Free ion yield of trimethyl bismuth used as sensitive medium for high-energy photon detection

Abstract : 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 primary photon. To do so, we developed a low-noise measuring system to determine the current induced by a $^60$Co 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.
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M. Farradèche, G. Tauzin, J.P. Mols, J.P. Dognon, V. Dauvois, et al.. Free ion yield of trimethyl bismuth used as sensitive medium for high-energy photon detection. Journal of Instrumentation, IOP Publishing, 2018, 13 (11), pp.P11004. ⟨10.1088/1748-0221/13/11/P11004⟩. ⟨cea-01918229⟩

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