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}$.
We estimate rates of solar neutrino-induced neutrons in a DAMA/LIBRA-like detector setup, and find that the needed contribution to explain the annual modulation would require neutrino-induced neutron cross sections several orders of magnitude larger
than current calculations indicate. Although these cross sections have never been measured, it is likely that the solar-neutrino effect on DAMA/LIBRA is negligible.
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.
CoGeNT has taken data for over 3 years, with 1136 live days of data accumulated as of April 23, 2013. We report on the results of a maximum likelihood analysis to extract any possible dark matter signal present in the collected data. The maximum like
lihood signal extraction uses 2-dimensional probability density functions (PDFs) to characterize the anticipated variations in dark matter interaction rates for given observable nuclear recoil energies during differing periods of the Earths annual orbit around the Sun. Cosmogenic and primordial radioactivity backgrounds are characterized by their energy signatures and in some cases decay half-lives. A third parameterizing variable -- pulse rise-time -- is added to the likelihood analysis to characterize slow rising pulses described in prior analyses. The contribution to each event category is analyzed for various dark matter signal hypotheses including a dark matter standard halo model and a case with free oscillation parameters (i.e., amplitude, period, and phase). The best-fit dark matter signal is in close proximity to previously reported results. We find that the significance of the extracted dark matter signal remains well below evidentiary at 1.7 $sigma$.
Weakly Interacting Massive Particles (WIMPs) are well-established dark matter candidates. WIMP interactions with sensitive detectors are expected to display a characteristic annual modulation in rate. We release a dataset spanning 3.4 years of operat
ion from a low-background germanium detector, designed to search for this signature. A previously reported modulation persists, concentrated in a region of the energy spectrum populated by an exponential excess of unknown origin. Its phase and period agree with phenomenological expectations, but its amplitude is a factor $sim$4-7 larger than predicted for a standard WIMP galactic halo. We consider the possibility of a non-Maxwellian local halo velocity distribution as a plausible explanation, able to help reconcile recently reported WIMP search anomalies.
Fifteen months of cumulative CoGeNT data are examined for indications of an annual modulation, a predicted signature of Weakly Interacting Massive Particle (WIMP) interactions. Presently available data support the presence of a modulated component of
unknown origin, with parameters prima facie compatible with a galactic halo composed of light-mass WIMPs. Unoptimized estimators yield a statistical significance for a modulation of ~2.8 sigma, limited by the short exposure.
A magnetically driven piston pump for xenon gas recirculation is presented. The pump is designed to satisfy extreme purity and containment requirements, as is appropriate for the recirculation of isotopically enriched xenon through the purification s
ystem and large liquid xenon TPC of EXO-200. The pump, using sprung polymer gaskets, is capable of pumping more than 16 standard liters per minute (SLPM) of xenon gas with 750 torr differential pressure.
We report on several features present in the energy spectrum from an ultra low-noise germanium detector operated at 2,100 m.w.e. By implementing a new technique able to reject surface events, a number of cosmogenic peaks can be observed for the first
time. We discuss several possible causes for an irreducible excess of bulk-like events below 3 keVee, including a dark matter candidate common to the DAMA/LIBRA annual modulation effect, the hint of a signal in CDMS, and phenomenological predictions. Improved constraints are placed on a cosmological origin for the DAMA/LIBRA effect.
