Fluorescence imaging of individual ions and molecules in pressurized noble gases for barium tagging in 136Xe

Abstract

The imaging of individual Ba²⁺ ions in high pressure xenon gas is one possible way to attain background-free sensitivity to neutrinoless double beta decay and hence establish the Majorana nature of the neutrino. In this paper we demonstrate selective single Ba²⁺ ion imaging inside a high-pressure xenon gas environment. Ba²⁺ ions chelated with molecular chemosensors are resolved at the gas-solid interface using a diffraction-limited imaging system with scan area of 1 × 1 cm² located inside 10 bar of xenon gas. This form of microscopy represents key ingredient in the development of barium tagging for neutrinoless double beta decay searches in ¹³⁶Xe. This also provides a new tool for studying the photophysics of fluorescent molecules and chemosensors at the solid-gas interface to enable bottom-up design of catalysts and sensors.

Fig. 1. Picture of the apparatus developed for this study.

a The head of the high-pressure microscope that sits inside the pressure chamber. b Optical paths and components in the high-pressure single-molecule microscope. SP and LP refer to short-pass and long-pass optical filters, respectively.

Fig. 2. Large-scale raw data image of BODIPY molecules drip-coated onto a slide surface.

a Full 1 mm × 1 mm image; (b) zoom into one raster frame of 33 μm × 33 μm; (c) resolved single molecule in a 2.5 μm  × 2.5 μm square. The image is resolved with a point-spread function close to the Abbe Diffraction Limit.

Fig. 3. Effect of Ba²⁺ addition to the sensor system.

Images show a slide spin-coated in IPG-1 chemosensor, showing the activity with (a) and without (b) added Ba²⁺.

Fig. 4. Statistical response of chemosensors to the addition of Ba²⁺ in xenon gas.

a Pixel intensity distribution for images taken using IPG vs IPG + Ba in 10 bar xenon gas. b reconstructed single molecule candidate brightnesses and step times for IPG vs IPG + Ba in 10 bar xenon gas. The plotted points are obtained over seven exposure regions on a single slide, and the trend is found to be repeatable over multiple slides. The horizontal line shows a cut that we place on this distribution for selecting single-ion candidate spots.

Fig. 5. Single barium ions chelated with IPG-1 turn-on chemosensor imaged in 10 bar xenon gas.

a time trace of fluorescence for the identified emitters, with discrete photobleaching and photoblinking steps indicative of single molecule origin. The intensity is taken from the central pixel after applying a double-Gaussian filter as described in the text and thus represents the baseline-subtracted integral over the diffraction-limited fluorescent spot. b 2D spatial map of fluorescence around each emitter, integrated for 20 frames before photo-bleaching, showing a well-localized peak in each case. In the case of the second and fourth figures, nearby peaks from adjacent ions are also visible in the 3D image histograms.

Fig. 6. Photo-bleaching time distribution for Ba²⁺ candidates chelated in IPG dyes in xenon and air.

a in xenon gas; (b) in air. The X-axis is shared between the top and bottom histogram. Error bars indicate representative 1σ statistical errors calculated as $\sqrt{N}$ of detected counts N.

Fig. 7. Illustration of the single molecule autofocus procedure.

An autofocus metric defined in terms of image brightness and kurtosis is applied that maximizes for an in-focus image. The left plot shows the Z dependence of the autofocus metric, with images at different focal depths provided on the right plot. A small signal is seen on the back face of the slide (A). No features are present when totally out of focus (B). Within around 2 microns of focus, some activity is seen (C), with sharp images only at the focal plane (D). The inset shows that the depth of the in-focus region is around 1 μm.

Fig. 8. Study of position resolution of the diffraction-limited microscope system.

a Measured point-spread function in X and Y-directions at five re-focused locations using single molecules in 7 bar argon gas. b Average X and Y PSF compared to the Abbe diffraction limit. The horizontal line on the right plot represents the baseline, as measured away from the bright spot.

Fig. 9. Illustration of the steps in the image series analysis used to identify and quantify the behavior of single molecule candidates.

a Frequency filtering of raw data to remove diffuse backgrounds and CCD speckle noise. b Bright spot identification by pixels over the threshold. c Step time series identification. d Comparison between images in different conditions.