Barium Fluoride Crystals for Future Hadron Colliders
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ABSTRACT This report summarizes the radiation resistance of the current production BaF 2 crystals. An approach to implement optical bleaching in situ is also presented. By using optical bleaching current production quality BaF 2 crystals could serve as an excellent candidate to construct a precision electromagnetic calorimeter at future hadron colliders.
INTRODUCTION Total absorption shower counters made of inorganic scintillating crystals have been known for decades for their superb energy resolution and detection efficiency. In high energy physics, many large arrays of such counters have been assembled for the precision detection of photons and electrons [1]. Crystal calorimeters containing 104 to more than 10i elements have been designed and studied intensively by a large sector of the high energy physics community planning experiments for multi-TeV hadron colliders, including the late Superconducting SuperCollider (SSC) in the U.S. [2, 3] and the Large Hadronic Collider (LHC) at CERN in Europe [4, 5]. The physics at future hadron colliders requires a high energy resolution of a crystal calorimeter [6]. In order to maintain the high resolution and the resultant sensitivity to new physics in situ, one key requirement to the scintillating material is the radiation hardness, i.e. the material must function well in a radiation environment up to few hundreds rad/hr. All known large crystal scintillators are affected by radiation damage [7]. The principal damage phenomenon, observed in all mass-produced crystals, is the appearance of absorption bands, caused by color center formation. The absorption bands reduce the transmission of scintillation light through the crystals to the photosensors, and hence the apparent light yield following irradiation. Additional effects observed in some crystals include reduced intrinsic yield of scintillation light, increased fluorescence (afterglow), and phosphorescence (spontaneous light emission over a long period). For crystals to be used in a high radiation environment, such as at multi-TeV hadron colliders, it is important that a crystal's scintillation mechanism not be damaged and that the radiation-induced phosphorescence does not affect the readout signal. By choosing crystals such as barium fluoride (BaF 2 ) [8, 9, 101 or Cerium Fluoride (CeF 3 ) [11, 12, 13], these criteria can be met However, the increased radiation-induced absorption (equivalently: a reduced light attenuation length), changes the light response uniformity, and thus may degrade the energy resolution. Figure 1 [10] shows the GEANT predictions for the energy fraction (top figure) and the intrinsic energy resolution (bottom figure) calculated by summing the energies deposited in a 3 x 3 subarray consisting of nine 50 cm (25 X0 ) long tapered (3 x 3 cm 2 to 5 x 5 cm 2 ) BaF 2 crystals, as a function of the light response uniformity. In this simulation, the light response (Y) of the crystal was parametrized as a normalized linear function:
Y = Y25 [1 + b(z/25 - 1)]
(1)
where Y25 represents the light response
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