分类： 物理学 >> 核物理学 提交时间： 2016-09-13
摘要： 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.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： 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 ± 0.14 (sys) ± 0.03 (stat) × 10−7cm−2 s−1, with a vertical intensity of Iv = 2.97+0.14−0.13 (sys) ± 0.02 (stat) × 10−7cm−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.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless double-beta decay of 136Xe to the ground state of 136Ba. We report here on a search for the two-neutrino double-beta decay of 136Xe to the first 0+ excited state, 0+1, of 136Ba based on a 100 kg⋅yr exposure of 136Xe. Using a specialized analysis employing a machine learning algorithm, we obtain a 90% CL half-life sensitivity of 1.7×1024 yr. We find no statistically significant evidence for the 2νββ decay to the excited state resulting in a lower limit of T2ν1/2 (0+→0+1) >6.9×1023 yr at 90% CL. This observed limit is consistent with the estimated half-life of 2.5×1025 yr.
分类： 物理学 >> 核物理学 提交时间： 2016-09-10
摘要： Alpha decays in the EXO-200 detector are used to measure the fraction of charged�18Po燼nd�14Bi燿aughters created from alpha and beta decays, respectively.�22Rn燼lpha decays in liquid xenon (LXe) are found to produce218Po+爄ons�0.3�.0%爋f the time, while the remainder of the�18Po燼toms are neutral. The fraction of�14Bi+爁rom�14Pb燽eta decays in LXe is found to be�6.4�.7%, inferred from the relative rates of�18Poand�14Po燼lpha decays in the LXe. The average velocity of�18Po爄ons is observed to decrease for longer drift times. Initially the ions have a mobility of�.390�.006燾m2/(kV爏), and at long drift times the mobility is�.219�.004燾m2/(kV爏). Time constants associated with the change in mobility during drift of the�18Po+爄ons are found to be proportional to the electron lifetime in the LXe.
分类： 物理学 >> 核物理学 提交时间： 2016-08-26
摘要： 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燵1.9641�.0039]%爐o燵1.5820�.0044]%.
分类： 物理学 >> 核物理学 提交时间： 2016-08-17
摘要： A search for Lorentz- and CPT-violating signals in the double beta decay spectrum of 136Xe has been performed using an exposure of 100 kg⋅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 −2.65×10−5GeV
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