Search

Here we publish three years of data for the UCNtau experiment performed at the Los Alamos Ultra Cold Neutron Facility at the Los Alamos Neutron Science Center. These data are in addition to our previously published data. Our goals in this paper are to better understand and quantify systematic uncertainties and to improve the lifetime statistical precision. We report a measured value for these runs from 2020-2022 for the neutron lifetime of 877.94+/-0.37 s; when all the data from UCNtau are averaged we report an updated value for the lifetime of 877.82+/-0.22 (statistical)+0.20-0.17 (systematic) s. We utilized improved monitor detectors, reduced our correction due to UCN upscattering on ambient gas, and employed four different main UCN detector geometries both to reduce the correction required for rate dependence and explore potential contributions due to phase space evolution.

The past two decades have yielded several new measurements and reanalysis of older measurements of the neutron lifetime. These have led to a 4.4 standard deviation discrepancy between the most precise measurements of the neutron decay rate producing protons in cold neutron beams and the most precise lifetime measured in neutron storage experiments. Here we publish an analysis of the recently published UCN aimed a searching for an explanation of this difference using the model proposed by Koch and Hummel.

We have developed a two-dimensional neutron imager based on a semiconductor pixelated sensor, especially designed for experiments measuring of a spatial and a temporal behavior of quantum bound states of ultra-cold neutrons. Through these measurements, we expect to measure the ratio between the inertial and gravitational masses of neutrons and to test the equivalence principle in the quantum regime. As one of the principal neutron imagers, we fabricated a sensor with a sandwich configuration, named 10B-INTPIX4-sw, and tested its response to ultra-cold neutrons at the Los Alamos Neutron Science Center (LANSCE). We observed simultaneous events on both sandwiching sensors without significant loss of detection efficiency. The efficiency was evaluated to be about 16%, relative to the 10B/ZnS reference detector. The coincidence condition reduces its efficiency by a factor of about 3.

The upcoming UCNProBe experiment at Los Alamos National Laboratory will measure the $\beta$-decay rate of free neutrons with different systematic uncertainties than previous beam-based neutron lifetime experiments. We have developed a new $^{10}$B-coated YAP:Ce scintillator whose properties are presented. The advantage of the YAP:Ce scintillator is its high Fermi potential, which reduces the probability for upscattering of ultracold neutrons, and its short decay time, which is important at high counting rates. Birks' coefficient of YAP:Ce was measured to be ($5.56^{+0.05}_{-0.30})\times 10^{-4}$ cm/MeV and light losses due to 120 nm of $^{10}$B-coating to be about 60%. The loss of light from YAP:Ce due to transmission through deuterated polystyrene scintillator was about 50%. The efficiency for counting neutrons that are captured on the $^{10}$B coating is (86.82 $\pm$ 2.61)%. Measurement with ultracold neutrons showed that YAP:Ce crystal counted 8% to 28% more UCNs compared to ZnS screen. This may be due to an uneven coating of $^{10}$B on the rough surface.

A study of the dead layer thickness and quenching factor of a plastic scintillator for use in ultracold neutron (UCN) experiments is described. Alpha spectroscopy was used to determine the thickness of a thin surface dead layer, and the relative light outputs from the decay of $^{241}$Am and Compton scattering of electrons were used to extract the quenching parameter. With these characteristics of the material known, the light yield of the scintillator can be calculated. The ability to make these scintillators deuterated, accompanied by its relatively thin dead layer, make it ideal for use in UCN experiment, where the light yield of decay electrons and alphas from neutron capture are critical for counting events.

High spatial resolution of ultracold neutron (UCN) measurement is of growing interest to UCN experiments such as UCN spectrometers, UCN polarimeters, quantum physics of UCNs, and quantum gravity. Here we utilize physics-informed deep learning to enhance the experimental position resolution and to demonstrate sub-micron spatial resolutions for UCN position measurements obtained using a room-temperature CMOS sensor, extending our previous work [1, 2] that demonstrated a position uncertainty of 1.5 microns. We explore the use of the open-source software Allpix Squared to generate experiment-like synthetic hit images with ground-truth position labels. We use physics-informed deep learning by training a fully-connected neural network (FCNN) to learn a mapping from input hit images to output hit position. The automated analysis for sub-micron position resolution in UCN detection combined with the fast data rates of current and next generation UCN sources will enable improved precision for future UCN research and applications.

