Towards Safe Operation of Liquid TriMethyl Bismuth for Use in a Drift Detector

TriMethyl Bismuth (TMBi) has been proposed as a detector medium with charge and Cherenkov photon readout for positron emission tomography. Bold-PET pursues the idea of a drift detector based on TMBi, a heavy metal-organic liquid, to read out both the charge and the light generated in the liquid. The high atomic number (Z
Bi
= 83) effectively converts TMBi 511 keV photons through the photoelectric effect (47% photo fraction). In addition, high optical density for visible wavelengths enables the emission of Cherenkov radiation generated by relativistic electrons in TMBi.In this work, we present studies on handling TMBi at different electric fields and under different environmental conditions to find suitable configurations to suppress electrical breakdown in TMBi at room temperature. A two-electrode glass cell filled with TMBi was constructed, and the breakdown behavior was tested under different operating conditions. At the vapor pressure of TMBi at room temperature of about 40 mbar and electric fields up to 20 kV/cm in the presence of low oxygen contamination, we found the formation of a discharge channel in the liquid and a steady increase in current. Further pressure reduction led to boiling of the TMBi and spontaneous ignition.We conclude from our experiments that TMBi remains stable against electric breakdown at room temperature in a pure argon atmosphere of 1 bar, at least up to electric field strengths of 20 kV/cm, presumably because the formation of gaseous TMBi was prevented.