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Single Optical Photon Detector with an Integrated Avalanche Amplifier



  Single Optical Photon Detector with an Integrated Avalanche Amplifier  
  The MAGIC Telescope on the Canary Island La Palma observes highly energetic cosmic gamma rays. Such photons interact in the upper atmosphere and generate a shower of secondary particles extending over kilometers. Cherenkov radiation and fluorescence light generated by such showers is detected by MAGIC (Major Atmospheric Gamma-Ray Imaging Cherenkov) and the energy and direction of the primary gamma can be reconstructed.   Schematic vertical section of an avalanche drift diode
  These kind of telescopes need photon sensors capable to detect single photons with good time resolution (about one nanosecond) and high efficiency. The detectors used so far, photo multiplier tubes, have limited quantum efficiency and should be replaced in future by more advanced devices, like silicon photomultipliers.  
  Our avalanche detector concept promises very high quantum efficiency and fast time response. Integrating an avalanche structure into the center of a drift diode a large area device is obtainable focusing the photoelectron produced at the shallow radiation entrance window onto a small “point-like” avalanche region placed on the opposite side. Such a device can be used as a building block for a macroscopic detector.  
  Avalanche drift diode concept The avalanche drift diode consists – as well as existing silicon photo mulitpliers (SiPM) – of many small area avalanche diodes working in the limited Geiger mode. Each of these micro cells provides a standard pulse when an avalanche is initiated by one (or several) electron(s). Composing several of these microcells to a macro-cell by adding their signals, provides a measure for the number of photons detected within the macro-cell.  
  The quantum efficiency of front illuminated SiPMs is limited by the insensitive regions between the microcells and by the presence of optically absorbing material needed for connections and circuitry on top of the radiation entrance side. In the new concept radiation enters from the back of a fully depleted wafer and the photoelectrons are focused onto a small “point-like” avalanche region located on the front side (figure on the right).
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Photo of the MAGIC telescope mirror array on La Palma, Canary Islands