Time-of-Flight Positron Emission Tomography (TOF-PET) scanners provide better signal-to-noise ratio (SNR)
and artifact reduction compared to conventional PET systems. The performance of TOF-PET scanners improves
with the timing precision of its detectors: the more accuracy in the time detection of gamma photons the better
the performance. The ultimate aim of TOF-PET is to reach a 10 ps full width at half maximum (FWHM)
coincidence time resolution (CTR) to resolve precisely the positron-electron annihilation point in 3 dimensions.
State-of-the-art PET detectors consist of scintillation crystals coupled to silicon photomultipliers (SiPM) and show
timing resolutions in the order of 100-200 ps FWHM.
In this project, we focus on improving dramatically the timing properties of the SiPMs, as such improvement
would have a strong impact on TOF-PET as it would improve the timing performance of most detectors that use
scintillation crystals and/or Cerenkov light emitters by several-fold. State-of-the-art SiPMs are optimized for a
narrow range of wavelengths (¿) because of the difference in penetration depth at different wavelengths. For
photons of ¿=450 nm and ¿=590 nm, the attenuation depth is 0.4 µm and 2 µm, respectively. The trade-off is to
either a) to have a thicker depletion layer to absorb a wider range of wavelengths but to increase the time jitter,
or b) to have a thinner depletion layer to reduce the time jitter but absorb only a narrow range of wavelengths.
Therefore, there is not a state-of-the-art SiPM that provides, simultaneously, very fast time response, and high
photon detection efficiency (PDE) across a wide range of wavelengths.
We propose to develop an SiPM prototype with photon-trapping microstructures integrated in the depletion layer
that disperses the light laterally and allows one to obtain high-detection efficiency for a wide range of wavelengths
within a depletion layer of 1 µm. With such a thin layer, the jitter in the electron drift time decreases to 10 ps and
the dark current is expected to decrease as well. This new photosensor could revolutionize TOF-PET.
The utilization of periodic microstructures to bend light in a perpendicular orientation and trapping photons for
enhanced interaction with materials, high detection efficiency and fast response have been recently shown for
wavelengths between 800-900 nm for optical communication. In this proposal, we will develop a new SiPM based
on this technology. First, we will simulate the optimum layer structure to integrate the hole-trapping
microstructures and an avalanche region to provide a gain of >105. Second, we will do an electronic
characterization for the different type of microcells, including a gain calibration and measure quantum efficiency
for different ¿ for each cell. Finally, we will manufacture a wafer with full-size SiPMs (3x3 mm2) and test the
SiPMs with scintillation crystals and Cerenkov emitters.