As neutrinoless double-beta decay experiments become more sensitive and intrinsic radioactivity in detector materials is reduced, previously minor contributions to the background must be understood ...and eliminated. With this in mind, cosmogenic backgrounds have been studied with the EXO-200 experiment. Using the EXO-200 TPC, the muon flux (through a flat horizontal surface) underground at the Waste Isolation Pilot Plant (WIPP) has been measured to be {\Phi} = 4.07 \(\pm\) 0.14 (sys) \(\pm\) 0.03 (stat) \(\times\) \(10^{-7}\)cm\(^{-2}\) s\(^{-1}\), with a vertical intensity of \(I_{v}\) = 2.97\(^{+0.14}_{-0.13}\) (sys) \(\pm\) 0.02 (stat) \(\times\) \(10^{-7}\)cm\(^{-2}\) s\(^{-1}\) sr\(^{-1}\). Simulations of muon-induced backgrounds identified several potential cosmogenic radionuclides, though only 137Xe is a significant background for the 136Xe 0{\nu}{\beta}{\beta} search with EXO-200. Muon-induced neutron backgrounds were measured using {\gamma}-rays from neutron capture on the detector materials. This provided a measurement of 137Xe yield, and a test of the accuracy of the neutron production and transport simulations. The independently measured rates of 136Xe neutron capture and of 137Xe decay agree within uncertainties. Geant4 and FLUKA simulations were performed to estimate neutron capture rates, and these estimates agreed to within ~40% or better with measurements. The ability to identify 136Xe(n,{\gamma}) events will allow for rejection of 137Xe backgrounds in future 0{\nu}{\beta}{\beta} analyses.
A search for Lorentz- and CPT-violating signals in the double beta decay spectrum of \(^{136}\)Xe has been performed using an exposure of 100 kg\(\cdot\)yr with the EXO-200 detector. No significant ...evidence of the spectral modification due to isotropic Lorentz-violation was found, and a two-sided limit of \(-2.65 \times 10^{-5 } \; \textrm{GeV} < \mathring{a}^{(3)}_{\text{of}} < 7.60 \times 10^{-6} \; \textrm{GeV}\) (90% C.L.) is placed on the relevant coefficient within the Standard-Model Extension (SME). This is the first experimental study of the effect of the SME-defined oscillation-free and momentum-independent neutrino coupling operator on the double beta decay process.
Phys. Rev. C 93, 035501 (2016) EXO-200 is a single phase liquid xenon detector designed to search for
neutrinoless double-beta decay of $^{136}$Xe to the ground state of $^{136}$Ba.
We report here on ...a search for the two-neutrino double-beta decay of $^{136}$Xe
to the first $0^+$ excited state, $0^+_1$, of $^{136}$Ba based on a 100
kg$\cdot$yr exposure of $^{136}$Xe. Using a specialized analysis employing a
machine learning algorithm, we obtain a 90% CL half-life sensitivity of $1.7
\times 10^{24}$ yr. We find no statistically significant evidence for the
$2\nu\beta\beta$ decay to the excited state resulting in a lower limit of
$T^{2\nu}_{1/2}$ ($0^+ \rightarrow 0^+_1$) $> 6.9 \times 10^{23}$ yr at 90% CL.
This observed limit is consistent with the estimated half-life of
$2.5\times10^{25}$ yr.
The search for neutrinoless double-beta decay (0{\nu}{\beta}{\beta}) 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{\nu}{\beta}{\beta} signal. We report on studies of various {\beta}- and {\gamma}-backgrounds in the liquid- xenon-based EXO-200 0{\nu}{\beta}{\beta} 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. Under current conditions, an image of \(\le10^4\) Ba atoms can be obtained. Prospects for imaging single Ba atoms in solid xenon are discussed.
EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless double-beta decay of \(^{136}\)Xe. Here we report on a search for various Majoron-emitting modes based on 100 ...kg\(\cdot\)yr exposure of \(^{136}\)Xe. A lower limit of \(T^{^{136}Xe}_{1/2} >1.2 \cdot 10^{24}\) yr at 90% C.L. on the half-life of the spectral index = 1 Majoron decay was obtained, corresponding to a constraint on the Majoron-neutrino coupling constant of \(|< g^{M}_{ee} >|<\) (0.8-1.7)\(\cdot\)10\(^{-5}\).
Many extensions of the Standard Model of particle physics suggest that neutrinos should be Majorana-type fermions, but this assumption is difficult to confirm. Observation of neutrinoless double-beta ...decay (\(0\nu \beta \beta\)), a spontaneous transition that may occur in several candidate nuclei, would verify the Majorana nature of the neutrino and constrain the absolute scale of the neutrino mass spectrum. Recent searches carried out with \(^{76}\)Ge (GERDA experiment) and \(^{136}\)Xe (KamLAND-Zen and EXO-200 experiments) have established the lifetime of this decay to be longer than \(10^{25}\) yr, corresponding to a limit on the neutrino mass of 0.2-0.4 eV. Here we report new results from EXO-200 based on 100 kg\(\cdot\)yr of \(^{136}\)Xe exposure, representing an almost fourfold increase from our earlier published datasets. We have improved the detector resolution at the \(^{136}\)Xe double-beta-decay Q-value to \(\sigma\)/E = 1.53% and revised the data analysis. The obtained half-life sensitivity is \(1.9\cdot10^{25}\) yr, an improvement by a factor of 2.7 compared to previous EXO-200 results. We find no statistically significant evidence for \(0\nu \beta \beta\) decay and set a half-life limit of \(1.1\cdot10^{25}\) yr at 90% CL. The high sensitivity holds promise for further running of the EXO-200 detector and future \(0\nu \beta \beta\) decay searches with nEXO.
EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless double-beta decay of \(^{136}\)Xe to the ground state of \(^{136}\)Ba. We report here on a search for the ...two-neutrino double-beta decay of \(^{136}\)Xe to the first \(0^+\) excited state, \(0^+_1\), of \(^{136}\)Ba based on a 100 kg\(\cdot\)yr exposure of \(^{136}\)Xe. Using a specialized analysis employing a machine learning algorithm, we obtain a 90% CL half-life sensitivity of \(1.7 \times 10^{24}\) yr. We find no statistically significant evidence for the \(2\nu\beta\beta\) decay to the excited state resulting in a lower limit of \(T^{2\nu}_{1/2}\) (\(0^+ \rightarrow 0^+_1\)) \(> 6.9 \times 10^{23}\) yr at 90% CL. This observed limit is consistent with the estimated half-life of \(2.5\times10^{25}\) yr.
