The aCORN experiment measures the electron-antineutrino $a$-coefficient in free neutron decay. We update the previous aCORN results to include radiative and recoil corrections to first order, and discuss a key issue in the comparison of results from different $a$-coefficient experimental methods when these effects are considered. The corrected combined result is $\overline{a} = -0.10779 \pm 0.00125\, ({\rm stat}) \pm 0.00133\, ({\rm sys})$, averaged over the full Fermi neutron beta spectrum. The corresponding corrected result for the ratio of weak coupling constants $\lambda = G_A/G_V$ is $\lambda = -1.2712 \pm 0.0061$. This improves agreement with previous $a$-coefficient experiments, in particular the 2020 aSPECT result

Fundamental neutron physics, combining precision measurements and theory, probes particle physics at short range with reach well beyond the highest energies probed by the LHC. Significant US efforts are underway that will probe BSM CP violation with orders of magnitude more sensitivity, provide new data on the Cabibbo anomaly, more precisely measure the neutron lifetime and decay, and explore hadronic parity violation. World-leading results from the US Fundamental Neutron Physics community since the last Long Range Plan, include the world's most precise measurement of the neutron lifetime from UCN$\tau$, the final results on the beta-asymmetry from UCNA and new results on hadronic parity violation from the NPDGamma and n-${^3}$He runs at the FNPB (Fundamental Neutron Physics Beamline), precision measurement of the radiative neutron decay mode and n-${}^4$He at NIST. US leadership and discovery potential are ensured by the development of new high-impact experiments including BL3, Nab, LANL nEDM and nEDM@SNS. On the theory side, the last few years have seen results for the neutron EDM from the QCD $\theta$ term, a factor of two reduction in the uncertainty for inner radiative corrections in beta-decay which impacts CKM unitarity, and progress on {\it ab initio} calculations of nuclear structure for medium-mass and heavy nuclei which can eventually improve the connection between nuclear and nucleon EDMs. In order to maintain this exciting program and capitalize on past investments while also pursuing new ideas and building US leadership in new areas, the Fundamental Neutron Physics community has identified a number of priorities and opportunities for our sub-field covering the time-frame of the last Long Range Plan (LRP) under development. This white paper elaborates on these priorities.

The neutron polarization of the NG-C beamline at the NIST Center for Neutron Research was measured as part of the aCORN neutron beta decay experiment. Neutron transmission through a polarized 3He spin filter cell was recorded while adiabatic fast passage (AFP) nuclear magnetic resonance (NMR) reversed the polarization direction of the 3He in an eight-step sequence to account for drifts. The dependence of the neutron transmission on the spin filter direction was used to calculate the neutron polarization. The time dependent transmission was fit to a model which included the neutron spectrum, and 3He polarization losses from spin relaxation and AFP-NMR. The neutron polarization averaged over the spectrum of the NG-C beam was found to be P n ≤ 4 × 10 - 4 with 90 % confidence.

The aCORN experiment measures the neutron-decay electron-antineutrino correlation (a coefficient) using a novel method based on an asymmetry in proton time-of-flight for events where the beta electron and recoil proton are detected in delayed coincidence. We report the data analysis and result from the second run at the NIST Center for Neutron Research, using the high-flux cold neutron beam on the new NG-C neutron guide end position: a = -0.10758 +/- 0.00136(stat) +/- 0.00148(sys). This is consistent within uncertainties with the result from the first aCORN run on the NG-6 cold neutron beam. Combining the two aCORN runs, we obtain a = -0.10782 +/- 0.00124(stat) +/- 0.00133(sys), which has an overall relative standard uncertainty of 1.7%. The corresponding result for the ratio of weak coupling constants lambda = G(A)/G(V) is lambda = -1.2796 +/- 0.0062.

The aCORN experiment uses a novel asymmetry method to measure the electron-antineutrino correlation (a-coefficient) in free neutron decay that does not require precision proton spectroscopy. aCORN completed two physics runs at the NIST Center for Neutron Research. The first run on the NG-6 beam line obtained the result a = 0.1090 +/- 0.0030 (stat) +/- 0.0028 (sys), the most precise to date. The second run on the new NG-C high flux beam line promises an improvement in precision to ¡ 2%. In addition we show that an improved measurement of the neutrino asymmetry (B-coefficient) can be made using the aCORN apparatus on a highly polarized neutron beam.

The aCORN experiment uses a novel asymmetry method to measure the electron-antineutrino correlation (a-coefficient) in free neutron decay that does not require precision proton spectroscopy. aCORN completed two physics runs at the NIST Center for Neutron Research. The first run on the NG-6 beam line in 2013--2014 obtained the result a = 0.1090 +/- 0.0030 (stat) +/- 0.0028 (sys), a total uncertainty of 3.8%. The second run on the new NG-C high flux beam line promises an improvement in precision to <2%.

