Demonstration of Single-Barium-Ion Sensitivity for Neutrinoless Double-Beta Decay Using Single-Molecule Fluorescence Imaging

A new method to tag the barium daughter in the double-beta decay of ${}^{136}\mathrm{Xe}$ is reported. Using the technique of single molecule fluorescent imaging (SMFI), individual barium dication (${\mathrm{Ba}}^{++}$) resolution at a transparent scanning surface is demonstrated. A single-step photobleach confirms the single ion interpretation. Individual ions are localized with superresolution ($\sim 2\mathrm{nm}$), and detected with a statistical significance of $12.9\sigma$ over backgrounds. This lays the foundation for a new and potentially background-free neutrinoless double-beta decay technology, based on SMFI coupled to high pressure xenon gas time projection chambers.

Figure 1

A single ${\mathrm{Ba}}^{++}$ candidate. A fixed region of the CCD camera is shown with a 0.5 s exposure before (top) and after (bottom) the photobleaching transition.

Figure 2

A schematic view of the TIRF system.

Figure 3

A sample image from the EM-CCD in one of the barium-spiked samples showing both near-surface (bright) and deeper (dim) fluorescent molecules.

Figure 4

Fluorescence trajectory for one candidate in a barium-spiked sample. “Signal” shows the average activity in $5\times 5\mathrm{pixels}$ centered on the local maximum. “Background” shows the average in the 56 surrounding pixels. The single step photobleach is characteristic of single molecule fluorescence.

Figure 5

Histogram showing the number of ion candidates per FOV in barium-free and barium-spiked samples. Entries are weighted by candidates detected.