This part of the tour takes us into a tunnel about a mile upstream of the detector. This picture shows a portion of this tunnel and a few of the magnets used to steer a beam of charged particles.
As you have already learned, when high energy particles interact new particles are often produced. We make use of this curious property of matter to use the proton beam from the Tevatron to create our neutrino beam in a two step process. Protons are steered to a target of berylium oxide, where new particles called mesons are produced. The mesons themselves continue on their way toward our neutrino detector, but they are unstable. Many of them spontaneously decay, producing a muon and a neutrino. The muons and undecayed mesons are removed by passing the beam through lots of dense matter, while most of the neutrinos continue to pass through our detector. Let's take a closer look.
The Tevatron delivers 800 GeV protons to our target in intense bursts called pings; each ping lasts 2 milliseconds and contains about two trillion protons. We get five pings every minute. These protons interact with the neucleons in our target, producing a thousand trillion mesons - mostly pions and kaons. These particles enter the Sign Selected Quadrapole Train (SSQT) - a group of magnets that select either positive or negative mesons and focus them into a beam. The 'wrong sign' mesons are steered away into a 'dump' to safely dissapate their energy.
This beam of mesons travels down a 350 meter decay pipe where a significant fraction of them decay into muons and neutrinos. Positive mesons decay into antimuons and neutrinos, whereas negative mesons decay into muons and antineutrinos, so by preselecting the sign of our mesons, we have selected either neutrinos or antineutrinos for the detector.
At the downstream end of the decay pipe we then have a beam of three billion neutrinos travelling toward the detector, but it is mixed with an equal number of muons as well as undecayed mesons. To filter these undesirable particles out, there is a lead and steel shield that stops the mesons followed by a 910 meter earthen berm to remove the much more penetrating muons.
Most of the neutrinos survive this trip and pass through the detector, where a tiny fraction of them interact as we have already discussed. We expect about twenty interactions each minute.
That's the tour for now. If you have questions, you can email me at the address below.