MACRO (Monopole, Astrophysics and Cosmic Ray Observatory) was aparticle physics experiment located at theLaboratori Nazionali del Gran Sasso inAbruzzo,Italy. MACRO was proposed by 6 scientific institutions in the United States and 6 Italian institutions.
The primary goal of MACRO was to search formagnetic monopoles. The active elements of MACRO were liquidscintillator andstreamer tubes, optimized for high resolution tracking and timing. This design also allowed MACRO to operate as aneutrino detector and as acosmic ray observatory.
The experiment operated from 1989 to 2000.[1] No monopole candidates were detected, meaning that theflux of monopoles is less than 1.4×10−16 per square centimetre persteradian per second (cm−2sr−1s−1) for velocities between0.0001 c and1 c (between30000 m/s and300000000 m/s).[2]
The magnetic monopole is a theorized particle that has not yet been observed. If detected, it would disproveGauss's law for magnetism, one of the fourMaxwell's equations which describe the well-established modern understanding of electricity and magnetism.
One researcher claimed to have observed a monopole with alight-bulb-sized detector.[citation needed] The fact that a detector the size of multiplefootball pitches (MACRO) has not yet duplicated this feat leads most to disregard the earlier claim.
The MACRO project included a large cavern, approximately 800 metres underground, which was further hollowed out and housed hundreds of long chambers filled withscintillating fluid – a fluid that gives off photons when a charged or magnetic particle passes through it. At opposing ends of the chamber were a pair ofphotomultiplier tubes. Photomultiplier tubes contain a number of small charged "plates". They look likeflood lights, but they are collectors that can take a handful ofphotons and "multiply" them. This multiplication begins by using thephoto-electric effect to convert photons that hit the first "plate" intoelectrons. 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 byThorn-EMI, and were sensitive to a signal as small as five photons. After decommissioning, MACRO donated about 800 photomultiplier tubes to theDaya Bay Reactor Neutrino Experiment. The exactvoltage put on each plate was determined by a customcircuit board designed by some of the scientists involved with the project.
The scintillating chambers were assembled into high stacks and long rows. When a signal was detected, it was usually detected in multiple chambers. The timing of each signal from each photomultiplier told the approximate path and speed of the particle. The type of signal and the speed through the "pool" of chambers told researchers if they had observed a monopole or merely some commoncharged particle.
Very important results were obtained by MACRO in other sectors:[3]
In particular, MACRO showed evidence of neutrino oscillations at the Takayama neutrino conference immediately before the announcement of the discovery of oscillations by theSuper-Kamiokande experiment.[4][5]