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The primary goal of MACRO is to search for magnetic monopoles. The active elements of MACRO are liquid scintillator and streamer tubes, optimized for high resolution tracking and timing. This design also allows MACRO to operate as a neutrino detector and as a cosmic ray observatory.
The magnetic monopole is a theorized particle that has not yet been observed. It is a possible solution to Maxwell's equations. One researcher claimed to have observed a monopole with a light-bulb-sized detector. The fact that a detector the size of multiple football pitches (MACRO) has not yet duplicated this feat leads most to disregard the earlier claim.
The MACRO project includes a large underground cavern, approximately 800 meters underground, which was further hollowed out and presently houses hundreds of long chambers filled with scintillating fluid – a fluid that gives off photons when a charged or magnetic particle passes through it. At opposing ends of the chamber are a pair of photomultiplier tubes. These tubes contain a number of small charged "plates." They look like flood lights, but they are collectors that can take a handful of photons and "multiply" them. This multiplication begins by using the photo-electric effect to convert photons that hit the first "plate" into electrons. These electrons are then attracted to the next plate which gives off more electrons that it receives. The next plate does the same, thus amplifying the signal more at each plate. The photomultipliers used in the MACRO experiment were produced by Thorn-EMI, and are sensitive to a signal as small as five photons. After decommissioning, MACRO donated about 800 photomultiplier tubes to the Daya Bay Reactor Neutrino Experiment. The exact voltage put on each plate was determined by a custom circuit board designed by some of the scientists involved with the project. The project leader at Boston University was Prof. James Stone.
The scintillating chambers were assembled into high stacks and long rows. When a signal is detected, it is usually detected in multiple chambers. The timing of each signal from each photomultiplier tells the approximate path and speed of the particle. The type of signal and the speed through the "pool" of chambers tells researchers if they have observed a monopole or merely some common charged particle.