  The CaLIPSO project is an innovative high-energy photon detector concept
using trimethylbismuth as sensitive medium in a liquid ionization chamber. The
detector, designed for high precision brain PET imaging, works as a
time-projection chamber and detects Cherenkov light and charge signal. We
measured the free ion yield of trimethylbismuth, which represents the number of
electron-ion pairs released by the incident photon. To do so, we developed a
low-noise measuring system to determine the current induced by a 60Co source in
the liquid with an accuracy better than 5 fA for an electric field up to 7
kV/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
measurements in 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 a high capacity of trimethylbismuth to
capture electrons which could explain the weak value. The consequences of a low
free ion yield in terms of high-energy photon detection and brain PET imaging
are finally discussed.
