# Ionization parameters of Trimethylbismuth for high-energy photon detection

## TL;DR

This study investigates the ionization properties of liquid trimethylbismuth for high-energy photon detection, revealing a significantly lower free ion yield than similar liquids, which impacts its charge transport characteristics in detector applications.

## Contribution

First measurement of free ion yield in liquid trimethylbismuth, providing new insights into its charge recombination behavior for photon detection.

## Key findings

- Zero-field free ion yield of 0.083 in TMBi, much lower than similar liquids.
- Validated measurement system with TMSi as benchmark.
- Quantum chemistry suggests high electron capture ability of TMBi.

## Abstract

CaLIPSO is an innovative photon detector concept designed for high precision brain PET imaging. For the first time, liquid trimethylbismuth is used as sensitive medium. The detector operates as a time-projection chamber and detects both Cherenkov light and charge signal. Indeed, each 511-keV photon releases a single primary electron that triggers a Cherenkov radiation and ionizes the medium. As trimethylbismuth has never been studied before, we measured its free ion yield defined as the number of electron-ion pairs released by the primary electron. To this end, we developed a low-noise measuring system to determine the weak current induced by a 60Co source in the liquid with an accuracy better than 5 fA for an electric field up to 7kV/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 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 the high ability of trimethylbismuth to capture electrons, which could explain this weak value. This recombination mechanism marks a new step in understanding charge transport in liquid detectors. Finally, to verify the detectability of individual charge pulses, we developed a charge pulse measurement system which has been successfully validated with TMSi. Measurements with TMBi are ongoing.