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

M. Farradèche 1, * G. Tauzin 1 J.P. Mols 1 J.P. Dognon 2 V. Dauvois 3, 4 V. Sharyy 1 P. Bard 1 X. Mancardi 1 P. Verrecchia 1 D. Yvon 1 
* Corresponding author
2 LSDRM - Laboratoire Structure et Dynamique par Résonance Magnétique (LCF)
NIMBE UMR 3685 - Nanosciences et Innovation pour les Matériaux, la Biomédecine et l'Energie (ex SIS2M)
3 LRMO - Laboratoire de Radiolyse et de la Matière Organique
SECR - Service d'Etudes du Comportement des Radionucléides : DEN/DPC
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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Submitted on : Saturday, November 10, 2018 - 10:58:45 AM
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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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