Abstract
Scintillation crystals have a wide range of applications in detectors for high energy and medical physics. They are recquired to have not only good energy resolution, but also excellent time resolution. In medical applications, L(Y)SO crystals are commonly used for time of flight positron emission tomography (TOF-PET). This study aims at determining the experimental and theoretical limits of timing using L(Y)SO based scintillators coupled to silicon photomultipliers (SiPMs). Measurements are based on the time-over-threshold method in a coincidence setup utilizing the ultra-fast amplifier-discriminator NINO and a fast oscilloscope. Using a 2 × 2 × 3 mm3 LSO:Ce codoped 0.4% Ca crystal coupled to a commercially available SiPM (Hamamatsu S10931-050P MPPC), we achieve a coincidence time resolution (CTR) of 108±5ps FWHM measured at E=511keV. We determine the influence of the data acquisition system to 27±2ps FWHM and thus negligible as compared to the CTR. This shows that L(Y)SO scintillators coupled to SiPM photodetectors are capable of achieving very good time resolution close to the desired 100ps FWHM for TOF-PET systems. To fully understand the measured values, we developed a simulation tool in MATLAB that incorporates the timing properties of the photodetector, the scintillation properties of the crystal and the light transfer within the crystal simulated by SLITRANI. The simulations are compared with measured data in order to determine their predictive power. Finally we use this model to discuss the influence of several important parameters on the time resolution like scintillation rise- and fall time and light yield, as well as single photon time resolution (SPTR) and the detection efficiency of the SiPM. In addition we find the influence of photon travel time spread in the crystal not negligible on the CTR, even for the used 2 × 2 × 3 mm3 geometry.
Highlights
∼3cm around the point of emission, sufficient to remove coincidence events outside the organ of interest
In the second part we present a complete Monte Carlo simulation tool modeling: (1) the scintillation statistics, (2) photon propagation and loss in the crystal, (3) photodetector properties like single photon time resolution (SPTR) and detection efficiency, (4) signal pile-up considering the single cell signal of the Silicon photomultipliers (SiPMs), (5) dark count rate and optical crosstalk of the SiPM and (6) the effect of the electronics, i.e. bandwidth limitations and noise
The decrease of the plot as a function of SiPM overvoltage is almost perfectly matched by the corresponding increase in photon detection efficiency (PDE), as we modeled the SPTR to be constant with bias voltage
Summary
∼3cm around the point of emission, sufficient to remove coincidence events outside the organ of interest. In the second part we present a complete Monte Carlo simulation tool modeling: (1) the scintillation statistics, (2) photon propagation and loss in the crystal (simulated by SLITRANI), (3) photodetector properties like single photon time resolution (SPTR) and detection efficiency, (4) signal pile-up considering the single cell signal of the SiPM, (5) dark count rate and optical crosstalk of the SiPM and (6) the effect of the electronics, i.e. bandwidth limitations and noise. A comparison of the simulation with experimental data shows good agreement for the influence of the bias voltage and NINO threshold on the CTR We will use this simulation tool to discuss the nature of several important parameters to the time resolution, like scintillation rise time and fall time and light yield, as well as single photon time resolution (SPTR) and the photo detection efficiency of the SiPM
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