Suppression of electrical breakdown phenomena in liquid TriMethyl Bismuth based ionization detectors

Abstract

Organometallic liquids provide good properties for ionization detectors. TriMethyl Bismuth (TMBi) has been proposed as a detector medium with charge and Cherenkov photon readout for Positron Emission Tomography. In this work, we present studies for the handling of TMBi at different electric fields and under different environmental conditions to find applicable configurations for the suppression of electrical breakdowns in TMBi at room temperature. A simple glass cell with two electrodes filled with TMBi was constructed and tested under different operation conditions. Working at the vapour pressure of TMBi at room temperature of about 40 mbar and electric fields of up to 20 kV/cm in presence of a small oxygen contamination we found the formation of a discharge channel in the liquid and a steady increase in the current. Further reduction of pressure by pumping caused the TMBi to boil and a spontaneous combustion. Eliminating the oxygen contamination led the TMBi under the same condition to only decompose. When operating the setup under an argon atmosphere of 1 bar we did not observe breakdowns of the electrical potential up to field strengths of 20 kV/cm. Still, in presence of a small oxygen contamination fluctuating currents in the nA range were observed, but no decomposition or combustion. We conclude from our experiments that TMBi at room temperature in a pure argon atmosphere of 1 bar remains stable against electrical breakdown at least up to electric field strengths of 20 kV/cm, presumably because the formation of gaseous TMBi was prevented.

Electro-purification studies and first measurement of relative permittivity of TMBi

“…TMBi, which contains the heavy element bismuth (Z=83), has been proposed as a detector medium with Cherenkov light and ionisation readout for the measurement of 511 keV annihilation quanta in a novel detector for position emission tomography. One difficulty in this detector design is the safe handling of TMBi [8], another challenge is the purification of TMBi from electro-negative impurities so that the resulting free electrons in TMBi can be drifted to the anode without losses.…”

“…The reactive nature of TMBi, which spontaneously decomposes at temperatures above 105 • C, poses strict limitations on the materials used in the test setup and the environmental conditions applied [8].…”

“…To reduce this risk, highly purified argon gas is used to pressurize the liquid at ≈ 1 bar after initial filling. This precaution has been shown to reliably suppress explosive behavior in separate test measurements described in [8].…”

“…Unfortunately, our test was doubly restricted. Firstly, we could only remove a small part of the liquid because we were required to work with a limited amount of TMBi in the apparatus due to safety concerns (see also [8]). A second drawback was that the geometry of the extraction port was designed primarily for the removal of impurities created by pick-up of electrons injected at the photocathode, while we actually started with a homogeneous distribution of charged impurities.…”

Characterization of the BOLDPET optical prototype, an innovative Cherenkov detector for 511 keV γ radiation

“…Detection of the ionization signal is the most challenging part of this project and requires an excellent purity of the liquid together with the special precautions to avoid the electrical break-down phenomena. At the initial stage of the project, the detection of the ionization signal is studied using simplified test devices, and detailed discussions of the results can be found elsewhere [27,[30][31][32]. In this article, we exclusively focus on the investigation of the optical Cherenkov signal detection, utilizing a dedicated optical prototype.…”