Speaker
Description
The future Circular Electron-Positron Collider (CEPC) is a large-scale experimental facility, which aims to accurately measure the Higgs boson, electroweak physics and the top quark. For the detector system in CEPC, a highly granular crystal electromagnetic calorimeter is proposed to achieve an EM energy resolution of less than 3%. It is a homogenous structure with long crystal scintillator bar as active material, and SiPM as the preferred photon sensor. There is a high requirement on the dynamic range of SiPM, since more than half million photoelectrons can be measured for one channel. However, the calibration for SiPMs with such a large dynamic range is challenging. We have explored a series of methods to measure the nonlinear behavior of SiPMs with extremely high pixel densities—25 μm, 10 μm and 6 μm pixel size—under different conditions.
Firstly, using a laser as the light source and a PMT as an auxiliary calibration device, we measured the SiPM response when the pixels are not repeatedly fired. The results from different SiPMs show that under these conditions, the maximum number of photoelectrons measurable by the SiPM approaches its intrinsic pixel number, and nonlinearity becomes apparent when the signal exceeds 10% of the intrinsic pixel number.
Furthermore, we designed a beam experiment to investigate the nonlinearity of SiPMs when measuring intense scintillation light signals. In this case, the decay time of the scintillator is longer than the pixel recovery time of the SiPM, allowing SiPM pixels to be fired multiple times within a single event. We used tungsten plates as a pre-shower and increased the incident angle of the beam particles to enhance the energy absorbed by the crystal unit. Taking advantage of the dual-ended readout of the crystal, we added an optical filter to one end of the crystal to calibrate the actual absorbed energy. Finally, we observed energy absorption in the crystal exceeding 80 GeV as well as the nonlinear response of the SiPM.
