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Register a new userDemonstration of Single Barium Ion Sensitivity for Neutrinoless Double Beta Decay using Single Molecule Fluorescence Imaging
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A new method to tag the barium daughter in the double beta decay of $^{136}$Xe is reported. Using the technique of single molecule fluorescent imaging (SMFI), individual barium dication (Ba$^{++}$) resolution at a transparent scanning surface has been demonstrated. A single-step photo-bleach confirms the single ion interpretation. Individual ions are localized with super-resolution ($sim$2~nm), and detected with a statistical significance of 12.9~$sigma$ over backgrounds. This lays the foundation for a new and potentially background-free neutrinoless double beta decay technology, based on SMFI coupled to high pressure xenon gas time projection chambers.
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Background rejection is key to success for future neutrinoless double beta decay experiments. To achieve sensitivity to effective Majorana lifetimes of $sim10^{28}$ years, backgrounds must be controlled to better than 0.1 count per ton per year, beyond the reach of any present technology. In this paper we propose a new method to identify the birth of the barium daughter ion in the neutrinoless double beta decay of $^{136}$Xe. The method adapts Single Molecule Fluorescent Imaging, a technique from biochemistry research with demonstrated single ion sensitivity. We explore possible SMFI dyes suitable for the problem of barium ion detection in high pressure xenon gas, and develop a fiber-coupled sensing system with which we can detect the presence of bulk Ba$^{++}$ ions remotely. We show that our sensor produces signal-to-background ratios as high as 85 in response to Ba$^{++}$ ions when operated in aqueous solution. We then describe the next stage of this R&D program, which will be to demonstrate chelation and fluorescence in xenon gas. If a successful barium ion tag can be developed using SMFI adapted for high pressure xenon gas detectors, the first essentially zero background, ton-scale neutrinoless double beta decay technology could be realized.
NEXT-100 is an electroluminescent high-pressure xenon gas time projection chamber that will search for the neutrinoless double beta ($beta beta 0 u$) decay of Xe-136. The detector possesses two features of great value for $beta beta 0 u$ searches: energy resolution better than 1% FWHM at the $Q$ value of Xe-136 and track reconstruction for the discrimination of signal and background events. This combination results in excellent sensitivity, as discussed in this paper. Material-screening measurements and a detailed Monte Carlo detector simulation predict a background rate for NEXT-100 of at most $4times10^{-4}$ counts keV$^{-1}$ kg$^{-1}$ yr$^{-1}$. Accordingly, the detector will reach a sensitivity to the bbonu-decay half-life of $2.8times10^{25}$ years (90% CL) for an exposure of 100 $mathrm{kg}cdotmathrm{year}$, or $6.0times10^{25}$ years after a run of 3 effective years.
A high pressure xenon gas time projection chamber with electroluminescent amplification (EL HPGXe TPC) searching for the neutrinoless double beta ($0 ubetabeta$) decay offers: excellent energy resolution ($0.5-0.7%$ FWHM at the $Q_{betabeta}$), by amplifying the ionization signal with electroluminescent light, and tracking capabilities, as demonstrated by the NEXT collaboration using two kg-scale prototypes. The NEXT collaboration is building an EL HPGXe TPC capable of holding 100 kg (NEXT-100) of xenon isotopically enriched in ${{}^{136}rm Xe}$. The installation and commissioning of the NEXT-100 detector at the Laboratorio Subterraneo de Canfranc (LSC) is planned for 2018. The current estimated background level for the NEXT-100 detector is of $4times10^{-4}$ counts/keV-kg-yr or less in the energy region of interest. Assuming an energy resolution of 0.75$%$ FWHM at the $Q_{betabeta}$ and a $0 ubetabeta$ signal efficiency of about 28$%$, this gives an expected sensitivity (at 90$%$ CL) to the $0 ubetabeta$ decay half life of $T^{0 u}_{1/2}>6.0times10^{25}$ yr for an exposure of 275 kg yr. A first phase of the NEXT experiment, called NEW, is currently being commissioned at the LSC. The NEW detector is a scale 1:2 in size (1:10 in mass) of the NEXT-100 detector using the same materials and photosensors and will be used to perform a characterization of the $0 ubetabeta$ backgrounds and a measurement of the standard double beta decay with neutrinos (${2 ubetabeta}$). An 8 sigma significance for the ${2 ubetabeta}$ signal in the NEW detector has been estimated for a 100-day run.
The DARWIN observatory is a proposed next-generation experiment to search for particle dark matter and for the neutrinoless double beta decay of $^{136}$Xe. Out of its 50$,$t total natural xenon inventory, 40$,$t will be the active target of a time projection chamber which thus contains about 3.6 t of $^{136}$Xe. Here, we show that its projected half-life sensitivity is $2.4times10^{27},$yr, using a fiducial volume of 5t of natural xenon and 10$,$yr of operation with a background rate of less than 0.2$~$events/(t$cdot$yr) in the energy region of interest. This sensitivity is based on a detailed Monte Carlo simulation study of the background and event topologies in the large, homogeneous target. DARWIN will be comparable in its science reach to dedicated double beta decay experiments using xenon enriched in $^{136}$Xe.
We present a new neutrinoless double beta decay concept: the high pressure selenium hexafluoride gas time projection chamber. Combining techniques pioneered in high pressure xenon gas such as topological discrimination, with the high Q-value afforded by double beta decay isotope $^{82}$Se, a promising new detection technique is outlined. Lack of free electrons in SeF$_6$ mandates the use of an ion TPC. The microphysics of ion production and drift, which have many nuances, are explored. Background estimates are produced suggesting such a detector may achieve background indices of better than 1 count per ton per year in the region of interest at the 100~kg scale, and still better at the ton-scale.
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