasher”?
You’ve all heard that (old-timey) phrase, “atom smasher” – I know that it’s been a part of American slang since the 1950s when I was aware of it. We don’t use it anymore but if we did, the closest device that fits that phrase is a cyclotron, like the earlier incarnation of the facility that’s on the MSU campus, NSCL (the National Superconducting Cyclotron Laboratory). The NSCL is a national laboratory serving the world community of nuclear physicists and something to be proud of. (I showed you how they work in the lecture.) The reason it’s the closest to the “atom smasher” term is because what a cyclotron accelerates are many different kinds of particles – nuclei of many of the members of the periodic table (the “atoms” part). One nucleus is sped up and then collided with another kind of element (the “smasher” part), and the result will typically be bits which are themselves nuclei of many other kinds of elements. The cyclotron concept is one of three kinds of particle accelerators—actually, the most conceptually complicated kind.
The other kinds of accelerators concentrate on creating beams of just one kind of particle. Typically these have been electrons and their antiparticles (positrons) and protons and antiprotons. (Cousins of the electrons called muons are also used, but in very specialized ways.) Protons and electrons are used because the scientific questions under examination may require one or the other or because they can be used to create exotic secondary and tertiary beams of many different kinds of particles.
A big reason to use protons and electrons is because they stick around! That is they appear to be completely—or mostly —stable. All other charged particles would decay before they could be used: you’d start a beam of such an ephemeral particle and by the time it was ready to be used, there would be nothing there! It would have decayed away.
Remember: Charged Particles in Fields
Particle accelerators are technically complicated devices, but the principles are simple. Accelerator physicists and engineers create machines that do two things:
1. accelerate particles up to high speeds (and hence, large kinetic energies) and
2. steer them where you want them to go.
Electric fields and magnetic fields do those jobs.
The collection of particles in an accelerator is called a “beam” and they come in various shapes and sizes depending on the scientific need. Sometimes the questions are purely exploratory – some beam or detection technology has become available and scientists want the highest energy beams that can be produced – the biggest bang. Sometimes the scientific questions are about rare reactions, so many, many particles in the beams are important in order to maximize the chances of something interesting happening. And sometimes the questions require particular species of beams. Accelerator physicists and engineers have become magicians at producing boutique beams of specific energies, particular numbers of particles in the beams, and species of particles for specialized uses. These can be complex chains. One can start with, say a proton beam, and create two or three additional beams by the time an experiment makes use of them yards or even miles away.
So, how do they do that?
Charged Particles in Electric Fields
Remember that a charged particle in an electric field, E, experiences a force in the direction of the field if the charge is positive and in the opposite direction, if negative. (Refer to the drawing’s you’ve made on your white boards and from lecture.) Here Newton’s laws are essentially at work: if a charged (or any) particle is in a region with no fields, it moves at a constant velocity. As soon as it encounters an electric field, it feels that force. The chain of reasoning is familiar:
- If there’s a force, there’s an acceleration.
- If there’s an acceleration, then the velocity goes up.
- If the velocity goes up, the kinetic energy increases.
(Need I note that if the particle is neutral like a neutron, that it experiences no force? No. I didn’t think I had to.)
In practice how this happens is that a beam of particles starts down a pipe which is nearly a vacuum —often of higher purity than that of outer space — and passes into a localized region where there is a field. Typically, these E field regions are inside of a metal container called an RF Cavity (RF for Radio Frequency). RF energy is “pumped” into it and creates a time-varying, but spatially uniform field on the inside. Uniformity of the field throughout the volume is crucial since irregularities would spoil the beam. “RF” stands for Radio Frequency, because the field is carefully tuned to rise and fall at a regular rate, at frequencies which are in the radio range. “Cavity” is just like it sounds: it’s a fancy metal can.
Engineering limitations make an accelerator with only one RF Cavity not very useful —fields can’t be created which are high enough to accelerate particles very much. So in order to get them to very high speeds, one of two designs are employed. Either cavities are stacked in a row, one after the other and the particle gets kicked all the way down a straight line (these are called Linear Accelerators, or “LINACs”) or the particles are sent through the same cavity over and over again, which means that the beams have to be bent into a closed loop. Typically these loops are close to being circular, so these are called Circular Accelerators called “Synchrotrons.” The radius of the beampipe (the channel in which the beams move without air in the way) doesn’t change, which is one of the defining characteristics of a “synchrotron.”
Charged Particles in Magnetic Fields
Bending the beam is where magnetic fields come in. Remember that when a charged particle encounters a magnetic field it feels a force which is oddly-directed: it’s perpendicular to the velocity of the particles and also perpendicular to the direction of the magnetic field. Both of those “perpendiculars” result in a bending since if the particle is moving forward, then the tug is to the left or right depending on the direction of the field. It’s also directly proportional to the signed charge—q— of the particle. So it it’s positive it’s bent one way and if the particle is negative, even of the same velocity and in the same magnetic field, it’s bent the opposite way.
Most of the magnets in a sychrotron do the bending and they might be a few feet long, or 10’s of feet long. Of course the longer the magnet the more time the particle spends in the gap, and so the larger bend by the time it emerges on the other side. So a beam might enter in the center of the gap and as a result of the bend, emerge a little bit to the side of center, continuing on its way in a straight line (Newton again) until it encounters the next magnet where it’s bent again. Lots of these straight magnets, placed just so, create nearly circular trajectories for the particles.
There are many complications in this picture. One of the most important is that when a beam is produced because of the RF kick, the beams tend to produce a string of bunches, like pearls on a necklace, where each perl is a collection of as many as particles. Now you get that many, say protons, close together and they’re going to want to repel one another. So they have to be constantly encouraged to get back into a tight bunch…continuously. The devices that accomplish this are many different kinds of magnets that focus and shape the bunches as they pass by. Those are the other kinds of magnets in the machines.