Division of Electronics, Detectors and Computing for Physics (DEDIP)

Motivations for CaLIPSO

The CaLIPSO project (French acronym for Calorimètre Liquide Ionisation Position Scintillation Organométallique) is based on the recent developments of the TriMethyl Bismuth (TMBi). This innovative liquid allows a very efficient and accurate detection of positron annihilation. Once fully developed and integrated with an acquisition system, it may become a key element of PET imagers. In particular, the detector performances would allow efficient 1 mm³ resolution PET imaging of the whole human brain, thus being an excellent complement of magnetic resonance imagers for diagnosis and research on neurodegenerative diseases. The development of such detector is a long term effort pursued at IRFU since 2009, partially funded by the Neuropôle de Recherche Francilien (NeRF), LabEx P2IO, and CEA interdisciplinary program TechnoSanté. This innovation is patent protected. The CaLIPSO detector uses the TriMethyl Bismuth to efficiently convert photons of energy less than 1 MeV, through the photoelectric effect. The photoelectric electron is relativistic in the liquid TMBi. Thus it produces a quasi-instantaneous flash of few tens of Cherenkov photons, as well as free charges in the liquid. Light produced is detected by an efficient photodetector and charges released drift along a strong electric field, pass through a Frisch grid, and are collected by a pixelated charge detector.

Scientific and technological challenges

The CaLIPSO project is very ambitious. At the project start, little was known about liquid TMBi for particle detection. Key science and technological issues were listed as:

• TMBi physical properties: namely light production yield, light absorption length at optical and UV wavelength, charge mobility in TMBi. We measured these properties using tailored devices designed for handling our mildly-pyrophoric liquid.
• The low light production efficiency does require careful optimization of the optical couplings. The first optical demonstrator, along with its detailed Monte Carlo simulation, allowed us to quantify the issue. The second optical prototype allowed us to demonstrate a very good efficiency of detection of 511 keV photons, especially, when they are converted to the electron via the photo-electric effect.
• The electronic readout density: Integrating an electronic density approaching one preamplifier per mm², has already been achieved in the laboratory (ASIC and 3D electronics), in the context of a spaceborne experiment. We have chosen to limit the resolution of our demonstrator to 2 x 2 mm², in the charge collection plane, in order to avoid having to use 3D electronics and to focus in the short term our efforts on the detector development.
• The TMBi must be ultra-purified to allow the charge drift and the measurement of gamma energies. We organized the detector cleaning and mounting work at the clean rooms. The materials, from which the detector is made, shall be carefully chosen to be compatible with ultra-high vacuum technologies.

ClearMind Project: study of a “scintronic” crystal targeting tens of picoseconds time resolution for gamma ray imaging

In this project we propose and study the concept of a new gamma ray “scintronic” detector targeting a time resolution of the order of 25 ps FWHM, with millimetric volume reconstruction and high detection efficiency. Its design consists of a monolithic large PbWO₄ scintillating crystal with an efficient photocathode directly deposited on it. With an index of refraction higher for the photocathode than for the crystal, this design negates the total reflection effect of optical photons at the crystal/photo-detector optical interface, and thus largely improves optical coupling between the crystal and the photodetector. This allows to detect efficiently the Cherenkov light produced by 511 keV photoelectric conversions in PbWO₄ , and to optimize the detector time resolution. Furthermore, the low-yield, fast scintillation light produced additionally by PbWO₄ increases the detected photon statistics by a factor 10, thus fostering accurate (3 dimensional) localization of the gamma ray interaction within the crystal and providing a fair measurement of the deposited energy.

For more details see: 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, arXiv:2006.14855 [physics.ins-det]