Neutrinoless double beta decay is one of the most sensitive probes for new physics beyond the Standard Model of particle physics. One of the isotopes under investigation is Xe 136 , which would double beta decay into Ba 136 . Detecting the single Ba 136 daughter provides a sort of ultimate tool in the discrimination against backgrounds. Previous work demonstrated the ability to perform single atom imaging of Ba atoms in a single-vacancy site of a solid xenon matrix. In this paper, the effort to identify signal from individual barium atoms is extended to Ba atoms in a hexa-vacancy site in the matrix and is achieved despite increased photobleaching in this site. Abrupt fluorescence turn-off of a single Ba atom is also observed. Significant recovery of fluorescence signal lost through photobleaching is demonstrated upon annealing of Ba deposits in the Xe ice. Following annealing, it is observed that Ba atoms in the hexa-vacancy site exhibit antibleaching while Ba atoms in the tetra-vacancy site exhibit bleaching. This may be evidence for a matrix site transfer upon laser excitation. Our findings offer a path of continued research toward tagging of Ba daughters in all significant sites in solid xenon. Published by the American Physical Society 2024
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 fewer than or equal to
104 Ba atoms can be obtained. Prospects for imaging single Ba atoms in solid xenon are discussed.
Excitation and emission spectra at 12 K temperature are presented for the first time for Ba+ in solid xenon. This is only the third such atomic ion studied in a solid noble gas matrix. Two groups of emission peaks, each with similar excitation spectra, photobleaching behavior and temperature dependence, are identified that may be associated with transitions from a common excited state in a particular solid xenon matrix site. Favorable conditions are found for imaging of single Ba+ ions in solid xenon, including reasonably strong emission efficiency, acceptable photobleaching and a method for overcoming background emission from the surface of the cooled sapphire window. A comparison of the new excitation spectrum of Ba+ in solid xenon with unpublished results on spectroscopy of Ba+ in liquid xenon helps confirm the identification of the latter.