Fluorescent bicolour sensor for low-background neutrinoless double β decay experiments

Observation of the neutrinoless double β decay is the only practical way to establish that neutrinos are their own antiparticles¹. Because of the small masses of neutrinos, the lifetime of neutrinoless double β 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 β decay². The most robust identification of neutrinoless double β decay requires the definition of a signature signal-such as the observation of the daughter atom in the decay-that cannot be generated by radioactive backgrounds, as well as excellent energy resolution. In particular, the neutrinoless double β decay of ¹³⁶Xe could be established by detecting the daughter atom, ¹³⁶Ba²⁺, in its doubly ionized state^3-8. Here we demonstrate an important step towards a 'barium-tagging' experiment, which identifies double β decay through the detection of a single Ba²⁺ ion. We propose a fluorescent bicolour indicator as the core of a sensor that can detect single Ba²⁺ ions in a high-pressure xenon gas detector. In a sensor made of a monolayer of such indicators, the Ba²⁺ dication would be captured by one of the molecules and generate a Ba²⁺-coordinated species with distinct photophysical properties. The presence of such a single Ba²⁺-coordinated indicator would be revealed by its response to repeated interrogation with a laser system, enabling the development of a sensor able to detect single Ba²⁺ ions in high-pressure xenon gas detectors for barium-tagging experiments.