The neutron electric dipole moment (nEDM) experiment that is currently being developed at Los Alamos National Laboratory (LANL) will use ultracold neutrons (UCN) and Ramsey's method of separated oscillatory fields to search for a nEDM. In this paper, we present measurements of UCN storage and UCN transport performed during the commissioning of a new beamline at the LANL UCN source and demonstrate a sufficient number of stored polarized UCN to achieve a statistical uncertainty of $\delta d_n = 2\times 10^{-27}$~$e\cdot\text{cm}$ in 5 calendar years of running. We also present an analytical model describing data that provides a simple parameterization of the input UCN energy spectrum on the new beamline., Comment: 12 pages, 13 figures

Electroless nickel plating is an established industrial process that provides a robust and relatively low-cost coating suitable for transporting and storing ultracold neutrons (UCN). Using roughness measurements and UCN-storage experiments we characterized UCN guides made from polished aluminum or stainless-steel tubes plated by several vendors. All electroless nickel platings were similarly suited for UCN storage with an average loss probability per wall bounce of $2.8\cdot10^{-4}$ to $4.1\cdot10^{-4}$ for energies between 90 neV and 190 neV, or a ratio of imaginary to real Fermi potential $\eta$ of $1.7\cdot10^{-4}$ to $3.3\cdot10^{-4}$. Measurements at different elevations indicate that the energy dependence of UCN losses is well described by the imaginary Fermi potential. Some special considerations are required to avoid an increase in surface roughness during the plating process and hence a reduction in UCN transmission. Increased roughness had only a minor impact on storage properties. Based on these findings we chose a vendor to plate the UCN-production vessel that will contain the superfluid-helium converter for the new TRIUMF UltraCold Advanced Neutron (TUCAN) source, achieving acceptable UCN-storage properties with ${\eta=3.5(5)\cdot10^{-4}}$., Comment: 8 pages, 7 figures

The past two decades have yielded several new measurements and reanalyses of older measurements of the neutron lifetime. These have led to a 4.4 standard deviation discrepancy between the most precise measurements of the neutron decay rate producing protons in cold neutron beams and the lifetime measured in neutron storage experiments. Measurements using different techniques are important for investigating whether there are unidentified systematic effects in any of the measurements. In this paper we report a new measurement using the Los Alamos asymmetric magneto-gravitational trap where the surviving neutrons are counted external to the trap using the fill and dump method. The new measurement gives a free neutron lifetime of . Although this measurement is not as precise, it is in statistical agreement with previous results using in situ counting in the same apparatus.

We report an improved measurement of the free neutron lifetime $\tau_{n}$ using the UCN$\tau$ apparatus at the Los Alamos Neutron Science Center. We counted a total of approximately $38\times10^{6}$ surviving ultracold neutrons (UCN) after storing in UCN$\tau$'s magneto-gravitational trap over two data acquisition campaigns in 2017 and 2018. We extract $\tau_{n}$ from three blinded, independent analyses by both pairing long and short storage-time runs to find a set of replicate $\tau_{n}$ measurements and by performing a global likelihood fit to all data while self-consistently incorporating the $\beta$-decay lifetime. Both techniques achieve consistent results and find a value $\tau_{n}=877.75\pm0.28_{\text{ stat}}+0.22/-0.16_{\text{ syst}}$~s. With this sensitivity, neutron lifetime experiments now directly address the impact of recent refinements in our understanding of the standard model for neutron decay., Comment: 7 pages, 2 figures, 3 tables

In this paper we report studies of the Fermi potential and loss per bounce of ultracold neutron (UCN) on a deuterated scintillator (Eljen-299-02D). These UCN properties of the scintillator enables a wide variety of applications in fundamental neutron research., Comment: 6 pages, 6 figures

The Ultracold Neutron Asymmetry (UCNA) experiment was designed to measure the $\beta$-decay asymmetry parameter, $A_0$, for free neutron decay. In the experiment, polarized ultracold neutrons are transported into a decay trap, and their $\beta$-decay electrons are detected with $\approx 4\pi$ acceptance into two detector packages which provide position and energy reconstruction. The experiment also has sensitivity to $b_{n}$, the Fierz interference term in the neutron $\beta$-decay rate. In this work, we determine $b_{n}$ from the energy dependence of $A_0$ using the data taken during the UCNA 2011-2013 run. In addition, we present the same type of analysis using the earlier 2010 $A$ dataset. Motivated by improved statistics and comparable systematic errors compared to the 2010 data-taking run, we present a new $b_{n}$ measurement using the weighted average of our asymmetry dataset fits, to obtain $b_{n} = 0.066 \pm 0.041_{\text{stat}} \pm 0.024_{\text{syst}}$ which corresponds to a limit of $-0.012 < b_{n} < 0.144$ at the 90% confidence level., Comment: 6 pages, 3 figures. Submission to PRC

A cryogenic apparatus is described that enables a new experiment, nEDM@SNS, with a major improvement in sensitivity compared to the existing limit in the search for a neutron Electric Dipole Moment (EDM). It uses superfluid $^4$He to produce a high density of Ultra-Cold Neutrons (UCN) which are contained in a suitably coated pair of measurement cells. The experiment, to be operated at the Spallation Neutron Source at Oak Ridge National Laboratory, uses polarized $^3$He from an Atomic Beam Source injected into the superfluid $^4$He and transported to the measurement cells as a co-magnetometer. The superfluid $^4$He is also used as an insulating medium allowing significantly higher electric fields, compared to previous experiments, to be maintained across the measurement cells. These features provide an ultimate statistical uncertainty for the EDM of $2-3\times 10^{-28}$ e-cm, with anticipated systematic uncertainties below this level.

