Abstract
The emergence of new solid-state avalanche photodetectors, e.g. SiPMs, with unprecedented timing capabilities opens new ways to profit from ultrafast and prompt photon emission in scintillators. In time of flight positron emission tomography (TOF-PET) and high energy timing detectors based on scintillators the ultimate coincidence time resolution (CTR) achievable is proportional to the square root of the scintillation rise time, decay time and the reciprocal light yield, CTR∝τrτd∕LY. Hence, the precise study of light emission in the very first tens of picoseconds is indispensable to understand time resolution limitations imposed by the scintillator. We developed a time correlated single photon counting setup having a Gaussian impulse response function (IRF) of 63ps sigma, allowing to precisely measure the scintillation rise time of various materials with 511keV excitation. In L(Y)SO:Ce we found two rise time components, the first below the resolution of our setup <10 ps and a second component being ∼380 ps. Co-doping with Ca2+ completely suppresses the slow rise component leading to a very fast initial scintillation emission with a rise time of <10ps. A very similar behavior is observed in LGSO:Ce crystals. The results are further confirmed by complementary measurements using a streak-camera system with pulsed X-ray excitation and additional 511 keV excited measurements of Mg2+ co-doped LuAG:Ce, YAG:Ce and GAGG:Ce samples.
Highlights
There is an increasing demand for fast timing in high energy physics and molecular positron emission tomography (PET) imaging
If only scintillation statistics is considered, the coincidence time resolution (CTR) of a time of flight positron emission tomography (TOF-PET) detector would be directly proportional to the square root of the scintillation rise time, as shown in Eq (1)
Co-doping LSO:Ce with Ca2+ suppresses this slow component completely leading to an overal rise time below 10 ps, which was as well observed for LGSO:Ce crystals
Summary
There is an increasing demand for fast timing in high energy physics and molecular positron emission tomography (PET) imaging. Future efforts will further go towards highest time resolution to reach 10 ps in TOF-PET, corresponding to 1.5 mm resolution along the line of response (LOR) This additional timing information will solve the inverse problem which allows for new PET geometries as a full 360◦ coverage is not a necessity anymore. Another advantage of 10 ps in PET will be a more than hundred-fold sensitivity gain which most likely will change the view on PET in the clinical routine, being able to allow for lowest dose PET/MR images and/or much faster scanning times. The factor 2.18 is a consequence of determining the time variance of the first scintillation photon emitted and includes the transformation into FWHM of the standard deviation [1]
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