Multiwire proportional chamber from J-particle experiment of S. Ting at Brookhaven
date: label: Date made
content: 1972-1973
name: label: designer
content: Becker, Ulrich
notes: label: Description
content: This object consists of a rectangular frame (steel, copper) holding signal wires (gold plated wires) separated by planes of high voltage wires (Cu-Be wire). Three planes of signal wires oriented at 60 degree increments; at +80, +20 and at -40. In operation, the entire chamber was filled with gas: 80% argon to provide an ionization medium for creating a detectable electrical signal; and 20% methylal, both as a spark extinguishing and as a cleaning agent (to prevent ageing of the wires due to carbon deposits). This chamber is one of four from left arm of the spectrometer setup at the Brookhaven Alternating Gradient Synchrotron (AGS), which measured electrons and positrons resulting from decay of a hypothesized massive "J" particle.
label: Description
content: Basic Principles and History
label: Description
content: A multiwire proportional chamber (MWPC) is constructed with alternating planes of high voltage wires (cathode) and sense wires (anode), which are at ground. All the wires are placed in a special gas environment. Spacing between planes is usually on the order of millimeters and voltage differences are typically in the kilovolt range. When a charged particle passes through the gas in the chamber, it will ionize gas molecules. The freed electrons are accelerated towards the sense wire (anode) by the electric field, ionizing more of the gas. In this way a cascade of charge develops and is deposited on the sense wires. The smaller the diameter of the sense wires, the higher the field gradient near the wire becomes. This in turn causes a larger cascade, increasing the efficiency of the chamber.
label: Description
content: Georges Charpak built the first MWPC in 1968. Unlike earlier particle detectors, such as the bubble chamber and the first generation of spark chambers, which can record the tracks left by particles at the rate of only one or two per second, the multiwire chamber records up to one million tracks per second and sends the data directly to a computer for analysis. In 1992 Charpak received the Nobel Prize for Physics in acknowledgment of his invention of the MWPC, an electronic particle detector that revolutionized high-energy physics experiments and has had applications in medical physics.
label: Description
content: The MWPC in the J-particle experiment of S.C.C. Ting at Brookhaven
label: Description
content: The 1976 Nobel Prize in physics was shared by a Massachusetts Institute of Technology physicist who used Brookhaven's Alternating Gradient Synchrotron (AGS) to discover a new particle and confirm the existence of the charmed quark. Samuel C.C. Ting was credited for finding what he called the "J" particle, the same particle as the "psi" found at nearly the same time at the Stanford Linear Accelerator Center by a group led by Burton Richter. The particle is now known as the J/psi.
label: Description
content: Ting's experiment took advantage of the AGS's high-intensity, 30 GeV proton beams, which bombarded a stationary beryllium target to produce showers of particles. The decay modes of these particles were identified using a two-arm spectrometer detection system. J particles decay into various combinations of lighter particles; one of these combinations is an electron and a positron. A small fraction of these enter the detection system, one particle in each arm of the spectrometer. Then dipole magnets deflect them out of the plane of the intense beam and measure their momentum; Cerenkov counters measure their velocity; multi-wire proportional chambers their position; scintillator hodoscopes their moment of passage; lead-glass and lead-lucite shower counters their total energy.
label: Description
content: In each spectrometer arm there are 4 MWPCs (Ao, A, B, C) with 2 mm wire spacing and a total of 4,000 wires on each arm. There are eleven planes of proportional wires (2 in Ao, 3 each in A, B, & C), and in A, B, & C the planes are rotated 20 degrees with respect to each other to reduce multitrack ambiguities. To ensure the chambers have 100% uniform efficiency at low voltage and a long live time in the highly radioactive environment, a special argon-methylal gas mixture at 2 deg. C was used.
label: Description
content: The identification of the J-particle and its significance
label: Description
content: A strong peak in electron and positron production at an energy of 3.1 billion electron volts (GeV) led Ting to suspect the presence of a new particle, the same one found by Richter. Their discoveries not only won the Nobel Prize; they also helped confirm the existence of the charmed quark -- the J/psi is composed of a charmed quark bound to its antiquark.
label: Description
content: The J/ψ (or J/psi) is a very special particle. Its discovery was announced in 1974 independently by two groups: one lead by Samuel Ting at Brookhaven National Laboratory (BNL) in New York and the second lead by Burton Richter at Stanford Linear Accelerator Center (SLAC) in California. J/ψ is special because it established the quark model as a credible description of nature. Having been invented by Gell-Man and Zweig as a bookkeeping tool, it was not until Glashow, Iliopoulos and Maiani (GIM) that the concept of quarks as real particles was taken seriously. GIM predicted that if quarks were real, then they should come in pairs, like the up and down quarks. Candidates for the up, down, and strange were identified, but there was no partner for the strange quark. J/ψ was the key.
label: Description
content: Like the proton or an atom, the J/ψ is a composite particle. This means that J/ψ is made of smaller, more elementary particles. Specifically, it is a bound state of one charm quark and one anti-charm quark. Since it is made of quarks, it is a “hadron“. But since it is made of exactly one quark and one antiquark, it is specifically a “meson.”
label: Description
content: For further details, see
label: Description
content: http://hitoshi.berkeley.edu/129A/Cahn-Goldhaber/chapter9.pdf
label: Description
content: http://www.nobelprize.org/nobel_prizes/physics/laureates/1976/ting-lecture.pdf
label: Location
content: Currently not on view
place: label: place made
content: United States: Massachusetts, Cambridge, Massachusetts Institute of Technology
topic: label: general subject association
content: Science & Scientific Instruments
setName: label: See more items in
content: Medicine and Science: Modern Physics
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content: Energy & Power
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content: Science & Mathematics
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content: Modern Physics
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content: Measuring & Mapping
creditLine: label: Credit Line
content: M.I.T. Laboratory for Nuclear Science, Cambridge, MA
dataSource: label: Data Source
content: National Museum of American History
identifier: label: ID Number
content: 1989.0050.01.1
label: accession number
content: 1989.0050
label: catalog number
content: 1989.0050.01.1
objectType: label: Object Name
content: chamber, multiwire proportional
label: Object Name
content: Wire Chamber, Proportional
physicalDescription: label: Physical Description
content: steel; copper, gold-plated wire, copper-berylium wire (overall material)
label: Measurements
content: overall: 54 in x 54 in x 9 in; 137.16 cm x 137.16 cm x 22.86 cm