The physics model of a next-generation spallation-driven high-current ultracold neutron (UCN) source capable of delivering an extracted UCN rate of around an-order-of-magnitude higher than the strongest proposed sources, and around three-orders-of-magnitude higher than existing sources, is presented. This UCN-current-optimized source would dramatically improve cutting-edge UCN measurements that are currently statistically limited. A novel "Inverse Geometry" design is used with 40 L of superfluid $^4$He (He-II), which acts as a converter of cold neutrons (CNs) to UCNs, cooled with state-of-the-art sub-cooled cryogenic technology to $\sim$1.6 K. Our design is optimized for a 100 W maximum heat load constraint on the He-II and its vessel. In our geometry, the spallation target is wrapped symmetrically around the UCN converter to permit raster scanning the proton beam over a relatively large volume of tungsten spallation target to reduce the demand on the cooling requirements, which makes it reasonable to assume that water edge-cooling only is sufficient. Our design is refined in several steps to reach $P_{UCN}=2.1\times10^9\,/$s under our other restriction of 1 MW maximum available proton beam power. We then study effects of the He-II scattering kernel as well as reductions in $P_{UCN}$ due to pressurization to reach $P_{UCN}=1.8\times10^9\,/$s. Finally, we provide a design for the UCN extraction system that takes into account the required He-II heat transport properties and implementation of a He-II containment foil that allows UCN transmission. We estimate a total useful UCN current from our source of $R_{use}=5\times10^8\,/$s from a 18 cm diameter guide 5 m from the source. Under a conservative "no return" approximation, this rate can produce an extracted density of $>1\times10^4\,/$cm$^3$ in $<$1000~L external experimental volumes with a $^{58}$Ni (335 neV) cut-off potential., Comment: Submitted to Journal of Applied Physics

The UCNA experiment was designed to measure the neutron $\beta$-asymmetry parameter $A_0$ using polarized ultracold neutrons (UCN). UCN produced via downscattering in solid deuterium were polarized via transport through a 7 T magnetic field, and then directed to a 1 T solenoidal electron spectrometer, where the decay electrons were detected in electron detector packages located on the two ends of the spectrometer. A value for $A_0$ was then extracted from the asymmetry in the numbers of counts in the two detector packages. We summarize all of the results from the UCNA experiment, obtained during run periods in 2007, 2008--2009, 2010, and 2011--2013, which ultimately culminated in a 0.67\% precision result for $A_0$., Comment: 6 pages, 7 figures, to appear in proceedings of the International Workshop on Particle Physics at Neutron Sources 2018

Novel experimental techniques are required to make the next big leap in neutron electric dipole moment experimental sensitivity, both in terms of statistics and systematic error control. The nEDM experiment at the Spallation Neutron Source (nEDM@SNS) will implement the scheme of Golub & Lamoreaux [Phys. Rep., 237, 1 (1994)]. The unique properties of combining polarized ultracold neutrons, polarized $^3$He, and superfluid $^4$He will be exploited to provide a sensitivity to $\sim 10^{-28}\,e{\rm \,\cdot\, cm}$. Our cryogenic apparatus will deploy two small ($3\,{\rm L}$) measurement cells with a high density of ultracold neutrons produced and spin analyzed in situ. The electric field strength, precession time, magnetic shielding, and detected UCN number will all be enhanced compared to previous room temperature Ramsey measurements. Our $^3$He co-magnetometer offers unique control of systematic effects, in particular the Bloch-Siegert induced false EDM. Furthermore, there will be two distinct measurement modes: free precession and dressed spin. This will provide an important self-check of our results. Following five years of "critical component demonstration," our collaboration transitioned to a "large scale integration" phase in 2018. An overview of our measurement techniques, experimental design, and brief updates are described in these proceedings., Comment: Submitted to proceedings of PPNS 2018 - International Workshop on Particle physics at Neutron Sources (https://www.webofconferences.org/epj-web-of-conferences-forthcoming-conferences/1148-ppns-2018)

A new boron-coated CCD camera is described for direct detection of ultracold neutrons (UCN) through the capture reactions $^{10}$B (n,$\alpha$0$\gamma$)$^7$Li (6%) and $^{10}$B(n,$\alpha$1$\gamma$)$^7$Li (94%). The experiments, which extend earlier works using a boron-coated ZnS:Ag scintillator, are based on direct detections of the neutron-capture byproducts in silicon. The high position resolution, energy resolution and particle ID performance of a scientific CCD allows for observation and identification of all the byproducts $\alpha$, $^7$Li and $\gamma$ (electron recoils). A signal-to-noise improvement on the order of 10$^4$ over the indirect method has been achieved. Sub-pixel position resolution of a few microns is demonstrated. The technology can also be used to build UCN detectors with an area on the order of 1 m$^2$. The combination of micrometer scale spatial resolution, few electrons ionization thresholds and large area paves the way to new research avenues including quantum physics of UCN and high-resolution neutron imaging and spectroscopy., Comment: 10 pages, 8 figures

