Enabling Devices for Sensors to Detect Neutrinoless Double Beta Decay at nEXO

The nEXO collaboration is searching for neutrinoless double beta decay (NLDBD), the observation of which would confirm the Majorana nature of the neutrino and lepton number violation. Double beta decay is a process whereby a nucleus decays into another one and emits two electrons and two antineutrinos. A special version of double beta decayŚthe ôneutrinolessö versionŚemits only two electrons and no antineutrinos. NLDBD is at the boundaries of human knowledge since it has not yet been observed and cannot occur according to the laws of physics as we know them. The nEXO is developing a low-background liquid xenon (LXe) time projection chamber (TPC) to search for NLDBD in xenon-136. Our TPC design uses application specific integrated circuits (ASICs) in the detector to measure the charge and light energy resulting from ionizing radiation interacting with the LXe, allowing a reconstruction of the decay process. The ASICs must operate near the sensitive volume of the TPC and are thus highly sensitive to radioactive contamination. These ASICs require micro-farad buffering capacitors to operate in an ultra-clean LXe (165K) environment and are exceptionally small for the required capacitance. Currently no capacitor on the market meets nEXOs requirements for size-specific capacitance and voltage, radiopurity, and LXe cryogenic compatibility. Mainstream Engineering Corporation has developed a novel electrochemical capacitor and capacitor electrode that is fabricated from ultra-high purity precursors and materials. Ultra-high purity encasement materials have been 3D printed and designer electrolytes have been demonstrated for operation in cryogenic, on-chip, environments through a pick-and-place electrode and separator manufacturing process. With the possible capacitance exceeding 300 mF/cm2 and a selection of large potential window cryogenic electrolytes, our technology is uniquely suited to become the enabling device for ASICs. In Phase I, Mainstream will produce and manufacture capacitors for SLAC to demonstrate their operation in the relevant environment. Beyond the applications in particle physics, such devices could find applications in the burgeoning world of quantum computing, where cryogenics are widely used and where it has been shown that radioactivity can potentially limit the coherence time of qubits storage.