As you might have read, before joining the IT profession, I spent time as a research assistant in the Intermediate-Energy Physics group at the University of Kentucky. As a whole, the group has experiments in progress at particle accelerators around the world. The stuff I worked on, however, is at the Thomas Jefferson National Accelerator Facility (formerly known as CEBAF) in Virginia.

Okay, so what sorts of stuff does one do at a particle accelerator? Well, essentially, you smash particles together (imagine that!) and look at what happens. There are a number of different types of particle accelerators: some collide protons and anti-protons, some collide heavier atomic nuclei while still others collide electrons. The accelerator at Jefferson Lab is of the electron variety. It acclerates electrons up to enormous energies at which point they're sent through a cool "switching station" where they're directed to one of three experiment halls where they collide with various targets. This switching station is very fast so that you can potentially have 3 experiments going on (one in each hall) at the same time.

Cool...but why? Well, a physicist uses a particle accelerator just like a biologist would use a microscope. In fact, an accelerator is a giant microscope -- it can be used to study particles and processes much too small to be seen by traditional microscopes or even advanced scanning-tunnelling microscopes. By accelerating a particle to very high energies and slamming it into something and looking at what happens, you can learn alot about the processes the occur during the collision.

Angular Momentum and Spin

Ever wonder why a gyroscope tends to keep its orientation even though the thing that's holding it (a hand, an airplane, a rocket...) is moving around? Or why a moving bicycle or spinning toy top manages to remain upright? The answer deals with a property known as angular momentum. So, even if we're not physicists or engineers and can't quantify it, we all have a hands-on experience with angular momentum and its effects.

In the macroscopic world we live in, angular momentum is related to the angular motion of an object about some axis: the spinning wheel inside a gyroscope or on a bicycle or the spinning of a top. In the world of particle physics and quantum mechanics there exists another form of angular momentum: spin.

You might have heard someone mention this thing called "spin" when talking about physics. Spin is an intrinsic property of particles (A general rule of thumb: scientists use the word "intrinsic" when describing something that nobody quite understands). Oddly enough, this thing called spin causes the particle to behave just like it's spinning like a top or a bicycle wheel about some axis. You have to understand, though, that this is just a rough analogy: in the macroscopic world, there actually is something spinning while in the microscopic world of particles, there isn't (there is even a philosophical argument against even asking the question of "what's spinning." ) Still, since spin behaves like an angular momentum, you'll often see reference to the direction or alignment of the spin. This will be important later.

One of the features of Jefferson Lab is that it is able to provide a polarized beam of electrons to the experiment halls. This means that the spins of all the electrons in the beam are supposed to be aligned in a particular direction. This is important because when you smash two particles together, it turns out that the relative spins of the particles involved can have an measurable effect on what happens. In order to interpret the results of such an interaction, the scientist must have an accurate idea of the particles' spin orientations before the collision. A device that measures spin alignment, or polarity, is called a polarimeter.

Moller Polarimeter

I was part of a team tasked with designing and constructing what's called an electron polarimeter for one of the experiment halls at Jefferson Lab. The electron polarimeter provides a way to measure exactly how well the spins of the electrons in the beam are aligned. This is critical information if the experiment you're performing is sensitive to spin alignment.

So how do you measure the polarity of an electron beam? It turns out that when electrons collide, the results depend on the relative alignment of the spins of the electrons involved. That is, if these spins are aligned in the same direction, one thing will happen. If they're aligned in different directions, then something slightly different will happen. So how do you measure the polarity of the beam? You make it collide with electrons that that you've placed in a known state. You can do this by magnetizing a piece of iron foil and letting the beam electrons slam into it. You measure the results and deduce what the spin of the beam electrons must have had in order to produce those results.

To measure the polarity of the beam, our polarimeter consists of several large magnets, a little piece of iron foil and a bunch of electronics and detectors to measure the results of the collisions. By magnetizing the foil, we can place the foil's electrons in a known state. This magnetized foil is placed in the beam where beam electrons collide with electrons in the foil. Two sets of detectors measure the results of the collisions. By adjusting the magnetization of the foil, we can vary what each set of detectors measure. Differences in the number of events one set of detectors measure compared to what the other set of detectors measure is called an asymmetry. By measuring this asymmetry, we can deduce the polarity of the beam.

Questions or comments? Drop me a line.