The Nab experiment will measure the ratio of the weak axial-vector and vector coupling constants $\lambda=g_A/g_V$ with precision $\delta\lambda/\lambda\sim3\times10^{-4}$ and search for a Fierz term $b_F$ at a level $\Delta b_F<10^{-3}$. The Nab detection system uses thick, large area, segmented silicon detectors to very precisely determine the decay proton's time of flight and the decay electron's energy in coincidence and reconstruct the correlation between the antineutrino and electron momenta. Excellent understanding of systematic effects affecting timing and energy reconstruction using this detection system are required. To explore these effects, a series of ex situ studies have been undertaken, including a search for a Fierz term at a less sensitive level of $\Delta b_F<10^{-2}$ in the beta decay of $^{45}$Ca using the UCNA spectrometer., Comment: Proceedings of the 7th International Syposium on Symmetries in Subatomic Physics SSP2018, Aachen (Germany), 10 - 15 Jun 2018. This is a pre-print of an article published in Hyperfine Interactions. The final authenticated version is available online at: https://doi.org/10.1007/s10751-018-1538-7

Neutron beta decay is one of the most fundamental processes in nuclear physics and provides sensitive means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, $\lambda = g_A / g_V$, through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlation parameter $a$ with a precision of $\delta a / a = 10^{-3}$ and the Fierz interference term $b$ to $\delta b = 3\times10^{-3}$ in unpolarized free neutron beta decay. These results, along with a more precise measurement of the neutron lifetime, aim to deliver an independent determination of the ratio $\lambda$ with a precision of $\delta \lambda / \lambda = 0.03\%$ that will allow an evaluation of $V_{ud}$ and sensitively test CKM unitarity, independent of nuclear models. Nab utilizes a novel, long asymmetric spectrometer that guides the decay electron and proton to two large area silicon detectors in order to precisely determine the electron energy and an estimation of the proton momentum from the proton time of flight. The Nab spectrometer is being commissioned at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source at Oak Ridge National Lab. We present an overview of the Nab experiment and recent updates on the spectrometer, analysis, and systematic effects., Comment: Presented at PPNS2018

In the UCN{\tau} experiment, ultracold neutrons (UCN) are confined by magnetic fields and the Earth's gravitational field. Field-trapping mitigates the problem of UCN loss on material surfaces, which caused the largest correction in prior neutron experiments using material bottles. However, the neutron dynamics in field traps differ qualitatively from those in material bottles. In the latter case, neutrons bounce off material surfaces with significant diffusivity and the population quickly reaches a static spatial distribution with a density gradient induced by the gravitational potential. In contrast, the field-confined UCN -- whose dynamics can be described by Hamiltonian mechanics -- do not exhibit the stochastic behaviors typical of an ideal gas model as observed in material bottles. In this report, we will describe our efforts to simulate UCN trapping in the UCN{\tau} magneto-gravitational trap. We compare the simulation output to the experimental results to determine the parameters of the neutron detector and the input neutron distribution. The tuned model is then used to understand the phase space evolution of neutrons observed in the UCN{\tau} experiment. We will discuss the implications of chaotic dynamics on controlling the systematic effects, such as spectral cleaning and microphonic heating, for a successful UCN lifetime experiment to reach a 0.01% level of precision., Comment: 18 pages, 19 figures

Solid deuterium (sD_2) is used as an efficient converter to produce ultracold neutrons (UCN). It is known that the sD_2 must be sufficiently cold, of high purity and mostly in its ortho-state in order to guarantee long lifetimes of UCN in the solid from which they are extracted into vacuum. Also the UCN transparency of the bulk sD_2 material must be high because crystal inhomogeneities limit the mean free path for elastic scattering and reduce the extraction efficiency. Observations at the UCN sources at Paul Scherrer Institute and at Los Alamos National Laboratory consistently show a decrease of the UCN yield with time of operation after initial preparation or later treatment (`conditioning') of the sD_2. We show that, in addition to the quality of the bulk sD_2, the quality of its surface is essential. Our observations and simulations support the view that the surface is deteriorating due to a build-up of D_2 frost-layers under pulsed operation which leads to strong albedo reflections of UCN and subsequent loss. We report results of UCN yield measurements, temperature and pressure behavior of deuterium during source operation and conditioning, and UCN transport simulations. This, together with optical observations of sD_2 frost formation on initially transparent sD_2 in offline studies with pulsed heat input at the North Carolina State University UCN source results in a consistent description of the UCN yield decrease., Comment: 15 pages, 22 figures, accepted by EPJ-A

