Realization of an advanced super-mirror solid-state neutron polarizer for the instrument PF1B at the Institut Laue-Langevin.

In this last of a series of three papers on the development of an advanced solid-state neutron polarizer, we present the final construction of the polarizer and the results of its commissioning. The polarizer uses spin-selective reflection of neutrons by interfaces coated with polarizing super-mirrors. The polarizer is built entirely in-house for the PF1B cold neutron beam facility at the Institut Max von Laue-Paul Langevin (ILL). It has been installed in the PF1B casemate and tested under real conditions. The average transmission for the "good" spin component is measured to be >30%. The polarization averaged over the capture spectrum reaches a record value of Pn ≈ 0.997 for the full angular divergence in the neutron beam, delivered by the H113 neutron guide, and the full wavelength band λ of 0.3-2.0 nm. This unprecedented performance is due to a series of innovations in the design and fabrication in the following domains: choice of the substrate material, super-mirror and anti-reflecting multilayer coatings, magnetizing field, and assembling process. The polarizer is used for user experiments at PF1B since the last reactor cycle in 2020.

Searching for Hidden Neutrons with a Reactor Neutrino Experiment: Constraints from the STEREO Experiment.

Different extensions of the standard model of particle physics, such as braneworld or mirror matter models, predict the existence of a neutron sterile state, possibly as a dark matter candidate. This Letter reports a new experimental constraint on the probability p for neutron conversion into a hidden neutron, set by the STEREO experiment at the high flux reactor of the Institut Laue-Langevin. The limit is p<3.1×10^{-11} at 95% C.L. improving the previous limit by a factor of 13. This result demonstrates that short-baseline neutrino experiments can be used as competitive passing-through-walls neutron experiments to search for hidden neutrons.

Accurate Measurement of the Electron Antineutrino Yield of

We report a measurement of the antineutrino rate from the fission of ^{235}U with the STEREO detector using 119 days of reactor turned on. In our analysis, we perform several detailed corrections and achieve the most precise single measurement at reactors with highly enriched ^{235}U fuel. We measure an IBD cross section per fission of σ_{f}=(6.34±0.06[stat]±0.15[sys]±0.15[model])×10^{-43} cm^{2}/fission and observe a rate deficit of (5.2±0.8[stat]±2.3[sys]±2.3[model])% compared to the model, consistent with the deficit of the world average. Testing ^{235}U as the sole source of the deficit, we find a tension between the results of lowly and highly enriched ^{235}U fuel of 2.1 standard deviations.

Limit on the Fierz Interference Term b from a Measurement of the Beta Asymmetry in Neutron Decay.

In the standard model of particle physics, the weak interaction is described by vector and axial-vector couplings only. Nonzero scalar or tensor interactions would imply an additional contribution to the differential decay rate of the neutron, the Fierz interference term. We derive a limit on this hypothetical term from a measurement using spin-polarized neutrons. This method is statistically less sensitive than the determination from the spectral shape but features much cleaner systematics. We obtain a limit of b=0.017(21) at 68.27% C.L., improving the previous best limit from neutron decay by a factor of four.

A project of advanced solid-state neutron polarizer for PF1B instrument at Institut Laue-Langevin.

Among polarizers based on the neutron reflection from Super-Mirrors (SMs), solid-state neutron-optical devices have many advantages. The most relevant is the 5-10 times smaller size along the neutron beam direction compared to more traditional air-gap devices. An important condition for a good SM polarizer is the matching of the substrate SLD (Scattering Length Density) with the SM coating SLD for the spin-down component. For traditional Fe/Si SM on the Si substrate, this SLD step is positive when a neutron goes from the substrate to the SM, which leads to a significant degradation of the polarizer performance in the small Q region. This can be solved by replacing single-crystal Si substrates by single-crystal sapphire or quartz substrates. The latter shows a negative SLD step for the spin-down neutron polarization component at the interface with Fe and, therefore, avoid the total reflection regime in the small Q region. In order to optimize the polarizer performance, we formulate the concept of sapphire V-bender. We perform ray-tracing simulations of sapphire V-bender, compare results with those for traditional C-bender on Si, and study experimentally V-bender prototypes with different substrates. Our results show that the choice of substrate material, polarizer geometry, as well the strength and quality of magnetizing field have dramatic effect on the polarizer performance. In particular, we compare the performance of polarizer for the applied magnetic field strength of 50 mT and 300 mT. Only the large field strength (300 mT) provides an excellent agreement between the simulated and measured polarization values. For the double-reflection configuration, a record polarization >0.999 was obtained in the neutron wavelength band of 0.3-1.2 nm with only 1% decrease at 2 nm. Without any collimation, the polarization averaged over the full outgoing capture spectrum, 0.997, was found to be equal to the value obtained previously using only a double polarizer in the "crossed" (X-SM) geometry. These results are applied in a full-scale polarizer for the PF1B instrument.

