NuTeV - A High Energy Physics Experiment

A Tour for the Non-Physicist


Welcome to NuTeV! My name is Len, and I will be your tourguide today. I am a high school science teacher who has joined the NuTeV collaboration, and I am giving this tour because I think, to paraphrase an old saw, physics is too much fun to be left to the physicists.

I am going to begin by showing you our detector and explaining what its component parts do. Since my work here has been on various aspects of the detector I think of it as the heart of the experiment, but the folks working on our beamline would quickly point out that without them we would have nothing to detect, and of course the test beam people are always telling us that calibration is the most important part of any experiment! If we have time, I will try to tell you a bit about all these aspects of this experiment.

Standing here on this little loading dock the top of the detector is almost at eye level, and the neutrino beam is coming in from our right. The first thing you probably notice is that this baby is BIG! The front face is ten feet square, and the overall length from front face to the blue cart is about 120 feet. The next thing to notice is that there are three distinct chunks, the target-calorimeter, the toroid magnet (muon spectrometer), and the blue cart way over to your left.

Let's first take a closer look at the target-calorimeter. You can see that this is a large "Dagwood Sandwich" (for those of you old enough to know who Dagwood is), made up of three kinds of slices. The first and heaviest type of slice is a two inch thick ten foot square steel plate. Altogether there are 690 tons of these, and they comprise the target -- that is, they provide some protons and neutrons for our neutrinos to interact with. It is worth remembering here that this seemingly solid steel is in fact mostly empty space. You probably know that Ernest Rutherford, back in 1910, shot some alpha particles through a gold foil, and discovered from the way these particles scattered that an atom has a nucleus that contains almost all of the atom's mass, but takes up only a tiny fraction of the apparent size of the atom. The rest of the space is where the electrons orbit, and the fact that electrons repel each other keeps the atom from getting squashed. In iron, if the individual nucleii were the size of golf balls, two "nearest neighbor" nucleii would be over a mile apart! It is no wonder then that most of the neutrinos entering our target go right on through -- that is why this detector has to be so large.

The next sort of slice in our sandwich is a scintillation counter, again ten feet square, and an inch thick. There are 84 of them in our target/calorimeter. These are in fact plastic boxes, filled with scintillation oil, and with a photomultiplier tube looking in at each corner. Any charged particle that passes through the oil will cause it to scintillate; electrons are disturbed by the passage of the charged particle, and as they settle down, they give off photons of light. The photons are detected by the phototubes and an electrical signal is produced. The signal strength is proportional to the energy deposited by the particle, and thus this acts as a calorimeter, or energy measuring device. A second important feature of scintillation counters is that they respond very quickly, allowing us to pinpoint the time of a particular event accurately.

The third sort of slice is a drift chamber, used to determine the position of a charged particle passing through; we have 42 of them in the target, and can thus determine the track of a particle, by linking up positions from chamber to chamber. To get a handle on how a drift chamber senses position, think about shooting a BB gun through a harp! Someone with perfect pitch could tell you which string the BB hit, just by listening. Our drift chambers are a bit more complicated than that, but I will try to make their operation clear for you. First of all, each chamber is more like two harps, with the wires at right angles, so we can get both an X and a Y position for the chamber. Secondly there are only 24 wire assemblies in each plane, so we certainly don't expect the particle to hit the wire in order to be detected. What really happens is that as a charged particle passes through the gas in the chamber it ionizes some gas molecules - that is it knocks some electrons loose. The wire is held at a high positive voltage and thus attracts the electrons, and the electrons arriving at the wire produce a signal. The time it takes the electron to drift to the wire is proportional to the distance from the wire the particle passed through. Remember we already know from the scintillation counter WHEN the particle passed, so by measuring the time delay until the drift chamber signal occurs we find out WHERE it passed. Those of you who are still paying attention are going to ask "how do you know which side of the wire the electrons came from?" The chambers in our target actually have two sense wires very close together in each cell, to answer this question for us. Additionally, there are some field shaping elements in each cell, but you don't really have to know about them at this point.

That then is the target/calorimeter. In this close-up picture, the scintillation counters have phototubes on the upper corner, the iron plates are red primer colored, and the drift chambers are the silvery slices without phototubes. The various cables bring power to and data from the detector elements.You can see that what is measured in the target is the energy deposited by any charged particle either stopping or passing through, and the track or path a particle follows. I will talk more about this when we get around to talking about exactly what kind of interactions we are looking for at NuTeV. First though, let's look at the other big part of this detector - the muon spectrometer.

The muon spectrometer is in fact a large toroidal magnet - a few big iron doughnuts with coils of current-carrying copper wire to produce a magnetic field. There are scintillation counters and drift chambers interspersed along the axis of the toroid to track particles passing through. As a charged particle passes through a magnetic field, it feels a force and its path is therefore bent. The amount of bend indicates the momentum of the particle, with low momentum particles being bent more than high momentum particles. (It is really the path that is bent, not the particle, but physicists talk about this so often that they have adopted this shorter way of saying things!) Our toroid is used to measure the momentum of muons passing through the detector, thus we call it a muon spectrometer. The blue cart sitting way over to the left holds more drift chambers and is really a part of the spectrometer. It gives us a longer lever arm for determining how much a particular muon was bent.

One last thing to notice out here is all those blue cables leading from the detector back into our trigger room; everything we "see" in a modern high energy physics experiment is seen electronically, and computers are absolutely indispensible for collecting, sorting, and analyzing the data that we get. We will go into the trigger room shortly, but first I should talk a bit about the particular kinds of events we are looking for on this experiment.

OOPS! Do you hear that sort of high-pitched whine? That is our "Droege Trip Alarm" telling me one of the drift chamber power supplies has tripped because it was drawing too much current. It's my responsibility to go take care of it, so I'll have to suspend the tour for now, but I will continue at this URL soon. In the meantime you can take heart in knowing that it is not just in your high school physics lab that these sort of things happen - it is a part of The Joy of Physics!

Continue the tour.

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Len Bugel: bugel@miranda.fnal.gov