It has been proposed recently that a previously unobserved neutron decay branch to a dark matter particle ($\chi$) could account for the discrepancy in the neutron lifetime observed in experiments that use two different measurement techniques. One of the possible final states discussed includes a single $\chi$ along with an $e^{+}e^{-}$ pair. We use data from the UCNA (Ultracold Neutron Asymmetry) experiment to set limits on this decay channel. Coincident electron-like events are detected with $\sim 4\pi$ acceptance using a pair of detectors that observe a volume of stored Ultracold Neutrons (UCNs). The summed kinetic energy ($E_{e^{+}e^{-}}$) from such events is used to set limits, as a function of the $\chi$ mass, on the branching fraction for this decay channel. For $\chi$ masses consistent with resolving the neutron lifetime discrepancy, we exclude this as the dominant dark matter decay channel at $\gg~5\sigma$ level for $100~\text{keV} < E_{e^{+}e^{-}} < 644~\text{keV}$. If the $\chi+e^{+}e^{-}$ final state is not the only one, we set limits on its branching fraction of $< 10^{-4}$ for the above $E_{e^{+}e^{-}}$ range at $> 90\%$ confidence level., Comment: 5 pages, 5 figures

In a recent paper submitted to Physical Review Letters, Fornal and Grinstein have suggested that the discrepancy between two different methods of neutron lifetime measurements, the beam and bottle methods can be explained by a previously unobserved dark matter decay mode, n$\rightarrow$ X+$\gamma$ where X is a dark matter particle. We have performed a search for this decay mode over the allowed range of energies of the monoenergetic gamma ray for X to be a dark matter particle. We exclude the possibility of a sufficiently strong branch to explain the lifetime discrepancy with greater than 4 sigma confidence., Comment: 6 pages 3 figures

The neutron $\beta$-decay asymmetry parameter $A_0$ defines the correlation between the spin of the neutron and the momentum of the emitted electron, which determines $\lambda=\frac{g_{A}}{g_{V}}$, the ratio of the axial-vector to vector weak coupling constants. The UCNA Experiment, located at the Ultracold Neutron facility at the Los Alamos Neutron Science Center, is the first to measure such a correlation coefficient using ultracold neutrons (UCN). Following improvements to the systematic uncertainties and increased statistics, we report the new result $A_0 = -0.12054(44)_{\mathrm{stat}}(68)_{\mathrm{syst}}$ which yields $\lambda\equiv \frac{g_{A}}{g_{V}}=-1.2783(22)$. Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces $A_0=-0.12015(34)_{\mathrm{stat}}(63)_{\mathrm{syst}}$ and $\lambda\equiv \frac{g_{A}}{g_{V}}=-1.2772(20)$., Comment: 9 pages, 7 figures, updated to as-published version

The ultracold neutron (UCN) source at Los Alamos National Laboratory (LANL), which uses solid deuterium as the UCN converter and is driven by accelerator spallation neutrons, has been successfully operated for over 10 years, providing UCN to various experiments, as the first production UCN source based on the superthermal process. It has recently undergone a major upgrade. This paper describes the design and performance of the upgraded LANL UCN source. Measurements of the cold neutron spectrum and UCN density are presented and compared to Monte Carlo predictions. The source is shown to perform as modeled. The UCN density measured at the exit of the biological shield was $184(32)$ UCN/cm$^3$, a four-fold increase from the highest previously reported. The polarized UCN density stored in an external chamber was measured to be $39(7)$ UCN/cm$^3$, which is sufficient to perform an experiment to search for the nonzero neutron electric dipole moment with a one-standard-deviation sensitivity of $\sigma(d_n) = 3\times 10^{-27}$ $e\cdot$cm., Comment: 6 pages, 6 figures

The precise value of the mean neutron lifetime, $\tau_n$, plays an important role in nuclear and particle physics and cosmology. It is a key input for predicting the ratio of protons to helium atoms in the primordial universe and is used to search for new physics beyond the Standard Model of particle physics. There is a 3.9 standard deviation discrepancy between $\tau_n$ measured by counting the decay rate of free neutrons in a beam (887.7 $\pm$ 2.2 s) and by counting surviving ultracold neutrons stored for different storage times in a material trap (878.5$\pm$0.8 s). The experiment described here eliminates loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls and neutrons in quasi-stable orbits rapidly exit the trap. As a result of this approach and the use of a new in situ neutron detector, the lifetime reported here (877.7 $\pm$ 0.7 (stat) +0.4/-0.2 (sys) s) is the first modern measurement of $\tau_n$ that does not require corrections larger than the quoted uncertainties., Comment: 9 pages, 3 figures, 2 tables

Precision measurements of free neutron $\beta$-decay have been used to precisely constrain our understanding of the weak interaction. However the neutron Fierz interference term $b_n$, which is particularly sensitive to Beyond-Standard-Model tensor currents at the TeV scale, has thus far eluded measurement. Here we report the first direct constraints on this term, finding $b_n = 0.067 \pm 0.005_{\text{stat}} {}^{+0.090}_{- 0.061}{}_{\text{sys}}$, consistent with the Standard Model. The uncertainty is dominated by absolute energy reconstruction and the linearity of the beta spectrometer energy response.

