Free ion yield of Trimethyl Bismuth used as sensitive medium for high-energy photon detection

• Published in: arXiv.org
• Main Authors: Farradèche, M, Tauzin, G, J-Ph Mols, J-P Dognon, Dauvois, V, Sharyy, V, J-P Bard, Mancardi, X, Verrecchia, P, Yvon, D
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 07.11.2018
• Subjects: Bismuth; Brain; Electric fields; Ion pairs; Ionization chambers; Noise measurement; Organic chemistry; Physics - Instrumentation and Detectors; Positron emission; Quantum chemistry; Sensors; Tomography
• DOI: 10.48550/arxiv.1809.08115

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.