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

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.

Double beta decay experiments at Canfranc Underground Laboratory

“…This "barium tagging" strategy has been investigated for a long time within different projects, including the NEXT collaboration, being the main difficulty the method for extraction and identification of barium ions. A new method to tag the doubly ionized barium daughter in the DBD of 136 Xe in high pressure xenon gas TPCs was reported in [251,252], based on single molecule fluorescent imaging (SMFI) techniques; individual ions were localized with superresolution (∼2 nm) and detected with a statistical significance of 12.9σ over backgrounds, confirming the very good prospects of the technique. Ba ++ ions are captured in a monolayer of fluorescent organic molecules; intense pulsed laser illuminating the molecule make the molecule give light in a different color.…”

Mitigation of Backgrounds from Cosmogenic $^{137}$Xe in Xenon Gas Experiments using $^{3}$He Neutron Capture

“…KamLAND-Zen has also found a 7% dead-time from spallation products, the largest source being 137 Xe with (3.9 ± 2.0) × 10 −3 tonne −1 day −1 produced [19]. Finally, it is notable that 137 Xe activation provides a slow but non-zero source of non-double-betadecay related barium production through the chain 137 Xe→ 137 Cs→ 137 Ba, a potentially relevant consideration for barium tagging [20][21][22][23][24][25]. In the NEXT detectors these may be largely rejected by time-coincidence cuts with energy deposits of interest as the Q β for 137 Cs→ 137 Ba is lower than the Q ββ of interest.…”

NEXT-CRAB-0: a high pressure gaseous xenon time projection chamber with a direct VUV camera based readout

“…The NEXT-100 experiment [35] is now being constructed, which will contain 100 kg of enriched xenon gas underground at the Laboratorio Subterráneo de Canfranc (LSC) with a sensitivity to 0𝜈𝛽𝛽 of 2.8 × 10 25 years with three years of operation. Ton-scale conventional phases of this program are under development [22] with a sensitivity of 1.4 × 10 27 years after 5 years of operation, as well as an active R&D program to enable detection of the Ba 2+ daughter ions emitted in the decay through single molecule fluorescence imaging [36][37][38][39][40][41][42][43].…”