Making Neutrinos

The Booster Neutrino Experiment starts by producing a beam of neutrinos. In a multi-stage process, protons from one of the Fermilab proton accelerators -- the Booster -- are used to generate muon neutrinos, one of the three types of neutrinos presently known.

In the first stage of the production, a pre-accelerator speeds hydrogen ions up to an energy of 750,000 electron volts. The ions then enter a linear accelerator; this device gives the particles even more energy before zipping them off to the circular Booster. At Booster injection, the electrons from the ions are stripped away; the next stage boosts the energy of resulting protons up to 8 GeV (8 billion electron volts).

Next, the BooNE beamline transports the 8 GeV protons from the Booster accelerator to the BooNE target, where they collide with the beryllium target nuclei and send a fountain of secondary particles down a 50 meter pipe. Among the spray of debris is a large number of lighter particles called pions. The pions are unstable and decay in flight into antimuons and muon neutrinos.

The resulting mix of debris, antimuons, undecayed pions, and muon neutrinos is filtered through a steel absorber, which stops all but the weakly-interacting neutrinos. They travel unaffected throught the steel and another quarter-mile of dirt before reaching the MiniBooNE tank, where a tiny, tiny fraction will interact and be detected.

Since neutrinos talk to other particles so very rarely, it is important to direct as many of them as possible toward the MiniBooNE detector. One device that helps to increase the number of neutrinos is called a horn, which uses intense magnetic fields to focus the pion fountain into a tighter stream. When the pions decay, the neutrinos fly off in the same direction, concentrating the neutrino beam onto the detector and increasing by a factor of eight the total number of neutrino interactions that MiniBooNE sees.

MiniBooNE can choose to operate with either a neutrino or an anti-neutrino beam by selecting the charge of the decaying pions. Positive pions produce neutrinos, negative pions decay into anti-neutrinos. Changing the operating conditions of the horn makes it possible to focus either positive or negative pions, resulting in a beam of neutrinos or anti-neutrinos.