The 10ps TOFPET Challenge, Myth or Reality

Paul Lecoq (CERN)
Event Date and Time: 
Mon, 2018-05-07 14:00 - 15:00
TRIUMF Auditorium

The future generation of radiation detectors is more and more demanding on timing performance for a wide range of applications, such as time of flight (TOF) techniques for PET cameras and particle identification in nuclear physics and high energy physics detectors, precise event time tagging in high luminosity accelerators and a number of photonic applications based on single photon detection. A target of 10ps coincidence time resolution in TOFPET scanners would introduce a paradigm shift in PET imaging. Besides resulting in on-line image formation, the localisation of annihilation events directly from their TOF provides ultimate use of the dose delivered to the patient to get the best Signal to Noise Ratio into the resulting image and offers a potential reduction of the scan duration and a direct access to the image during the scan itself. Reconstructionless TOF-PET also reduces efficiently undesired effects inherent to the PET detection, namely randoms and scatters when appropriately correlated to energy discrimination, hence contributing to reduce dose, scan duration and possibly scan cost while using very short-lived positron emitting isotopes.

The time resolution of a scintillator-based detector is directly driven by the density of photoelectrons generated in the photodetector at the detection threshold. At the scintillator level it is related to the intrinsic light yield, the pulse shape (rise time and decay time) and the light transport from the gamma-ray conversion point to the photodetector. When aiming at 10ps time resolution fluctuations in the thermalization and relaxation time of hot electrons and holes generated by the interaction of ionization radiation with the crystal become important. This talk will review the different processes at work and evaluate if some of the transient phenomena taking place during the fast thermalization phase can be exploited to extract a time tag with a precision in the few ps range. Some considerations will also be given on the possibility to exploit quantum confinement for the production of ultrafast spontaneous or stimulated emission in semi-conductors.

The light transport in the crystal is also an important source of time jitter. In particular light bouncing within the scintillator must be reduced as much as possible as it spreads the arrival time of photons on the photodetector and strongly reduces the light output by increasing the effect of light absorption within the crystal. A possible solution to overcome these problems is to improve the light extraction efficiency at the first hit of the photons on the crystal/photodetector coupling face by means of photonic crystals (PhCs) specifically designed to couple light propagation modes inside and outside the crystal at the limit of the total reflection angle.
Finally the present limitations of the photodetectors, and in particular the SiPMs will be discussed and some R&D lines to meet the 10ps challenge will be presented.

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