Abstract
Silicon Photomultipliers (SiPM) are solid-state photodetectors made of a matrix of Avalanche Photodiodes working in Geiger mode (GM-APD). Each APD behaves as a binary device, giving in output a standardised signal if fired by one or more photons. However, the device as a whole is an analog device, giving in output a pulse proportional to the number of incident photons. For this reason, the SiPM can be considered as a solid-state equivalent of the traditional Photomultiplier Tube (PMT), with the advantage of single photon counting capability, improved detection efficiency at wavelengths that better suit scintillating crystals’ output and of compactness, ruggedness as well as easier production technology. Moreover, being a solid state device, the SiPM is also insensitive to magnetic fields and characterised by an intrinsic timing resolution which makes it a good candidate for fast timing applications. The motivation behind this thesis is the study of a system made of inorganic scintillating crystals readout by various types of SiPMs with specific interest in timing applications. The characterisation of various SiPMs with different total area and pixel sizes is discussed. The main result of this analysis is the measurement of the parameters of these photodetectors that allows the simulation of their output pulses, which ultimately determine the single photon response and hence the intrinsic performance limit of the device. In particular, it was found that coherent light improves the timing performances of SiPMs, due to an improvement in signal to noise ratio, while almost maintaining the single photon response. For this reason, the Intrinsic and Coincidence Time Resolution (ITR and CTR respectively), i. e. the variance on the time the detector takes to respond to a light stimulus and the uncertainty introduced in the measurement of the time coincidence, were measured using a femtosecond laser. For both quantities, values of the order of tens of ps were found optimising the parameters of a low pass Butterworth filter and of a time stamp pick-up algorithm which is the digital equivalent of a Constant Fraction Discriminator, for two Hamamatsu MPPCs with different total area and same pixel size. Finally the characterisation of various size LYSO crystals readout by SiPMs is also reported. This study gives information on the timing and energy properties of a Gamma sensitive system and allows the understanding of the characteristics that the optimum scintillating crystal readout by SiPMs should have to optimise the coincidence time and energy resolution. Values of the CTR for a system made of two LYSO crystals with size 3 x 3 x 10 mm3 readout by a pair of MPPCs with 25 um pixel size and 3x3 mm2 total area, were found of the order of 300 ps. Hence, this detector system appears as a good candidate for timing applications involving the detection of coincident gamma rays, particularly in applications like the Time-of-Flight Positron Emission Tomography, where an excellent timing resolution imply very good spatial resolution and hence superior quality of the final image.