  The nature of the neutrino is one of the major open questions in experimental
nuclear and particle physics. The most sensitive known method to establish the
Majorana nature of the neutrino is detection of the ultra-rare process of
neutrinoless double beta decay. However, identification of one or a handful of
decay events within a large mass of candidate isotope, without obfuscation by
backgrounds is a formidable experimental challenge. One hypothetical method for
achieving ultra-low-background neutrinoless double beta decay sensitivity is
the detection of single $^{136}$Ba ions produced in the decay of $^{136}$Xe
(`barium tagging'). To implement such a method, a single-ion-sensitive barium
detector must be developed and demonstrated in bulk liquid or dry gaseous
xenon. This paper reports on the development of two families of dry-phase
barium chemosensor molecules for use in high pressure xenon gas detectors,
synthesized specifically for this purpose. One particularly promising
candidate, an anthracene substituted aza-18-crown-6 ether, is shown to respond
in the dry phase with almost no intrinsic background from the unchelated state,
and to be amenable to barium sensing through fluorescence microscopy. This
interdisciplinary advance, paired with earlier work demonstrating sensitivity
to single barium ions in solution, opens a new path toward single ion detection
in high pressure xenon gas.
