Speaker
Description
A high-resolution electromagnetic calorimeter typically consists of an array of inorganic scintillators in crystalline form (cells), read out by Photo-Multiplier Tubes (PMTs) or Avalanche Photo-Diodes (APDs). An energy resolution of $\simeq 2\%$ at 1 GeV is considered excellent performance.
When a particle hits the scintillator, it loses energy through Bremsstrahlung and $e^+e^-$ pair production, generating an electromagnetic cascade that spreads from the main cell (seed) to several adjacent scintillators. For a particle in the 1-10 GeV range, most of the energy is released in the seed (up to $\simeq 6$ GeV). To reconstruct the total energy of the particle, the deposited energy in the neighboring cells must be measured down to a few MeV. This is necessary to reduce the uncertainty to a value comparable with the statistical fluctuations of the electromagnetic cascade. PMTs and APDs are well-known technologies, with readout chains developed and optimized over many years. Nevertheless, they have some limitations: high cost and complexity, sensitivity to magnetic fields (PMTs), and the need for complex high-gain signal amplification (APDs). The commercial development of a new, cheaper, high-performance photo-sensor, the Silicon Photo-Multiplier (SiPM), opens the possibility of a new readout approach. The SiPM is a solid-state sensor consisting of a matrix of micrometer-size APDs (pixels). Its appealing features include high quantum efficiency, low bias voltage, and high gain, but some limitations arise in calorimetry applications: the saturation effect impacts energy resolution if more than 10-15\% of pixels are activated; the gain depends on external factors such as temperature and bias voltage; and the active area of a single photo-sensor is limited.
A matrix of SiPMs with asymmetric segmentation can address these issues: it consists of two sub-matrices optimized for low- and high-energy signals, with an overlap region allowing cross-calibration. One proposal is to develop a 9-photo-sensor matrix composed of 2 SiPMs with large (50-75 $\mu$m) pixels, optimized for signals in the range 5-200 MeV, and 7 SiPMs with small (10-25 $\mu$m) pixels, optimized for signals in the range 20 MeV-6 GeV. This design will provide a large dynamic range ($\sim 1000$), allowing continuous single-photoelectron measurements to monitor sensor gain, while minimizing saturation. The poster will show the idea, the simulation and the preliminary study performed at INFN Genova.
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