The MiniBooNE Horn
When the protons from the Booster hit the beryllium target, all sorts of particle debris (including pions) come spewing forth. To help maximize the number of neutrinos that get to the MiniBooNE tank, it helps to do some selective particle steering. The focusing device MiniBooNE uses to accomplish this is called a "horn;" it uses large magnetic fields to aim the pions of choice toward the detector. Running with the horn in operation increases the number of neutrinos at the detector by a factor of ten.
The MiniBooNE horn is an amazing engineering feat. Five times every second, this device will be zapped with 170,000 amperes of current for 150 microseconds -- and the horn has to absorb 100 million of these pulses over its lifetime! Previous horns were pulsed at 300,000 amperes -- but only once every 1.25 seconds, and for only a fifth of the number of pulses.
The horn focuses pions into an air-filled steel decay pipe, 2 meters in diameter and 50 meters in length. At the end of the decay pipe, a concrete and steel absorber will stop all the particles except the neutrinos; they will go zipping through 440 meters of earth to arrive at the MiniBooNE detector. The energies of the neutrinos will range from 300 MeV to 1.5 GeV.