Purification and characterization studies of TMBi for a novel PET-detector concept

A recent proposal has introduced a new type of PET detector that utilizes TriMethyl Bismuth (TMBi) – a heavy organo-metallic liquid – as the target material [1]. TMBi is a transparent dielectric liquid that is 82% bismuth by weight. Its high effective Z number, resulting from bismuth being the central atom in TMBi, allows for efficient conversion of 511 keV photons into photo-electrons within the detector material. These photo-electrons produce both Cherenkov light and free charge carriers in the liquid, with the optical component providing fast timing and charges drifted towards a segmented anode enabling accurate position and energy reconstruction through charge measurement. However, the purity of TMBi must be carefully maintained to avoid signal degradation and noise within the detector, electro-negative impurities will capture free electrons from the liquid and decomposing impurities will increase the self conductivity. In addition, the reactive nature of TMBi presents several challenges that must be overcome before a fully functional PET detector using this material can be realized. This thesis will present a purification and measurement setup for the characterisation of organometallic liquids. It will show first purification studies performed with an electrostatic filter that was developed as part of this thesis. In addition to presenting the first direct measurement of the relative permittivity εr of TMBi ε_r = 2.59 ± 0.003 stat ± 0.026sys (0.1) estimates of other important characteristics of TMBI, based on the evaluation of literature values of other hydrocarbons will be given. One of these characteristics is the electron mobility μe. The estimated value of μ^TMBi_e ≈ 0.09 ± 0.08 cm2V−1s−1. (0.2) is much lower than previously anticipated, which could result in significant dead times of the proposed BOLD-PET detector, drastically decreasing its photosensitivity. This discrepancy towards other hydrocarbons like TetraMethyl Silane (TMSi) which have much larger electron mobilities (μ^TMSi_e = 100 cm2V−1s−1) might be caused by the molecular shape of TMBi, which is further discussed in this thesis. Based on these findings TMBi does not seem to be the best option for the use in a PET detector. A better alternative could be TetraMethyl Lead (TMPb) which probably has a similar free ion yield and electron mobility as TMSi and only a slightly lower density and effective z-number compared to TMBi.