We describe an apparatus used to measure the electron-antineutrino angular correlation coefficient in free neutron decay. The apparatus employs a novel measurement technique in which the angular correlation is converted into a proton time-of-flight asymmetry that is counted directly, avoiding the need for proton spectroscopy. Details of the method, apparatus, detectors, data acquisition, and data reduction scheme are presented, along with a discussion of the important systematic effects.

We report the first result for the electron-antineutrino angular correlation (a coefficient) in free neutron β decay from the aCORN experiment. aCORN uses a novel method in which the a coefficient is proportional to an asymmetry in proton time of flight for events where the β electron and recoil proton are detected in delayed coincidence. Data are presented from a 15 month run at the NIST Center for Neutron Research. We obtained a=-0.1090±0.0030(stat)±0.0028(sys), the most precise measurement of the neutron a coefficient reported to date.

The aCORN experiment is designed to make a precision (<1%) measurement of the electron–antineutrino angular correlation (a-coefficient) in neutron beta decay. It uses a new method proposed in 1996 by Yerozolimsky and Mostovoy. Electrons and recoil protons from neutron decay in a cold beam are detected in coincidence. The momenta of the particles are selected so that the protons form two kinematically distinct time-of-flight groups as a function of electron energy. The count rate asymmetry in these two groups is proportional to the a-coefficient. Precision spectroscopy of the protons is not required. The apparatus is currently under construction. It will be integrated and tested at the Indiana University Cyclotron Facility (IUCF) and then moved to the NIST Center for Neutron Research for the initial physics run.

We have tested two 3He neutron spin filters (NSF), one for the polarizer and one for the analyzer, in conjunction with a doubly focusing pyrolytic graphite (PG) monochromator on the state-of-the-art BT-7 thermal triple axis spectrometer (TAS) at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). This system will provide significantly better neutronic performance for polarization analysis over a conventional TAS with Heusler crystals. We discuss the scheme for employing NSFs on the TAS instrument, including the 3He cell design, spin-exchange optical pumping (SEOP) of these large 3He cells, and the holding fields on the spectrometer. Using Rb/K hybrid SEOP, we have produced 75% 3He polarization for the 11 cm diameter cells for TAS in less than two days.

Spin filters based on the large spin dependence of the neutron absorption cross-section by 3He are currently being applied in neutron scattering. We report here the construction and test of a 3He neutron spin filter that incorporates (1) in situ continuous optical pumping to maximize the time-averaged polarization and maintain a stable 3He polarization during experiments, and (2) low-loss adiabatic-fast-passage inversion of the 3He polarization to eliminate the need for a neutron spin flipper. The device was successfully tested at the single-crystal diffractometer at the Intense-Pulsed Neutron Source, Argonne National Laboratory. This device can be used in measurements of static magnetic-materials as well as magnetic-relaxation phenomena with long relaxation times.

Polarized 3 He can be used as a spin filter to polarize a broad energy spectrum of neutrons. As a prototype for use on the single-crystal diffractometer (SCD) at the spallation neutron source (SNS), we have built a compact system to continuously polarize a 3 He spin filter by spin-exchange optical pumping. Polarizing in the neutron beam provides a constant neutron polarization and reduces the sensitivity to relaxation mechanisms. The capability to operate in the presence of non-optimal magnetic field homogeneity allowed us to employ a highly compact solenoid only 9.5 cm in diameter and 20 cm long. Using only 7 W of laser light we maintained 44% 3 He polarization in an 11 cm 3 cell, despite an overall cell relaxation time of ≈ 10 h . Results from a test on the SCD at IPNS are discussed.

This paper characterizes an imaging x-ray detector formed by coupling a gadolinium oxysulphide phosphor to the input of an optical imaging photon detector. The device is small, light, easy to use, and features a direct digital readout. It exhibits fairly high efficiency (40%–80%) and high resolution (160 μm-width point spread function) near the center of its 40-mm active area. There is a small amount of pincushion distortion which seems to be associated with a loss in resolution toward the edge of the active area. The device has very low noise and can be used at x-ray fluxes down to about 0.1 x-ray/mm2/s without loss of accuracy but it is count rate limited at 105 x rays/s over the active area and so is not usable in high-flux situations such as are often found at synchrotrons. We have used the device to record good diffraction patterns from striated rabbit muscle in 30 min on a rotating anode x-ray generator: less than one tenth the time needed under similar conditions when using film.