  CaLIPSO is an innovative photon detector concept designed for high precision
brain PET imaging. For the first time, liquid trimethylbismuth is used as
sensitive medium. The detector operates as a time-projection chamber and
detects both Cherenkov light and charge signal. Indeed, each 511-keV photon
releases a single primary electron that triggers a Cherenkov radiation and
ionizes the medium. As trimethylbismuth has never been studied before, we
measured its free ion yield defined as the number of electron-ion pairs
released by the primary electron. To this end, we developed a low-noise
measuring system to determine the weak current induced by a 60Co source in the
liquid with an accuracy better than 5 fA for an electric field up to 7kV/cm. We
used tetramethylsilane as benchmark liquid to validate the apparatus and we
measured a zero-field free ion yield of 0.53 +/- 0.03 in agreement with
literature. However, we found a zero-field free ion yield of 0.083 +/- 0.003
for trimethylbismuth, which is a factor 7 lower than the typical values for
similar dielectric liquids. Quantum chemistry computations on heavy atoms tend
to demonstrate the high ability of trimethylbismuth to capture electrons, which
could explain this weak value. This recombination mechanism marks a new step in
understanding charge transport in liquid detectors. Finally, to verify the
detectability of individual charge pulses, we developed a charge pulse
measurement system which has been successfully validated with TMSi.
Measurements with TMBi are ongoing.
