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 search for neutrinoless double-beta decay (0{ u}{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{ u}{beta}{beta} signal. W
e report on studies of various {beta}- and {gamma}-backgrounds in the liquid- xenon-based EXO-200 0{ u}{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.
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 ($0 u beta beta$), 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 $1.9cdot10^{25}$ yr, an improvement by a factor of 2.7 compared to previous EXO-200 results. We find no statistically significant evidence for $0 u beta beta$ decay and set a half-life limit of $1.1cdot10^{25}$ yr at 90% CL. The high sensitivity holds promise for further running of the EXO-200 detector and future $0 u beta beta$ decay searches with nEXO.
We report on an improved measurement of the 2 u beta beta half-life of Xe-136 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-beta decay searches with EXO-200, while emphasizing those directly related to the present measurement. The Xe-136 2 u beta beta half-life is found to be 2.165 +- 0.016 (stat) +- 0.059 (sys) x 10^21 years. This is the most precisely measured half-life of any 2 u beta beta decay to date.