Measurement of the Weak Axial-Vector Coupling Constant in the Decay of Free Neutrons Using a Pulsed Cold Neutron Beam.

We present a precision measurement of the axial-vector coupling constant g_{A} in the decay of polarized free neutrons. For the first time, a pulsed cold neutron beam was used for this purpose. By this method, leading sources of systematic uncertainty are suppressed. From the electron spectra we obtain λ=g_{A}/g_{V}=-1.27641(45)_{stat}(33)_{sys}, which confirms recent measurements with improved precision. This corresponds to a value of the parity violating beta asymmetry parameter of A_{0}=-0.11985(17)_{stat}(12)_{sys}. We discuss implications on the Cabibbo-Kobayashi-Maskawa matrix element V_{ud} and derive a limit on left-handed tensor interaction.

Constraints on the Dark Matter Interpretation n→χ+e

Discrepancies from in-beam- and in-bottle-type experiments measuring the neutron lifetime are on the 4σ standard deviation level. In a recent publication Fornal and Grinstein proposed that the puzzle could be solved if the neutron would decay on the one percent level via a dark decay mode, one possible branch being n→χ+e^{+}e^{-}. With data from the Perkeo II experiment we set limits on the branching fraction and exclude a one percent contribution for 95% of the allowed mass range for the dark matter particle.

Sterile Neutrino Constraints from the STEREO Experiment with 66 Days of Reactor-On Data.

The reactor antineutrino anomaly might be explained by the oscillation of reactor antineutrinos toward a sterile neutrino of eV mass. In order to explore this hypothesis, the STEREO experiment measures the antineutrino energy spectrum in six different detector cells covering baselines between 9 and 11 m from the compact core of the ILL research reactor. In this Letter, results from 66 days of reactor turned on and 138 days of reactor turned off are reported. A novel method to extract the antineutrino rates has been developed based on the distribution of the pulse shape discrimination parameter. The test of a new oscillation toward a sterile neutrino is performed by comparing ratios of cells, independent of absolute normalization and of the prediction of the reactor spectrum. The results are found to be compatible with the null oscillation hypothesis and the best fit of the reactor antineutrino anomaly is excluded at 97.5% C.L.

Determination of the weak axial vector coupling λ = gA/gV from a measurement of the ß-asymmetry parameter A in neutron beta decay.

We report on a new measurement of the neutron ß-asymmetry parameter A with the instrument Perkeo II. The main enhancements are the high neutron polarization of P = 99.7(1)% from a novel arrangement of supermirror polarizers and reduced background from improvements in beam line and shielding. The leading corrections were thus reduced by a factor of 4, pushing them below the level of statistical error and resulting in a significant reduction of systematic uncertainty compared to our previous experiments. We derive the ß-asymmetry parameter A0 = -0.11972(45)(stat)((-44)(+32))(sys) = -0.11972((-65)(+53)) and the ratio of the axial vector to the vector coupling constant λ = gA/gV = -1.2761(12)(stat)((-12)(+9))(sys) = -1.2761((-17)(+14)).

Neutron beam effects on spin-exchange-polarized 3He.

We have observed depolarization effects when high intensity cold neutron beams are incident on alkali-metal spin-exchange-polarized 3He cells used as neutron spin filters. This was first observed as a reduction of the maximum attainable 3He polarization and was attributed to a decrease of alkali-metal polarization, which led us to directly measure alkali-metal polarization and spin relaxation over a range of neutron fluxes at Los Alamos Neutron Science Center and Institute Laue-Langevin. The data reveal a new alkali-metal spin-relaxation mechanism that approximately scales as sqrt[phi_{n}], where phi_{n} is the neutron capture-flux density incident on the cell. This is consistent with an effect proportional to the concentration of electron-ion pairs but is much larger than expected from earlier work.

Measurement of the proton asymmetry parameter in neutron beta decay.

The proton asymmetry parameter C in neutron decay describes the angular correlation between neutron spin and proton momentum. In this Letter, the first measurement of this quantity is presented. The result C= -0.2377(26) agrees with the standard model expectation. The coefficient C provides an additional parameter for new and improved standard model tests.

Measurement of the neutrino asymmetry parameter B in neutron decay.

A new measurement of the neutrino asymmetry parameter B in neutron decay, the angular correlation between neutron spin and antineutrino momentum, is presented. The result, B=0.9802(50), confirms earlier measurements but features considerably smaller corrections. It agrees with the standard model expectation and permits updated tests on "new physics" in neutron decay.

The Beta-, Neutrino- and Proton-Asymmetry in Neutron ß-Decay.

This article describes measurements of angular-correlation coefficients in the decay of free neutrons with the superconducting spectrometer PERKEO II. A method for measuring the ß-asymmetry coefficient A is presented, as well as a new method for determining the neutrino-asymmetry coefficient B, which allows a value for the proton-asymmetry coefficient C to be obtained for the first time. An ongoing experiment is trying to improve the accuracy of these quantities.