A new measurement of the neutron β-decay asymmetry A 0 has been carried out by the UCNA Collaboration using polarized ultracold neutrons (UCNs) from the solid deuterium UCN source at the Los Alamos Neutron Science Center. Improvements in the experiment have led to reductions in both statistical and systematic uncertainties leading to A 0 = −0.11954(55) stat (98) Precision measurements of neutron β decay are an essential ingredient in understanding the electro-weak interaction in the light quark sector. In particular the axial-vector weak coupling constant, g A , is an important input to understanding the spin and flavor structure of the nucleon [1,2] and is being actively studied in detailed lattice QCD calculations [3,4]. It also plays an important role in a variety of astrophysical processes, including solar fusion cross sections important for energy and neutrino production in the Sun [5].The angular distribution of emitted electrons from decays of a polarized neutron ensemble can be expressed as [6]where A(E) specifies the decay asymmetry for electron energy E, v ≡ βc is the electron velocity, P is the mean neutron polarization, and θ is the angle between the neutron spin and the electron momentum. can be expressed aswhere λ ≡ g A /g V is the ratio of the vector to axial-vector weak coupling constants. Combining g A with independent measurements of the Fermi coupling constant G F , the Cabibbo-Kobayashi-Maskawa matrix element V ud , and the neutron lifetime τ n allows a precision test of the consistency of measured neutron β-decay observables [7]. The ultracold neutron asymmetry (UCNA) experiment is the first experiment to use ultracold neutrons (UCNs) in a precision measurement of neutron decay correlations. Following the publication of our earlier results [7][8][9], the UCNA Collaboration implemented a number of experimental improvements that led to reductions in both statistical and systematic uncertainties. These improvements, described below, include enhanced UCN storage, improved electron energy reconstruction, and continuous monitoring of the magnetic field in the spectrometer. This refined treatment of the systematic corrections and uncertainties begins to address issues of consistency in the world data set for A 0 .The UCNA experiment ran in 2010 using the "thin window geometry D" as described in [7,9], and collected a total of 20.6 × 10 6 β-decay events after all cuts were applied. We used the UCN source [10] Copyright by the American Physical Society. Mendenhall, M. P. ; Pattie, R. W., Jr. ; Bagdasarova, Y. ; et al., Mar 25, 2013. "Precision measurement of the neutron beta-decay asymmetry," PHYSICAL REVIEW C 87(3): 032501.

A.; Makela, M.; Bagdasarova, Y.; et al., "Performance of the Los Alamos National Laboratory spallation-driven solid-deuterium ultra-cold neutron source," Rev. Sci. Instrum. 84, 013304 (2013); http://dx.doi.org/10.1063/1.4770063 REVIEW OF SCIENTIFIC INSTRUMENTS 84, 013304 (2013) Performance of the Los Alamos National Laboratory spallation-driven solid-deuterium ultra-cold neutron source In this paper, we describe the performance of the Los Alamos spallation-driven solid-deuterium ultracold neutron (UCN) source. Measurements of the cold neutron flux, the very low energy neutron production rate, and the UCN rates and density at the exit from the biological shield are presented and compared to Monte Carlo predictions. The cold neutron rates compare well with predictions from the Monte Carlo code MCNPX and the UCN rates agree with our custom UCN Monte Carlo code. The source is shown to perform as modeled. The maximum delivered UCN density at the exit from the biological shield is 52(9) UCN/cc with a solid deuterium volume of ∼1500 cm 3 .

We demonstrate neutron beam focusing by axisymmetric mirror systems based on a pair of mirrors consisting of a confocal ellipsoid and hyperboloid. Such a system, known as a Wolter mirror configuration, is commonly used in x-ray telescopes. The axisymmetric Wolter geometry allows nesting of several mirror pairs to increase collection efficiency. We have implemented a system containing four nested Ni mirror pairs, which was tested by focusing a polychromatic neutron beam at the MIT Reactor. In addition, we have carried out extensive ray-tracing simulations of the mirrors and their performance in different situations. The major advantages of the Wolter mirrors are nesting for large angular collection, and aberration-free performance. We discuss how these advantages can be utilized to benefit various neutron scattering methods, such as imaging, SANS, and time-of-flight spectroscopy.

The accurate determination of atomic final states following nuclear β decay plays an important role in several experiments. In particular, the charge state distributions of ions following nuclear β decay are important for determinations of the β − ν angular correlation with improved precision. Beyond the hydrogenic cases, the decay of neutral 6 He presents the simplest case. Our measurement aims at providing benchmarks to test theoretical calculations. The kinematics of Li n+ ions produced following the β decay of 6 He within an electric field were measured using 6 He atoms in the metastable (1s2s, 3 S1) and in the (1s2p, 3 P2) states confined by a magneto-optical trap. The electron shake-off probabilities were deduced including their dependence on ion energy. We find significant discrepancies on the fractions of Li ions in the different charge states with respect to a recent calculation.

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We have developed a position response calibration method for a micro-channel plate (MCP) detector with a delay-line anode position readout scheme. Using an in situ calibration mask, an accuracy of 8 µm and a resolution of 85 µm (FWHM) have been achieved for MeV-scale α particles and ions with energies of ∼10 keV.At this level of accuracy, the difference between the MCP position responses to high-energy α particles and low-energy ions is significant. The improved performance of the MCP detector can find applications in many fields of AMO and nuclear physics. In our case, it helps reducing systematic uncertainties in a high-precision nuclear β-decay experiment.

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