The BooNE experiment will create its own beam of neutrinos using the Fermilab
proton accelerator. In a multi-stage process, protons are converted into
muon neutrinos - one of the three types of neutrinos known to physicists.
The first stage of the production process is to accelerate negatively charged hydrogen ions up to 750,000 volts in the pre-accelerator. The ions then enter the linear section of the accelerator where they gain more energy before passing into the circular Booster. At that point, the electrons from the hydrogen are stripped away and the remaining protons are further accelerated, bringing their energy up to 8 GeV (8 billion electron volts).
Next, the protons enter the BooNE beamline and strike a beryllium target resulting in a spray of smaller particles called pions. The pions are unstable particles and do not last for very long. They subsequently decay as they pass through a 50-meter pipe, breaking up into muon and muon neutrino pairs.
The final stage is to pass the beam through a steel absorber and dirt. This stops the heavier, electron-like muons while allowing the weakly-interacting neutrinos to pass through unaffected.
Since neutrinos interact very rarely, it is important to direct as many of them as possible toward the MiniBooNE detector. This is accomplished with a device called a horn. The horn uses intense magnetic fields to focus the pions in a particular direction. In this way, the resulting neutrinos are concentrated in a path directly through the detector, maximizing (by a factor of 10) the total number of neutrino interactions that MiniBooNE sees.
An interesting fact is that BooNE can select to use either a neutrino or an anti-neutrino beam. This is because the positive pions produce pairs of positive muons and neutrinos while the negative pions decay into negative muons and anti-neutrinos. And so, by changing the operating conditions of the horn, it is possible to focus either positive or negative pions, resulting in either neutrinos or anti-neutrinos.
Note: BooNE's name derives from the Fermilab Booster which is used to produce its neutrinos beam. In fact, BooNE stands for Booster Neutrino Experiment.