Electromagnetic calorimeters for the BNL muon (g−2) experiment
Twenty-four detectors that caught the muons’ decay positrons. Without them, there is no g−2 measurement.
P. T. Debevec, D. W. Hertzog, C. J. G. Onderwater, R. M. Carey, M. Hare, J. P. Miller, J. Ouyang, O. Rind, P. Cushman, S. Giron, J. Kindem, D. Zimmerman, V. Druzhinin. Nuclear Instruments and Methods in Physics Research A 455, 346–360 (2000).
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You can’t watch a muon’s spin directly. But when a muon decays, the positron it emits tends to fly out along the spin. Count the high-energy positrons over time and the count rises and falls as the spin turns. That rhythm is the g−2 signal.
These are the instruments that did the counting: 24 calorimeters made of lead and scintillating fibers, spaced around the inside of the storage ring. They had to measure each positron’s energy well, time it precisely, and stay stable for hundreds of microseconds after every beam injection.
My part One of thirteen authors. This is the detector I helped build in graduate school.
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What they had to do
Measure positron energy with good resolution up to about 3 GeV, with precise timing, uniform response, and gain and timing that stayed steady over short periods, while ignoring the neutron background from the beam.
The design
Lead interleaved with scintillating fibers: the lead makes the positron shower, the fibers turn the shower into light, and photomultipliers read it out. Those demands led to several unusual design features, which the paper describes.
Testing and calibration
Before installation the calorimeters were tested and calibrated with electron beams from 0.5 to 4.0 GeV, then mounted inside the muon storage ring.
How they were used
These calorimeters recorded the positrons behind every g−2 result from E821, including the 2002 measurement and the 2006 final report.
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