We report on the evaluation of commercial electroless nickel phosphorus (NiP) coatings for ultracold neutron (UCN) transport and storage. The material potential of 50~$\mu$m thick NiP coatings on stainless steel and aluminum substrates was measured to be $V_F = 213(5.2)$~neV using the time-of-flight spectrometer ASTERIX at the Lujan Center. The loss per bounce probability was measured in pinhole bottling experiments carried out at ultracold neutron sources at Los Alamos Neutron Science Center and the Institut Laue-Langevin. For these tests a new guide coupling design was used to minimize gaps between the guide sections. The observed UCN loss in the bottle was interpreted in terms of an energy independent effective loss per bounce, which is the appropriate model when gaps in the system and upscattering are the dominate loss mechanisms, yielding a loss per bounce of $1.3(1) \times 10^{-4}$. We also present a detailed discussion of the pinhole bottling methodology and an energy dependent analysis of the experimental results., Comment: 13 pages, 14 figures

The neutron lifetime is important in understanding the production of light nuclei in the first minutes after the big bang and it provides basic information on the charged weak current of the standard model of particle physics. Two different methods have been used to measure the neutron lifetime: disappearance measurements using bottled ultracold neutrons and decay rate measurements using neutron beams. The best measurements using these two techniques give results that differ by nearly 4 standard deviations. In this paper we describe a new method for measuring surviving neutrons in neutron lifetime measurements using bottled ultracold neutrons that provides better characterization of systematic uncertainties and enables higher precision than previous measurement techniques. We present results obtained using our method.

We describe a detection system designed for precise measurements of angular correlations in neutron $\beta$ decay. The system is based on thick, large area, highly segmented silicon detectors developed in collaboration with Micron Semiconductor, Ltd. The prototype system meets specifications for $\beta$ electron detection with energy thresholds below 10 keV, energy resolution of $\sim$3 keV FWHM, and rise time of $\sim$50 ns with 19 of the 127 detector pixels instrumented. Using ultracold neutrons at the Los Alamos Neutron Science Center, we have demonstrated the coincident detection of $\beta$ particles and recoil protons from neutron $\beta$ decay. The fully instrumented detection system will be implemented in the UCNB and Nab experiments, to determine the neutron $\beta$ decay parameters $B$, $a$, and $b$., Comment: Copyright 2016. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

Position-sensitive detection of ultracold neutrons (UCNs) is demonstrated using an imaging charge-coupled device (CCD) camera. A spatial resolution less than 15 $\mu$m has been achieved, which is equivalent to an UCN energy resolution below 2 pico-electron-volts through the relation $\delta E = m_0g \delta x$. Here, the symbols $\delta E$, $\delta x$, $m_0$ and $g$ are the energy resolution, the spatial resolution, the neutron rest mass and the gravitational acceleration, respectively. A multilayer surface convertor described previously is used to capture UCNs and then emits visible light for CCD imaging. Particle identification and noise rejection are discussed through the use of light intensity profile analysis. This method allows different types of UCN spectroscopy and other applications., Comment: 12 figures, 28 pages, accepted for publication in NIMA

We report a measurement of the spin-flip probabilities for ultracold neutrons interacting with surfaces coated with nickel phosphorus. For 50~$\mu$m thick nickel phosphorus coated on stainless steel, the spin-flip probability per bounce was found to be $\beta_{\rm NiP\;on\;SS} = (3.3^{+1.8}_{-5.6}) \times 10^{-6}$. For 50~$\mu$m thick nickel phosphorus coated on aluminum, the spin-flip probability per bounce was found to be $\beta_{\rm NiP\;on\;Al} = (3.6^{+2.1}_{-5.9}) \times 10^{-6}$. For the copper guide used as reference, the spin flip probability per bounce was found to be $\beta_{\rm Cu} = (6.7^{+5.0}_{-2.5}) \times 10^{-6}$. The results on the nickel phosphorus-coated surfaces may be interpreted as upper limits, yielding $\beta_{\rm NiP\;on\;SS} < 6.2 \times 10^{-6}$ (90\% C.L.) and $\beta_{\rm NiP\;on\;Al} < 7.0 \times 10^{-6}$ (90\% C.L.) for 50~$\mu$m thick nickel phosphorus coated on stainless steel and 50~$\mu$m thick nickel phosphorus coated on aluminum, respectively. Nickel phosphorus coated stainless steel or aluminum provides a solution when low-cost, mechanically robust, and non-depolarizing UCN guides with a high-Fermi-potential are needed., Comment: 12 pages, 5 figures

We have constructed an apparatus to study DC electrical breakdown in liquid helium at temperatures as low as 0.4 K and at pressures between the saturated vapor pressure and $\sim$600 torr. The apparatus can house a set of electrodes that are 12 cm in diameter with a gap of $1-2$ cm between them, and a potential up to $\pm 50$ kV can be applied to each electrode. Initial results demonstrated that it is possible to apply fields exceeding 100 kV/cm in a 1 cm gap between two electropolished stainless steel electrodes 12 cm in diameter for a wide range of pressures at 0.4 K. We also measured the current between two electrodes. Our initial results, $I<1$ pA at 45 kV, correspond to a lower bound on the effective volume resistivity of LHe of $\rho_V > 5\times10^{18}$ $\Omega\cdot$cm. This lower bound is 5 times larger than the bound previously measured. We report the design, construction, and operational experience of the apparatus, as well as initial results., Comment: 15 pages, 14 figures, accepted for publication in Review of Scientific Instruments

