The Mark I, also known as the

SLAC SLAC National Accelerator Laboratory, originally named the Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S. Departm ...

- LBL

Magnetic Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particle ...

Detector, was a

particle detector In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify ionizing particles, such as those produced by nuc ...

that operated at the

interaction point In particle physics, an interaction point (IP) is the place where particles collide in an accelerator experiment. The ''nominal'' interaction point is the design position, which may differ from the ''real'' or ''physics'' interaction point, where ...

of the

SPEAR A spear is a pole weapon consisting of a shaft, usually of wood, with a pointed head. The head may be simply the sharpened end of the shaft itself, as is the case with fire hardened spears, or it may be made of a more durable material fasten ...

collider A collider is a type of particle accelerator which brings two opposing particle beams together such that the particles collide. Colliders may either be ring accelerators or linear accelerators. Colliders are used as a research tool in particle ...

from 1973 to 1977. It was the first 4π detector, i.e. the first detector to uniformly cover as much of the 4π

steradian The steradian (symbol: sr) or square radian is the unit of solid angle in the International System of Units (SI). It is used in three-dimensional geometry, and is analogous to the radian, which quantifies planar angles. Whereas an angle in radian ...

s (units of

solid angle In geometry, a solid angle (symbol: ) is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point. The poi ...

) around the interaction point as possible with different types of component

s arranged in layers. This design proved quite successful, and the detector was used in discoveries of the particle and

tau lepton The tau (), also called the tau lepton, tau particle, tauon or tau electron, is an elementary particle similar to the electron, with negative electric charge and a spin of . Like the electron, the muon, and the three neutrinos, the tau is a l ...

, which both resulted in

Nobel prize The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel's will of 1895, are awarded to "those who, during the preceding year, have conferred the greatest benefit to humankind." Alfr ...

s (for

Burton Richter Burton Richter (March 22, 1931 – July 18, 2018) was an American physicist. He led the Stanford Linear Accelerator Center (SLAC) team which co-discovered the J/ψ meson in 1974, alongside the Brookhaven National Laboratory (BNL) team led by S ...

in 1976 and

Martin Lewis Perl Martin Lewis Perl (June 24, 1927 – September 30, 2014) was an American chemical engineer and physicist who won the Nobel Prize in Physics in 1995 for his discovery of the tau lepton. Life and career Perl was born in New York City, New York. Hi ...

in 1995). This basic design philosophy continues to be used in all modern collider detectors.

Details of the detector

The detector was enormous for the early 1970's, weighing in at ~150 tons, with a length of 12 feet and a height of 20 feet. The colliding electron and positron beams were contained within a vacuum chamber of about 6 inches in diameter. The beam pipe was constructed from a very thin (0.008 inch) corrugated stainless steel tube. The two counter-rotating beams were collided at the center of the detector. A solenoid coil generated a magnetic field roughly parallel to the beam direction, which enabled measurement of the transverse momentum of particles emerging from the collision point. The steel flux return was constructed from 8 pieces of steel arranged in an octagon around the detector; and two removable steel end caps, one at each end of the detector. Construction of the original detector, designed by Bill-Davies White, took about a year, and was completed in 1973. The original detector consisted of a series of components in cylindrical layers.

Inner Trigger Scintillation Counters

Four inner trigger scintillation counters were positioned around the beam pipe. Charged particles traversing these counters generated light pulses, detected by photo-multiplier tubes and associated electronics.

Multi-Wire Proportional Chambers

SLAC collaborators developed the MWPC system.

Cylindrical Wire Spark Chambers

There were 4 concentric sets of electronic read-out wire spark chambers. Design and construction of these detectors were overseen by

Roy Schwitters Roy F. Schwitters, (June 20, 1944 – January 10, 2023) was an American physicist who was professor of physics at the University of Texas at Austin. He was formerly a professor of physics at Harvard and Stanford, as well as director of the Supe ...

of the SLAC collaboration

Outer Trigger Counters

Sandwiched between the outermost cylindrical wire chamber and the magnetic coil were 48 scintillation counters. Again, light pulses generated by the passage of charged particles traversing these counters were detected by photo-multiplier tubes at each end and associated electronics. Time-of-arrival of the pulse was also recorded for each photomultiplier.

Magnet Coil

A solenoid coil was powered with DC current to produce a .4 T (check) magnetic field, to bend charged particles in the plane perpendicular to the beam. This made it possible to detect tracks in three dimensions, and measure charged particles, to determine if they originated from the interaction region of the beam pipe.

Lead-Scintillator Shower Detectors

Just outside of the magnet coil were 24 shower counter. Each counter was roughly 4 feet wide by 12 feet long. 10 plates of .25 inch lead were alternated with 10 plastic scintillators. Electrons or photons passing through this sandwich detector produced electromagnetic cascade showers. Light pulses from the scintillator plates were guided to a photomultiplier tube at each end, using plastic (lucite) light guides. These counters, plus one spare, were designed and constructed at LBL, and transported to SLAC.

Iron Flux Return

Eight 8 inch (25 cm) iron plates, plus two endcap steel pieces, completed the magnetic flux return path. The eight iron plates form an octagon.

Muon Spark Chambers

References

Article on SPEAR history
Particle experiments {{particle-stub