Results from a search for neutrinoless double-beta decay (0νββ) of ^{136}Xe are presented using the first year of data taken with the upgraded EXO-200 detector. Relative to previous searches by EXO-200, the energy resolution of the detector has been improved to σ/E=1.23%, the electric field in the drift region has been raised by 50%, and a system to suppress radon in the volume between the cryostat and lead shielding has been implemented. In addition, analysis techniques that improve topological discrimination between 0νββ and background events have been developed. Incorporating these hardware and analysis improvements, the median 90% confidence level 0νββ half-life sensitivity after combining with the full data set acquired before the upgrade has increased twofold to 3.7×10^{25}  yr. No statistically significant evidence for 0νββ is observed, leading to a lower limit on the 0νββ half-life of 1.8×10^{25}  yr at the 90% confidence level.

We report on an improved measurement of the 2νββ half-life of 136 Xe 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 136 Xe 2νββ half-life is found to be T 2νββ 1/2 = 2.165 ± 0.016(stat) ± 0.059(sys) · 10 21 years. This is the most precisely measured half-life of any 2νββ decay to date.

Studies in heterogeneous catalysis have long speculated on or have provided indirect evidence for the role of hydrogen embedded in the catalyst bulk as a primary reactant. This report describes experiments carried out under single-collision conditions that document the distinctive reactivity of hydrogen embedded in the bulk of the metal catalyst. Specifically, the bulk H atom is shown to be the reactive species in the hydrogenation of CH(3) adsorbed on Ni(111) to form CH(4), while the H atoms bound to the surface were unreactive. These results unambiguously demonstrate the importance of bulk species to heterogeneous catalytic chemistry.

A search for neutrinoless double-β decay (0νββ) in 136 Xe is performed with the full EXO-200 dataset using a deep neural network to discriminate between 0νββ and background events. Relative to previous Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP 3 .

The stability and chemistry of methyl radicals adsorbed on Ni (111) A detailed analysis of the vibrational spectra of CH 3 , CH 2 D, and CD 3 adsorbed on Ni͑111͒ and the products of their reactions is presented. The synthesis of adsorbed methyl radicals from CH 4 , CH 3 D, or CD 4 is effected by molecular beam techniques. The ability to measure these spectra by high-resolution electron energy loss spectroscopy ͑HREELS͒ at higher resolution ͑35 cm Ϫ1 ͒ and higher sensitivity ͑5ϫ10 6 counts/s͒ has allowed new features to be observed and a symmetry analysis to be carried out. It is concluded that the CH 3 radical is adsorbed with C 3v symmetry on a threefold hollow site. The symmetric C-H stretch mode of CH 3 and the overtone of the antisymmetric deformation mode are observed to be in Fermi resonance. At temperatures above 150 K, CH 3 dissociates to form adsorbed CH. Confirmation for the assignment to a CH species is found in the observation that the spectrum measured after thermal decomposition of CH 2 D is a superposition of those from the decomposition of CH 3 and CD 3 . The adsorption site of the CH species is concluded to be a threefold hollow site and the geometry of the Ni 3 -C-H is concluded to be pyramidal. At temperatures above 250 K, carbon-carbon bond formation between the CH species is observed to yield C 2 H 2 . Low coverages of C 2 H 2 are shown to dehydrogenate at 400 K. High coverages of C 2 H 2 are shown conclusively to trimerize to form adsorbed benzene in contrast to a literature report of C 2 H 2 dissociation to adsorbed CH at these temperatures. The relative stabilities of the hydrocarbon species on Ni͑111͒ are determined to be CH 3 ϽCHϩ2H Ͻ1/2C 2 H 2 ϩ2HϽ1/6C 6 H 6 ϩH 2͑g͒ .