  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.
