Precision Machining And Polishing Of Scintillating Crystals For Large Calorimeters And Hodoscopes
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INTRODUCTION New machining and polishing techniques have been developed for large barium fluoride scintillating crystals that provide crystalline surfaces without sub-surface damage or deformation
as verified by Atomic Force Microscopy (AFM) and Rutherford Back-scattering (RBS) analyses. Surface roughness of about 10-20 angstroms and sub-micron mechanical tolerances have been demonstrated on large crystal samples. Mass production techniques have also been developed for machining and polishing up to five 50 cm long crystals at one time. We present this technology along with surface studies of barium fluoride and cerium fluoride crystals polished with this technique. This technology is applicable for a number of new crystal detectors proposed at Colliders I including the Barium Fluoride Electromagnetic Calorimeter at SSC 2 , the Crystal Clear Collaboration's 3 cerium fluoride or lead tungstenate calorimeter at LHC, the cesium iodide Calorimeter for the BaBar Detector at SLAC 4 and the PHENIX scintillating hodoscope at RHIC 5. Lawrence Livermore National Laboratory (LLNL) has an active program of study on barium fluoride scintillating crystals for the Barium Fluoride Electromagnetic Calorimeter Collaboration, cerium fluoride and lead fluoride for the Crystal Clear Collaboration, and cesium iodide for the SLAC B Factory. This program has resulted in a number of significant improvements in the mechanical processing, polishing and coating of fluoride and iodide crystals. Techniques have been developed using diamond turning and diamond-loaded pitch lapping that can produce 15 angstrom RMS surface finishes over large areas. Also, special polishing fixtures have been designed based on mounting technology developed for the 1.1 m diameter optics used in LLNL's Nova Laser. These fixtures allow as many as five 25-50 cm long crystals to be pol545 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
ished and lapped at the same time with tolerances satisfying the stringent requirements of crystal calorimeters. We also discuss results on coating barium fluoride with UV reflective layers of magnesium fluoride and aluminum. BARIUM FLUORIDE SURFACE PREPARATION AND ANALYSIS Surface preparation is critical to the performance of barium fluoride and other fluoride and iodide crystals for a number of reasons. First, an improperly prepared (machined, ground, polished, lapped) crystal suffers from induced stresses and deformations in the first few hundred microns of the surface. These stresses can manifest themselves in the formation of cracks (crazing) over long times, or more quickly when subjected to extremes of heat, radiation, humidity, etc.. Surface stresses can be minimized using well-known polishing and lapping techniques that gently bring the surface to a final finish. These techniques have been developed at LLNL for barium fluoride and also applied to cerium fluoride and lead fluoride. Improper surface preparation can also introduce contaminants into the surface of the crystal. Under certain conditions these co
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