Development of a “scintronic" crystal for very fast timing gamma ray imaging applications – ClearMind

We want to make the ClearMind prototypes in two phases.
Phase 1: It consists in a proof of principle on a “thin” detector, ~10 mm thick, instrumented on one face. We will measure the performances of the prototypes and compare them to the predictions of a Monte Carlo simulation using the GATE simulation platform. This will allows us to develop and test all the techniques quickly and to understand issues related to them.
Phase 2: It consists in the realization of a ~20 mm thick detector, instrumented on both faces. This detector should reach the efficiency, spatial and temporal resolutions required for building the next generation of PET cameras.
We will study the possibility of instrumenting both sides of the crystal in “scintronic” technology. If this proves to be technologically difficult or too expensive, we will instrument the second face using a SiPM matrix or an MCP-PMT mounted by means of an optical joint. SiPM has the advantage of a smaller amount of matter in the path of the gamma photon and a relative simplicity of implementation. Its disadvantages are high dark count rates and a possible emission of light resulting from in the SiPM avalanches. Our study will decide the impact of these phenomena.
At the end of this second phase, the Monte Carlo simulation will be used to a estimate the potential of this technology to realize a TOF full body PET imager. Deadline 36 months.

- We have developed a technique of photocathode deposition on PbWO4 crystal, (subject to NDA). It has allowed the integration of a first prototype of ClearMind Phase 1 detector, with the Photek company.
- We have developed the analog and digital electronics necessary to read the MCP-PMT. These electronics give complete satisfaction.
- We have developed a test bench for fast photodetectors, which allows us to measure the performance of the MCP-PMTs associated with their reading electronics. Published.
- We have developed a bench for measuring the quantum efficiency of photocathodes. Functional.
- We have developed a bench to study the physical properties of the PbWO4 scintillating crystal. Published.
- We have developed a detailed physical simulation of the whole reading chain of a ClearMind Phase 1 prototype, under Geant4 and GATE software, followed by codes in C++ (publication is in preparation). This led us to develop new simulation tools under Geant4, (conference papers and report).
- We have installed the mechanical bench for tomographic experiments in the nuclear imaging room of the CPPM.
- First ClearMind Phase 1 prototype delivered at the end of October 2021. It was tested in the laboratory the following month. This validates the technological mastery of the entire detection chain, even if the performance of this first detector must be greatly improved.

Continue to make progress on our work program.

- D. Yvon , et al., «Design study of a scintronic crystal targeting tens of picoseconds time resolution for gamma ray imaging: the ClearMind detector,« 2020, JINST 15 P07029
- M. Follin, et al, «High resolution MCP-PMT Readout Using Transmission Lines,«NIMA,1027 (2022)166092, doi.org/10.1016/j.nima.2021.166092
- M. Follin, et al., «Scintillating properties of today available lead tungstate crystals«, JINST 16 (2021) P08040, doi.org/10.1088/1748-0221/16/08/P08040
- L. Cappellugola, et al. «Modelisation of light transmission through surfaces with thin film optical coating in Geant4«, IEEE-NSS/MIC Conf. Report, 2021.

The aim of the ClearMind project is to develop a monolithic gamma ray detector (0.5 MeV to few MeV) with a large surface area (= 25 cm2), high efficiency, high spatial accuracy (< 4 mm3 FWHM ) and high timing accuracy ( < 20 ps FWHM, excluding contributions of the collection and amplification of photoelectrons).

Our motivation is to improve the performance of Positron Emission Tomography scanners (PET). We propose to develop a position-sensitive detector consisting of a scintillating crystal on which is directly deposited a photo-electric layer of refractive index greater than that of the crystal. This "scintronic" crystal, which combines scintillation and photoelectron generation, optimizes the transmission of scintillation photons and Cherenkov light photons to the photoelectric layer. We expect a factor 4 gain on the probability of optical photon transmission between the crystal and the photoelectric layer, compared to conventional assemblies using optical contact gels. The crystal will be encapsulated with a micro-channel plate multiplier tube (MCP-MT) in order to amplify the signal and optimize the transit time of the photo-electrons towards the plane of detection anodes (densely pixelated) and thus the temporal and spatial resolutions of the detection chain.

The originality of our detector consists in:
- Improve the efficiency of light collection in a high-density, and high-effective atomic number crystal by depositing a photoelectric layer directly on the scintillating crystal.
- Use the Cherenkov light emission for detection. The gain in optical coupling optimizes the measurement of time based Cherenkov photons, inherently very fast.
- Use the map of photoelectrons produced at the surface of the crystal to reconstruct the properties of the gamma interactions by means of robust statistical estimators and information processing using machine learning algorithms. The scintillation photons provide the necessary statistics for a measurement of the energy deposited in the crystal, modest but compatible with a use on a PET imager, and a precise measurement of the coordinates of the interaction position of the gamma ray.
- The fast acquisition of signal shapes (SAMPIC technology), which facilitates the optimization of the detector.
- The effort to reduce the number of electronic channels (and associated constraints) while keeping optimal performance.

We propose to develop the ClearMind prototypes in two phases.
Phase 1 consists in producing a "thin" detector, ~ 10 mm, instrumented on one side. The objective is a proof of principle of the technology, the characterization of the performances of this prototype, and its confrontation with a Monte Carlo model, using the GATE simulation tool. This should allow us to set up all the technologies and to concretely understand their stakes. Deadline 18 months.
Phase 2 involves the production of a ~ 20 mm thick detector, instrumented on both sides. The objective will then be to produce a detector module of optimized efficiency, spatial and temporal resolutions, close to what would be used in future PET machines. Deadline 30 months.
The GATE Monte Carlo simulation will then allow us to assess the potential of the technology to design an enhanced cerebral Time-Of-Flight PET imager, (and alternatively whole body TOF-PET).
Our efforts with the manufacturers involved in the development of the prototypes resulted in quotations and delivery times compatible with the schedule and the budget presented in this project.