Liquid ionization chamber detector of γ photons for PET imaging

CaLIPSO is a photon detector concept designed for dedicated brain Positron Emission Tomography. It is a medical imaging technique based on the coincidence detection of two 511-keV photons. For the first time, the liquid trimethyl-bismuth is used as sensitive medium. Each 511-keV photon releases a primary electron that triggers a Cherenkov radiation and ionizes the medium. CaLIPSO operates as a time projection chamber and detects both Cherenkov light and charge signal. As the total number of released charges is proportional to the energy deposited by the initial photon, we were able to measure the charge production yield (or free ion yield) of the trimethyl-bismuth. To this end, we developed a purification bench associated with a low-noise measurement system for the current induced by a γ-ray source of photons with a precision ≤ 5 fA for an electric field up to 7 kV/cm. The tetramethylsilane was used as a benchmark liquid to validate the measurement. We obtained a free ion yield of trimethyl-bismuth 6 times lower than the typical values for similar dielectric liquids. Quantum chemistry computations on heavy atoms shown that this behavior is due to the geometry of the trimethyl-bismuth molecule. The bismuth atom acts as an electron trapping center which induces an additional recombination mechanism of the electrons near their parent cations. Finally, in order to verify this hypothesis and to quantify the mobility of charges in liquids, we developed an individual charge pulses measurement system which has been successfully validated with tetramethylsilane. (author)