Spectroscopy of Ba andBa+deposits in solid xenon for barium tagging in nEXO

Abstract

Progress on a method of barium tagging for the nEXO double beta decay experiment is reported. Absorption and emission spectra for deposits of barium atoms and ions in solid xenon matrices are presented. Excitation spectra for prominent emission lines, temperature dependence and bleaching of the fluorescence reveal the existence of different matrix sites. A regular series of sharp lines observed in Ba + deposits is identified with some type of barium hydride molecule. Lower limits for the fluorescence quantum efficiency of the principal Ba emission transition are reported. Under current conditions, an image of ≤ 10 4 Ba atoms can be obtained. Prospects for imaging single Ba atoms in solid xenon are discussed.

Measurements of the ion fraction and mobility ofα−andβ-decay products in liquid xenon using the EXO-200 detector

“…This result provides some basis for expecting high ionization fraction of 136 Ba from double beta decay, which is relevant to the design of possible tagging methods. Progress on some of these tagging methods have been reported recently [7,8]. The higher daughter ion fraction than hole ion fraction, particularly for alpha decay, was discussed, and a potential mechanism of charge transfer reactions with Xe holes and/or Penning ionization reactions with excitons was proposed to explain this.…”

“…The fraction of ions created from beta decays and the behavior of ions in the detector is of particular interest for nEXO, the next-generation EXO experiment, which is expected to contain 5 tons of LXe enriched in 136 Xe. A possible second phase of nEXO may include a system to "tag" the 136 Ba daughter of the 0νββ decay in order to eliminate all background except a negligible contribution from the two-neutrino mode [7,8]. There are several important unanswered questions relevant to the implementation of Ba tagging: 1) whether the 136 Ba daughter is mainly an ion or a neutral atom, 2) what the charge of the ion is in LXe, and 3) how the Ba will move in the detector over time scales required to capture it.…”

Mobility and clustering of barium ions and dications in high-pressure xenon gas

“…This is because the detection of such ions could serve as an unambiguous tag of the hypothetical, ultra-rare process of neutrinoless double beta decay [1]. So-called "barium tagging" has been a subject of intensive R&D for both liquid and gaseous xenon detectors [2][3][4]. However, only recently has single ion sensitivity been demonstrated using a method that appears compatible with in-situ ion identification [5].…”

Searching for 0νββ Decay in 136Xe: Toward the Ton-Scale and Beyond

“…Ba-tagging has been proposed by [11] and various approaches are pursued by the nEXO collaboration using a tip or cold probe to extract 136 Ba from the volume [12,13] (see [14] for a proposed technique to identify Ba inside the detector). An alternative approach proposes to move a capillary Figure 5: Schematic of the setup to extract Ba-ions from xenon gas and identify them by means of laser-fluorescence spectroscopy.…”