The properties of avalanche photodiodes and associated electronics required for photon counting in the Geiger and the sub-Geiger modes are reviewed. When the Geiger mode is used, there are significant improvements reported in overall photon detection efficiencies (approaching 70% at 633 nm), and a timing jitter (under 200 ps) is achieved with passive quenching at high overvoltages (20-30 V). The results obtained by using an active-mode fast quench circuit capable of switching overvoltages as high as 15 V (giving photon detection efficiencies in the 50% range) with a dead time of less than 50 ns are reported. Larger diodes (up to 1 mm in diameter) that are usable in the Geiger mode and that have quantum efficiencies over 80% in the 500-800-nm range are also reported.
The measurement of depth of interaction (DOI) within detectors is necessary to improve resolution uniformity across the FOV of small diameter PET scanners. DO1 encoding by pulse shape discrimination (PSD) has definite advantages as it requires only one readout per pixel and it allows DO1 measurement of photoelectric and Compton events. The PSD time characteristics of various scintillators were studied with avalanche photodiodes (APD) and the identification capability was tested in multi-crystal assemblies with up to four scintillators. In the PSD time spectrum of an APD-GSO/LSO/ BGO/CsI(Tl) assembly, four distinct time peaks at 45, 26, 88 and 150 ns relative to a fast test pulse, having resolution of 10.6, 5.2, 20 and 27 ns, can be easily separated. Whereas the number and position of scintillators in the multi-crystal assemblies affect detector performance, the ability to identify crystals is not compromised. Compton events have a significant effect on PSD accuracy, suggesting that photopeak energy gating should be used for better crystal identification. However, more sophisticated PSD techniques using parametric time-energy histograms can also improve crystal identification in cases where PSD time or energy discrimination alone is inadequate. These results c o n f i i the feasibility of PSD DO1 encoding with APD-based detectors for PET.
This paper reports an experimental characterization of the EG&G SPCM-AQ single-photon avalanche diode module with an active quenching bias circuit that gives a dead time of ∼35 ns for use in high count rate, fast timing applications. A quantum efficiency of ≳70% and an optimal timing jitter with a full width at one-thousandth maximum of 1.5 ns is obtained if the light is tightly focused to a spot of <25 μm in the center of the active region, if the signal from the diode before the active quenching circuitry is used as input to the timing electronics, and if an external dead time of ∼55 ns is imposed. Under these conditions, the total probability of obtaining an afterpulse is found to be ∼2×10−3. Limitations of existing time-correlated single-photon counting instrumentation for count rates exceeding 106 s−1 are discussed.
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