Mid-Term Report Summary - TICAL (TICAL: 4D total absorptionTime Imaging CALorimeter)
The ERC funded TICAL project for exploring the conditions to build a 4 dimensional Time Imaging CALorimeter with an unprecedented timing resolution of 10 picoseconds gives is typical example of the role of fundamental science to trigger the development of disruptive technologies to answer the formidable challenges of modern scientific experiments with a high societal impact, when applied to other domains (here medical imaging) through an efficient technology transfer policy.
Indeed the quest for 10 ps time resolution for scintillator-based detectors is of paramount importance for many applications and in particular for:
- particle physics detectors (calorimeters) for mitigating the pile-up of overlapping of complex events at high luminosity colliders and for improving the energy resolution of hadron calorimeters
- medical imaging PET scanners for a direct reconstruction at a millimetric resolution level of each event produced by the radioactive decay of the isotope labeling a specific biomarker.
In the first two years of the project important progress have been made on the following points, each essential to reach this very challenging goal:
- fundamental understanding of the mechanisms of energy deposition of ionizing radiation in scintillators at high energy (for particle physics) and at low energy (for PET scanners). Detailled Monte Carlo have been made to delineate the temporal sequence of the different energy deposition mechanisms, allowing us to identify the parameters of interest to focus the R&D effort for improving the time resolution of such detectors.
- Light generation in scintillators: it was established that the rate of production of scintillating photons in the leading edge of the scintillation pulse is the driving parameter for optimizing the scintillator for ultimate time resolution. This rate is directly correlated to the light yield, the decay time and very importantly to the rise time of the scintillator. A specific effort has therefore been spent to define the conditions for engineering scintillating materials for the fastest possible rise time. A first, coarse approach has allowed to indentify an interesting co-doping process with Cerium and Calcium in Luthetium Orthosicilicate crystals (LSO) allowing to reduce the rise time from 70ps to 20ps. A complementary approach is on the production of prompt photons in addition to the scintillation photons. A few prompt photons are generated by Cerenkov emission produced by the primary particle or by the secondary particles after the interaction in the scintillators but their number is too small to significantly contribute. Other mechanisms have therefore been looked at and two of them give promising preliminary results. The first one is based on the mechanism of Hot Intraband Luminescence (HIL) existing in specifically engineered scintillators with a non-uniform density of states in the conduction or in the valence band. The second one is based on the ultrafast (1ps or even less) and bright emission of nanoplatelets or nnocrystals deposited in thin layers at he surface of scintillator blocks.
- Light propagation in the crystal: heavy scintillators used for detecting ionizing radiation are generally characterized by a high index of refraction, which strongly limits the light extraction efficiency. The majority of photons are bouncing several times in the crystal before having a chance to be extracted if they are not absorbed in the meantime. This considerably increases their travel time jitter and reduces the light output efficiency. A new approach using a nano-structuration of the extraction face of the crystal by means of photonic crystals has been successfully tested and has shown an improvement of the light extraction efficiency by a factor up to 200% with a considerable reduction of the photon transit time jitter. This approach has been patented and a proof of concept for medical imaging has been launched in the frame of the ERC PoC ULTIMA project.
- Photodetection: A new concept of Silicon Photomultiplier (SiPM) with Silicon strips and differential readout has been proposed and patented in the frame of the TICAL project. It is presently developed in collaboration with the Japanese company Hamamatsu, world leader in the domain of photodetection. In parallel a fully digital SiPM is being designed in 3D electronics and should offer, if successful, the ultimate photodetection solution compatible with the challenging goal of 10ps resolution.
- Test beam results at high energy: a test has been made with minimum ionizing particles in a test beam facility at CERN with the identified LSO crystal co-doped with Cerium and Calcium, a new SiPM produced by industry and optimized readout electronics. A coincidence time resolution of 13ps between two crystals has been obtained, which is a world record and very close to our target of 10ps at high energy.
Indeed the quest for 10 ps time resolution for scintillator-based detectors is of paramount importance for many applications and in particular for:
- particle physics detectors (calorimeters) for mitigating the pile-up of overlapping of complex events at high luminosity colliders and for improving the energy resolution of hadron calorimeters
- medical imaging PET scanners for a direct reconstruction at a millimetric resolution level of each event produced by the radioactive decay of the isotope labeling a specific biomarker.
In the first two years of the project important progress have been made on the following points, each essential to reach this very challenging goal:
- fundamental understanding of the mechanisms of energy deposition of ionizing radiation in scintillators at high energy (for particle physics) and at low energy (for PET scanners). Detailled Monte Carlo have been made to delineate the temporal sequence of the different energy deposition mechanisms, allowing us to identify the parameters of interest to focus the R&D effort for improving the time resolution of such detectors.
- Light generation in scintillators: it was established that the rate of production of scintillating photons in the leading edge of the scintillation pulse is the driving parameter for optimizing the scintillator for ultimate time resolution. This rate is directly correlated to the light yield, the decay time and very importantly to the rise time of the scintillator. A specific effort has therefore been spent to define the conditions for engineering scintillating materials for the fastest possible rise time. A first, coarse approach has allowed to indentify an interesting co-doping process with Cerium and Calcium in Luthetium Orthosicilicate crystals (LSO) allowing to reduce the rise time from 70ps to 20ps. A complementary approach is on the production of prompt photons in addition to the scintillation photons. A few prompt photons are generated by Cerenkov emission produced by the primary particle or by the secondary particles after the interaction in the scintillators but their number is too small to significantly contribute. Other mechanisms have therefore been looked at and two of them give promising preliminary results. The first one is based on the mechanism of Hot Intraband Luminescence (HIL) existing in specifically engineered scintillators with a non-uniform density of states in the conduction or in the valence band. The second one is based on the ultrafast (1ps or even less) and bright emission of nanoplatelets or nnocrystals deposited in thin layers at he surface of scintillator blocks.
- Light propagation in the crystal: heavy scintillators used for detecting ionizing radiation are generally characterized by a high index of refraction, which strongly limits the light extraction efficiency. The majority of photons are bouncing several times in the crystal before having a chance to be extracted if they are not absorbed in the meantime. This considerably increases their travel time jitter and reduces the light output efficiency. A new approach using a nano-structuration of the extraction face of the crystal by means of photonic crystals has been successfully tested and has shown an improvement of the light extraction efficiency by a factor up to 200% with a considerable reduction of the photon transit time jitter. This approach has been patented and a proof of concept for medical imaging has been launched in the frame of the ERC PoC ULTIMA project.
- Photodetection: A new concept of Silicon Photomultiplier (SiPM) with Silicon strips and differential readout has been proposed and patented in the frame of the TICAL project. It is presently developed in collaboration with the Japanese company Hamamatsu, world leader in the domain of photodetection. In parallel a fully digital SiPM is being designed in 3D electronics and should offer, if successful, the ultimate photodetection solution compatible with the challenging goal of 10ps resolution.
- Test beam results at high energy: a test has been made with minimum ionizing particles in a test beam facility at CERN with the identified LSO crystal co-doped with Cerium and Calcium, a new SiPM produced by industry and optimized readout electronics. A coincidence time resolution of 13ps between two crystals has been obtained, which is a world record and very close to our target of 10ps at high energy.