The EXO-200 experiment searched for neutrinoless double-beta decay of \(^{136}\)Xe with a single-phase liquid xenon detector. It used an active mass of 110 kg of 80.6%-enriched liquid xenon in an ...ultra-low background time projection chamber with ionization and scintillation detection and readout. This paper describes the design and performance of the various support systems necessary for detector operation, including cryogenics, xenon handling, and controls. Novel features of the system were driven by the need to protect the thin-walled detector chamber containing the liquid xenon, to achieve high chemical purity of the Xe, and to maintain thermal uniformity across the detector.
The search for neutrinoless double-beta decay ( 0 ν β β ) requires extremely low background and a good understanding of their sources and their influence on the rate in the region of parameter space ...relevant to the 0 ν β β signal. We report on studies of various β and γ backgrounds in the liquid-xenon-based EXO- 200 0 ν β β experiment. With this work we try to better understand the location and strength of specific background sources and compare the conclusions to radioassay results taken before and during detector construction. Finally, we discuss the implications of these studies for EXO-200 as well as for the next-generation, tonne-scale nEXO detector.
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. Lastly, 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.
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.
In this work, we report on an improved measurement of the 2νββ half-life of 136 Xe performed by EXO-200. The use of a large and homogeneous time-projection chamber allows for the precise estimate of ...the fiducial mass used for the measurement, resulting in a small systematic uncertainty. We also discuss in detail the data-analysis methods used for double- β decay searches with EXO-200, while emphasizing those directly related to the present measurement. The 136 Xe 2νββ half-life is found to be T1/22νββ = 2.165±0.016(stat)±0.059(sys)×1021 yr. Finally, this is the most precisely measured half-life of any 2νββ decay to date.
Alpha decays in the EXO-200 detector are used to measure the fraction of charged \(^{218}\mathrm{Po}\) and \(^{214}\mathrm{Bi}\) daughters created from alpha and beta decays, respectively. ...\(^{222}\mathrm{Rn}\) alpha decays in liquid xenon (LXe) are found to produce \(^{218}\mathrm{Po}^{+}\) ions \(50.3 \pm 3.0\%\) of the time, while the remainder of the \(^{218}\mathrm{Po}\) atoms are neutral. The fraction of \(^{214}\mathrm{Bi}^{+}\) from \(^{214}\mathrm{Pb}\) beta decays in LXe is found to be \(76.4 \pm 5.7\%\), inferred from the relative rates of \(^{218}\mathrm{Po}\) and \(^{214}\mathrm{Po}\) alpha decays in the LXe. The average velocity of \(^{218}\mathrm{Po}\) ions is observed to decrease for longer drift times. Initially the ions have a mobility of \(0.390 \pm 0.006~\mathrm{cm}^2/(\mathrm{kV}~\mathrm{s})\), and at long drift times the mobility is \(0.219 \pm 0.004~\mathrm{cm}^2/(\mathrm{kV}~\mathrm{s})\). Time constants associated with the change in mobility during drift of the \(^{218}\mathrm{Po}^{+}\) ions are found to be proportional to the electron lifetime in the LXe.
The EXO-200 Collaboration is searching for neutrinoless double beta decay using a liquid xenon (LXe) time projection chamber. This measurement relies on modeling the transport of charge deposits ...produced by interactions in the LXe to allow discrimination between signal and background events. Here we present measurements of the transverse diffusion constant and drift velocity of electrons at drift fields between 20~V/cm and 615~V/cm using EXO-200 data. At the operating field of 380~V/cm EXO-200 measures a drift velocity of 1.705\(_{-0.010}^{+0.014}\)~mm/\(\mu\)s and a transverse diffusion coefficient of 55\(\pm\)4~cm\(^2\)/s.
The energy resolution of the EXO-200 detector is limited by electronics noise in the measurement of the scintillation response. Here we present a new technique to extract optimal scintillation energy ...measurements for signals split across multiple channels in the presence of correlated noise. The implementation of these techniques improves the energy resolution of the detector at the neutrinoless double beta decay Q-value from \(\left1.9641\pm 0.0039\right\%\) to \(\left1.5820\pm 0.0044\right\%\).
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