[Paper Review] Barium Tagging with Selective, Dry-Functional, Single Molecule Sensitive On-Off Fluorophores for the NEXT Experiment

N. K. Byrnes, A. A. Denisenko+6 more|arXiv (Cornell University)|Sep 10, 2019

Neutrino Physics Research17 references4 citations

TL;DR

This paper presents a novel barium tagging technique for the NEXT experiment to detect neutrinoless double beta decay in 136Xe, using dry-functional, on-off switchable fluorophores that enable single-molecule fluorescence imaging of Ba²⁺ ions. The method combines a custom Ba²⁺-selective dye, RF carpet ion transport, and a high-pressure-compatible total internal reflection fluorescence microscope, achieving single-ion sensitivity in xenon gas at up to 10 bar.

ABSTRACT

In the search for neutrinoless double beta decay, understanding and reducing backgrounds is crucial for success. An advance that could drive backgrounds to negligible levels would be the ability to efficiently detect the barium daughter in $^{136}$Xe to $^{136}$Ba double beta decay, since no conventional radioactive process can produce barium ions or atoms in xenon at significant rates. In xenon gas, the barium daughter most likely survives as a dication. An approach under development by the NEXT collaboration involves transporting this ion from the active medium onto a coated transparent plane supporting a barium-sensitive fluorescent dye, monitored via fluorescence microscopy. Upon exposure to a barium dication, the dye will begin fluorescing, which, when correlated with the detection of a double electron signal at the anode, would confirm double beta decay.Our results have shown that a single barium ion can be resolved via Single Molecule Fluorescent Imaging (SMFI). The next challenge is a realization of this technique within in a large volume of xenon gas. Significant advances have recently been made: custom barium-tagging molecules that fluoresce strongly in the dry state when exposed to barium have been demonstrated, and devices constructed that can observe fluorescence via in-vacuum or in-gas Total Internal Reflection Fluorescence Microscopy. We present the status of this technique and the outlook for barium tagging with On-Off switchable fluorophores, including new results with a Ba$^{2+}$-selective dye that functions under our desired conditions in the visible region and with single ion sensitivity.

Motivation & Objective

• To reduce backgrounds in neutrinoless double beta decay searches by detecting the barium daughter ion (136Ba) in 136Xe decay.
• To develop a dry-functional, selective fluorophore that fluoresces only upon binding Ba²⁺, enabling single-molecule sensitivity in gas-phase xenon.
• To enable single-ion detection in high-pressure xenon gas using a modified fluorescence microscopy system compatible with 10 bar pressure.
• To design and test an ion transport system (RF carpet) that guides Ba²⁺ ions from the active volume to the detection plane.
• To integrate all components into a functional system for real-time, coincidence detection of double electron signals and barium ion fluorescence.

Proposed method

• Design and synthesize Ba²⁺-selective, on-off switchable fluorophores based on 18-crown-6 conjugated to rigid, dry-state-active fluorophores like pyrene or anthracene.
• Use total internal reflection fluorescence (TIRF) microscopy with sapphire windows to image single molecules in high-pressure xenon gas (up to 10 bar).
• Implement an RF carpet system using radiofrequency fields to levitate and transport Ba²⁺ ions toward a central detection plane within a high-pressure chamber.
• Mount an electron-multiplying CCD (EMCCD) camera and external laser excitation system outside the pressure vessel to maintain vacuum integrity.
• Utilize a PVA matrix to immobilize fluorophores on a transparent substrate for stable, dry-state single-molecule imaging.
• Correlate fluorescence signals with double-electron events detected at the anode to confirm 0νββ decay.

Experimental results

Research questions

• RQ1 Can a dry-functional, Ba²⁺-selective fluorophore achieve single-molecule sensitivity in high-pressure xenon gas?
• RQ2 Can an RF carpet effectively transport Ba²⁺ ions from the drift volume to a detection plane under high-pressure conditions?
• RQ3 Can a high-pressure-compatible TIRF microscope resolve single fluorescent molecules at pressures up to 10 bar?
• RQ4 Can the fluorescence of a single Ba²⁺ ion be detected and distinguished from background noise in a xenon gas environment?
• RQ5 Can the combined system of ion transport, fluorophore tagging, and fluorescence imaging achieve coincidence detection with sufficient efficiency for background suppression?

Key findings

• A Ba²⁺-selective fluorophore based on 18-crown-6 conjugated to pyrene (18c6-py) demonstrates on-off switching and strong fluorescence in the dry state.
• The prototype high-pressure microscope achieved stable, high-resolution fluorescence imaging at 10 bar using 20x and 40x air-coupled objectives with external optics.
• Single Ba²⁺ ions were resolved via discrete photo-bleaching events in the FLUO-3 dye system, confirming single-molecule sensitivity in dry, PVA-embedded matrices.
• The RF carpet prototype successfully levitated and transported Ba²⁺ ions in high-pressure xenon gas, demonstrating feasibility for ion concentration to the detection plane.
• The system achieved sub-2 nm spatial resolution in single-molecule imaging, enabling unambiguous identification of individual barium ions.
• The NEXT-GodXilla program at UT Arlington is on track to demonstrate single Ba²⁺ ion detection in high-pressure xenon gas, with integration planned into the NEXT-100 detector.