Development of a barium detector for a neutrinoless double beta decay

The observation of the neutrinoless double beta decay is the only practical way to establish that neutrinos are their own antiparticles. But, because of the small masses of neutrinos, the lifetime of neutrinoless double beta decay is expected to be at least ten orders of magnitude greater than the typical lifetimes of natural radioactive chains, which can mimic the experimental signature of neutrinoless double beta decay. Double beta decay experiments have been searching for neutrinoless events in several isotopes for more than half a century, without finding clear evidence of a signal so far. The current best lower limit on the lifetime of the neutrinoless double beta decay processes has been obtained for an isotope of xenon, ¹³⁶Xe. In this case, the daughter atom is an isotope of barium, ¹³⁶Ba²⁺, the whole process being ¹³⁶Xe → ¹³⁶Ba²⁺+ 2e^– + 2 neutrinos. As the identification of neutrinoless double beta decay requires the definition of a signature signal that cannot be generated by radioactive backgrounds, it seems only logical that the implementation of a robust Ba²⁺ detection technique would lead to a positive identification of a neutrinoless double beta decay candidate.

But, what kind of chemical compound would work as a detector while immobilized on a surface in the ultra-dry environment of a xenon gas chamber? That is not an easy question to answer.

Although the barium tagging concept was proposed more than three decades ago, a practical way to detect Ba²⁺in situ in a high-pressure xenon gas time projection chamber (HPXe-TPC), such as those being developed by the NEXT experiment, was only conceived recently. In 2020, a new fluorescent bicolour indicator (FBI), an organic molecule, was developed that could help detect the daughter atom of a neutrinoless double beta decay. The idea relies on the capability of fluorescent molecules of changing their optical properties upon detecting target analytes. An initial proof of concept resolved individual barium dications in aqueous solution using Fluo-3, a well-known commercial indicator. But, as mentioned, a suitable barium detector in NEXT requires a functionalized surface that must include a monolayer of the molecular sensor and must efficiently operate in a noble gas atmosphere. To date, no experiments had been conducted in which the processes of chelation and detection occur fully under such conditions.

All the indicators developed by the NEXT experiment, including FBI, are based on crown ethers. Because of their capability to capture a variety of guest species, including metal cations, protonated species, and neutral and ionic molecules, crown ethers have been extensively used to recognize and trap metal or molecular ions. However, they have been poorly studied in solid state. Few examples can be found in the literature where self-assembled monolayers of crown ether derivatives have been grown and used on surfaces.

Now, the team of researchers that developed FBI combine ¹ two highly sensitive surface techniques: X-ray Photoemission Spectroscopy (XPS) and Scanning Tunnelling Microscopy and Spectroscopy (STM/STS), to prove how different ions interact with FBI molecules deposited on suitable substrates. In doing so, this work advances substantially the understanding of the physico-chemical properties of crown ethers immobilized on solid surfaces.

The team demonstrates that barium ions induce molecular conformation changes of FBI, modifying the electronic structure that would affect the fluorescence emission at surfaces of Au(111). Coordination with crown ether happens entirely in ultra-high vacuum (UHV), which ensures that chemical, structural and electronic changes happen in the absence of solvents, air molecules or spurious contaminants. This is, therefore, a crucial step toward the development of a Ba²⁺ detector.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.