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Avalanche Photodiodes (APD) are now widely used for the detection of weak optical signals. They find applications in a large number of fields of science and technology, from physics to medicine and environmental sciences. The request for sensitive detectors, capable to respond to weak radiations emitted from scintillation materials, has produced over the last decades an increasing number of studies on avalanche photodiodes and their applications as photo-sensors in particle detection. The first APD prototypes were developed more than 40 years ago. The initial size of such devices was however very small (below 1 mm2) and their spectral response confined to the near-infrared region. As a result, although available since several years, they did not receive much attention, also because of their initial high cost and low gain. However, large progresses have been made since then, and it has been possible to design and produce, at a reasonable cost, devices which have now a much larger area (tens of squared millimetres), with a high spectral sensitivity in the blue and near ultra-violet wavelength region. For such reasons, avalanche photodiodes are now widely used as sensitive light detectors in the construction of particle detectors in high energy physics. One of such examples is the impulse received by the design and construction of large scale electromagnetic calorimeters for the high energy experiments currently running in the world largest Laboratories. At present, APDs exhibit excellent quantum efficiency, with values around 80% in the near ultra-violet range, dropping to about 40% in the blue region, which is to be compared to typical values of 5-8 % in the blue for standard photomultipliers. Additional advantages which make them preferable over photomultipliers are discussed more specifically in the Chapter. The overall set of problems and solutions related to the use of Avalanche Photodiodes in the design, construction, test and operation of large electromagnetic calorimeters in nuclear and particle physics experiments, is described in this Chapter, as observed within a Collaboration at the CERN Large Hadron Collider. Section 2 briefly recalls the principles on which electromagnetic calorimetry for particle physics experiments is based. Relative merits of Avalanche Photodiodes in comparison to traditional devices, mostly photomultipliers, are discussed in Sect.3, in connection with the light collection from scintillation detectors and the readout and front-end electronics. A review of the large detectors which have employed in the recent past or are currently employing such devices as photo-sensors is given in Section 4. Sect.5 describes the overall set of procedures carried out to characterize a large

The Use of Avalanche Photodiodes in High Energy Electromagnetic Calorimetry