A multilayer surface detector for ultracold neutrons (UCNs) is described. The top $^{10}$B layer is exposed to the vacuum chamber and directly captures UCNs. The ZnS:Ag layer beneath the $^{10}$B layer is a few microns thick, which is sufficient to detect the charged particles from the $^{10}$B(n,$\alpha$)$^7$Li neutron-capture reaction, while thin enough so that ample light due to $\alpha$ and $^7$Li escapes for detection by photomultiplier tubes. One-hundred-nm thick $^{10}$B layer gives high UCN detection efficiency, as determined by the mean UCN kinetic energy, detector materials and others. Low background, including negligible sensitivity to ambient neutrons, has also been verified through pulse-shape analysis and comparisons with other existing $^3$He and $^{10}$B detectors. This type of detector has been configured in different ways for UCN flux monitoring, development of UCN guides and neutron lifetime research., Comment: 21 pages, 10 figures

We present the status of current US experimental efforts to measure the lifetime of the free neutron by the "beam" and "bottle" methods. BBN nucleosynthesis models require accurate measurements with 1 second uncertainties, which are currently feasible. For tests of physics beyond the standard model, future efforts will need to achieve uncertainties well below 1 second. We outline paths achieve both.

The radial spread of charged particles emitted from a point source in a magnetic field is a potential source of systematic error for any experiment where magnetic fields guide charged particles to detectors with finite size. Assuming uniform probability as a function of the phase along the particle's helical trajectory, an analytic solution for the radial probability distribution function follows which applies to experiments in which particles are generated throughout a volume that spans a sufficient length along the axis of a homogeneous magnetic field. This approach leads to the same result as a different derivation given by Dubbers et al. But the constant phase approximation does not strictly apply to finite source volumes or fixed positions, which lead to local maxima in the radial distribution of emitted particles at the plane of the detector. A simple method is given to calculate such distributions, then the effect is demonstrated with data from a $^{207}$Bi electron-conversion source in the superconducting solenoid magnet spectrometer of the Ultracold Neutron facility at the Los Alamos Neutron Science Center. Potential future applications of this effect are discussed., Comment: 6 pages, 7 figures, submitted to Review of Scientific Instruments

The UCN$\tau$ experiment is designed to measure the lifetime $\tau_{n}$ of the free neutron by trapping ultracold neutrons (UCN) in a magneto-gravitational trap. An asymmetric bowl-shaped NdFeB magnet Halbach array confines low-field-seeking UCN within the apparatus, and a set of electromagnetic coils in a toroidal geometry provide a background "holding" field to eliminate depolarization-induced UCN loss caused by magnetic field nodes. We present a measurement of the storage time $\tau_{store}$ of the trap by storing UCN for various times, and counting the survivors. The data are consistent with a single exponential decay, and we find $\tau_{store}=860\pm19$ s: within $1 \sigma$ of current global averages for $\tau_{n}$. The storage time with the holding field deactiveated is found to be $\tau_{store}=470 \pm 160$ s; this decreased storage time is due to the loss of UCN which undergo Majorana spin-flips while being stored. We discuss plans to increase the statistical sensitivity of the measurement and investigate potential systematic effects., Comment: 6 pages, 5 figures

It is generally accepted that the main cause of ultracold neutron (UCN) losses in storage traps is the upscattering to the thermal energy range by hydrogen adsorbed on the surface of the trap walls. However, the data on which this conclusion is based are poor and contradictory. Here, we report a measurement, performed at the Los Alamos National Laboratory UCN source, of the average energy of the flux of upscattered neutrons after the interaction of UCN with hydrogen bound in semicrystalline polymer PMP (tradename TPX), [C$_{6}$H$_{12}$]$_n$. Our analysis, performed with the MCNP code based on the application of the neutron scattering law to UCN upscattered by bound hydrogen in semicrystalline polyethylene, [C$_{2}$H$_{4}$]$_n$, leads us to a flux average energy value of 26$\pm3$ meV in contradiction with previously reported experimental values of 10 to 13 meV and in agreement with the theoretical models of neutron heating implemented in the MCNP code.

The study of neutron cross sections for elements used as efficient ``absorbers'' of ultracold neutrons (UCN) is crucial for many precision experiments in nuclear and particle physics, cosmology and gravity. In this context, ``absorption'' includes both the capture and upscattering of neutrons to the energies above the UCN energy region. The available data, especially for hydrogen, do not agree between themselves or with the theory. In this report we describe measurements performed at the Los Alamos National Laboratory UCN facility of the UCN upscattering cross sections for vanadium and for hydrogen in CH$_2$ using simultaneous measurements of the radiative capture cross sections for these elements. We measured $\sigma_{up}=1972\pm130$ b for hydrogen in CH$_2$, which is below theoretical expectations, and $\sigma_{up} < 25\pm9$ b for vanadium, in agreement with the expectation for the neutron heating by thermal excitations in solids., Comment: 6 pages 2 figures

A new measurement of the neutron $\beta$-decay asymmetry $A_0$ has been carried out by the UCNA collaboration using polarized ultracold neutrons (UCN) from the solid deuterium UCN source at the Los Alamos Neutron Science Center (LANSCE). Improvements in the experiment have led to reductions in both statistical and systematic uncertainties leading to $A_0 = -0.11954(55)_{\rm stat.}(98)_{\rm syst.}$, corresponding to the ratio of axial-vector to vector coupling $\lambda \equiv g_A/g_V = -1.2756(30)$.

