M. Danilov

Search for Majorana neutrinos with the first two years of EXO-200 data

Many extensions of the standard model of particle physics suggest that neutrinos should be Majorana-type fermions—that is, that neutrinos are their own anti-particles—but this assumption is difficult to confirm. Observation of neutrinoless double-β decay (0νββ), 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 76Ge (the GERDA experiment) and 136Xe (the KamLAND-Zen and EXO (Enriched Xenon Observatory)-200 experiments) have established the lifetime of this decay to be longer than 1025u2009years, corresponding to a limit on the neutrino mass of 0.2–0.4u2009electronvolts. Here we report new results from EXO-200 based on a large 136Xe exposure that represents an almost fourfold increase from our earlier published data sets. We have improved the detector resolution and revised the data analysis. The half-life sensitivity we obtain is 1.9u2009×u20091025u2009years, an improvement by a factor of 2.7 on previous EXO-200 results. We find no statistically significant evidence for 0νββ decay and set a half-life limit of 1.1u2009×u20091025u2009years at the 90 per cent confidence level. The high sensitivity holds promise for further running of the EXO-200 detector and future 0νββ decay searches with an improved Xe-based experiment, nEXO.

Search for Neutrinoless Double-Beta Decay in 136 Xe with EXO-200

We report on a search for neutrinoless double-beta decay of 136Xe with EXO-200. No signal is observed for an exposure of 32.5 kgu2009yr, with a background of ∼1.5×10(-3)u2009u2009kg(-1)u2009yr(-1) u2009keV(-1) in the ±1σ region of interest. This sets a lower limit on the half-life of the neutrinoless double-beta decay T(1/2)(0νββ)(136Xe)>1.6×10(25)u2009u2009yr (90% C.L.), corresponding to effective Majorana masses of less than 140-380 meV, depending on the matrix element calculation.

Systematic study of trace radioactive impurities in candidate construction materials for EXO-200

The Enriched Xenon Observatory (EXO) will search for double beta decays of 136Xe. We report the results of a systematic study of trace concentrations of radioactive impurities in a wide range of raw materials and finished parts considered for use in the construction of EXO-200, the first stage of the EXO experimental program. Analysis techniques employed, and described here, include direct gamma counting, alpha counting, neutron activation analysis, and high-sensitivity mass spectrometry.

Correlated fluctuations between luminescence and ionization in liquid xenon

The ionization of liquefied noble gases by radiation is known to be accompanied by fluctuations much larger than predicted by Poisson statistics. We have studied the fluctuations of both scintillation and ionization in liquid xenon and have measured, for the first time, a strong anti-correlation between the two at a microscopic level, with coefficient -0.80 < {rho}{sub ep} < -0.60. This provides direct experimental evidence that electron-ion recombination is partially responsible for the anomalously large fluctuations and at the same time allows substantial improvement of calorimetric energy resolution.

Study of scintillator strip with wavelength shifting fiber and silicon photomultiplier

The performance of the

An improved measurement of the 2\nu \beta \beta\ half-life of Xe-136 with EXO-200

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Quartz fibre calorimetry — Monte Carlo simulation

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Detection of scintillation light in liquid xenon by multipixel avalanche Geiger photodiode and wavelength shifter

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Registration of reactor neutrinos with the highly segmented plastic scintillator detector DANSSino

plastic scintillator strip with wavelength shifting fiber read-out by two novel photodetectors called Silicon PhotoMultipliers (SiPMs) is discussed. The advantages of SiPM relative to the traditional multichannel photomultiplier are shown. Light yield and light attenuation measurements are presented. This technique can be used in muon or calorimeter systems.

Recent developments in quartz fibre calorimetry

We report on an improved measurement of the 2nu 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 2nu 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 2nu beta beta decay to date.