Crystal Detectors in Particle Physics

Abstract

We review the principal characteristics driving the design of precision calorimeters composed of inorganic crystal scintillators now in operation (L3, CLEO II) or developed for the next generation of particle physics experiments. The unique discovery potential of these detectors (1.5 to 50 m^3 of crystals; 10^4 to > 10^5 elements) is the result of their high electron and photon energy resolution over a wide energy range, uniform hermetic acceptance and fine granularity. Experiments at CERN's multi-TeV Large Hadron Collider (LHC) will search for the Higgs particles thought to be responsible for mass, and for many other new physics processes. In order to exploit the intrinsically high resolution of crystal detectors, exceptionally high speed (1 to 30 ns decay time) and radiation resistance are required. BaF_2 and CeF_3 are currently the preferred choices, and higher density alternatives such as PbWO_4 are under investigation. Lower energy, high luminosity experiments that will measure rare particle decays, and explore the violation of the fundamental “CP” symmetry that may be related to the predominance of matter over antimatter in our universe, have chosen Cesium Iodide for its combination of high light output, speed, and radiation resistance. Recent developments by Caltech include the use of photons generated by an H− beam from an RFQ accelerator to calibrate and provide sub-percent resolution in the L3 BGO calorimeter, and an in situ optical bleaching technique that renders large BaF_2 crystals now mass produced in China radiation hard up to dose levels ≳l 10 MegaRads.