Precision measurements in neutron beta decay serve to determine the coupling constants of beta decay and allow for several stringent tests of the standard model. This paper discusses the design and the expected performance of the Nab spectrometer., Comment: Submitted to Proceedings of the Conference CIPANP12, St.Petersburg, Florida, May 2012

We present a detailed report of a measurement of the neutron $\beta$-asymmetry parameter $A_0$, the parity-violating angular correlation between the neutron spin and the decay electron momentum, performed with polarized ultracold neutrons (UCN). UCN were extracted from a pulsed spallation solid deuterium source and polarized via transport through a 7-T magnetic field. The polarized UCN were then transported through an adiabatic-fast-passage spin-flipper field region, prior to storage in a cylindrical decay volume situated within a 1-T $2 \times 2\pi$ solenoidal spectrometer. The asymmetry was extracted from measurements of the decay electrons in multiwire proportional chamber and plastic scintillator detector packages located on both ends of the spectrometer. From an analysis of data acquired during runs in 2008 and 2009, we report $A_0 = -0.11966 \pm 0.00089_{-0.00140} ^{+0.00123}$, from which we extract a value for the ratio of the weak axial-vector and vector coupling constants of the nucleon, $\lambda = g_A/g_V = -1.27590 \pm 0.00239_{-0.00377}^{+0.00331}$. Complete details of the analysis are presented., Comment: 54 pages, 41 figures, submitted to Phys. Rev. C

A precise measurement of the neutron decay $\beta$-asymmetry $A_0$ has been carried out using polarized ultracold neutrons (UCN) from the pulsed spallation UCN source at the Los Alamos Neutron Science Center (LANSCE). Combining data obtained in 2008 and 2009, we report $A_0 = -0.11966 \pm 0.00089_{-0.00140}^{+0.00123}$, from which we determine the ratio of the axial-vector to vector weak coupling of the nucleon $g_A/g_V = -1.27590_{-0.00445}^{+0.00409}$., Comment: 5 pages, 2 figures

We present the results of an Ultracold neutron (UCN) production experiment in a pulsed neutron beam line at the Los Alamos Neutron Scattering Center. The experimental apparatus allows for a comprehensive set of measurements of UCN production as a function of target temperature, incident neutron energy, target volume, and applied magnetic field. However, the low counting statistics of the UCN signal expected can be overwhelmed by the large background associated with the scattering of the primary cold neutron flux that is required for UCN production. We have developed a background subtraction technique that takes advantage of the very different time-of-flight profiles between the UCN and the cold neutrons, in the pulsed beam. Using the unique timing structure, we can reliably extract the UCN signal. Solid ortho-D$_2$ is used to calibrate UCN transmission through the apparatus, which is designed primarily for studies of UCN production in solid O$_2$. In addition to setting the overall detection efficiency in the apparatus, UCN production data using solid D$_2$ suggest that the UCN upscattering cross-section is smaller than previous estimates, indicating the deficiency of the incoherent approximation widely used to estimate inelastic cross-sections in the thermal and cold regimes.

Ultra-cold neutrons (UCN), neutrons with energies low enough to be confined by the Fermi potential in material bottles, are playing an increasing role in measurements of fundamental properties of the neutron. The ability to manipulate UCN with material guides and bottles, magnetic fields, and gravity can lead to experiments with lower systematic errors than have been obtained in experiments with cold neutron beams. The UCN densities provided by existing reactor sources limit these experiments. The promise of much higher densities from solid deuterium sources has led to proposed facilities coupled to both reactor and spallation neutron sources. In this paper we report on the performance of a prototype spallation neutron-driven solid deuterium source. This source produced bottled UCN densities of 145 +/-7 UCN/cm3, about three times greater than the largest bottled UCN densities previously reported. These results indicate that a production UCN source with substantially higher densities should be possible.

We present the first measurements of the survival time of ultracold neutrons (UCNs) in solid deuterium SD2. This critical parameter provides a fundamental limitation to the effectiveness of superthermal UCN sources that utilize solid ortho-deuterium as the source material. Superthermal UCN sources offer orders of magnitude improvement in the available densities of UCNs, and are of great importance to fundamental particle-physics experiments such as searches for a static electric dipole moment and lifetime measurements of the free neutron. These measurements are performed utilizing a SD2 source coupled to a spallation source of neutrons, providing a demonstration of UCN production in this geometry and permitting systematic studies of the influence of thermal up-scatter and contamination with para-deuterium on the UCN survival time., Comment: 5 pages, 3 figures, revtex4