Search

In experiments that measure the lifetime of trapped particles, in addition to loss mechanisms with exponential survival probability functions, particles can be lost by mechanisms with non-exponential survival probability functions. Failure to account for such loss mechanisms produces systematic measurement error and associated systematic uncertainties in these measurements. In this work, we develop a general competing risks survival analysis method to account for the joint effect of loss mechanisms with either exponential or non-exponential survival probability functions, and a method to quantify the size of systematic effects and associated uncertainties for lifetime estimates. As a case study, we apply our survival analysis formalism and method to the Ultra Cold Neutron lifetime experiment at NIST. In this experiment, neutrons can escape a magnetic trap before they decay due to a wall loss mechanism with an associated non-exponential survival probability function.

Accurately measuring the neutron beam polarization of a high flux, large area neutron beam is necessary for many neutron physics experiments. The Fundamental Neutron Physics Beamline (FnPB) at the Spallation Neutron Source (SNS) is a pulsed neutron beam that was polarized with a supermirror polarizer for the NPDGamma experiment. The polarized neutron beam had a flux of $\sim10^9$ neutrons per second per cm$^2$ and a cross sectional area of 10$\times$12~cm$^2$. The polarization of this neutron beam and the efficiency of a RF neutron spin rotator installed downstream on this beam were measured by neutron transmission through a polarized $^{3}$He neutron spin-filter. The pulsed nature of the SNS enabled us to employ an absolute measurement technique for both quantities which does not depend on accurate knowledge of the phase space of the neutron beam or the $^{3}$He polarization in the spin filter and is therefore of interest for any experiments on slow neutron beams from pulsed neutron sources which require knowledge of the absolute value of the neutron polarization. The polarization and spin-reversal efficiency measured in this work were done for the NPDGamma experiment, which measures the parity violating $\gamma$-ray angular distribution asymmetry with respect to the neutron spin direction in the capture of polarized neutrons on protons. The experimental technique, results, systematic effects, and applications to neutron capture targets are discussed., Comment: 13 pages, 14 figures, 2 tables

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

We describe the Fundamental Neutron Physics Beamline (FnPB) facility located at the Spallation Neutron Source at Oak Ridge National Laboratory. The FnPB was designed for the conduct of experiments that investigate scientific issues in nuclear physics, particle physics, astrophysics and cosmology using a pulsed slow neutron beam. We present a detailed description of the design philosophy, beamline components, and measured fluxes of the polychromatic and monochromatic beams.

The cryogenic design and performance of an apparatus used to magnetically confine ultracold neutrons (UCN) is presented. The apparatus is part of an effort to measure the beta-decay lifetime of the free neutron and is comprised of a high-current superconducting magnetic trap that surrounds ∼21 l of isotopically pure 4 He cooled to approximately 250 mK. A 0.89 nm neutron beam can enter the apparatus from one end of the magnetic trap and a light collection system allows visible light generated within the helium by decays to be transported to detectors at room temperature. Two cryocoolers are incorporated to reduce liquid helium consumption.

© 2019 IOP Publishing Ltd and Sissa Medialab. 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). This apparatus uses superfluid 4He 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 3He from an Atomic Beam Source injected into the superfluid 4He and transported to the measurement cells where it serves as a co-magnetometer. The superfluid 4He 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× 10-28 e-cm, with anticipated systematic uncertainties below this level.

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., 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)

Research efforts ranging from studies of solid helium to searches for a neutron electric dipole moment require isotopically purified helium with a ratio of ^3He to ^4He at levels below that which can be measured using traditional mass spectroscopy techniques. We demonstrate an approach to such a measurement using accelerator mass spectroscopy, reaching the 10^(−14) level of sensitivity, several orders of magnitude more sensitive than other techniques. Measurements of ^3He/^4He in samples relevant to the measurement of the neutron lifetime indicate the need for substantial corrections. We also argue that there is a clear path forward to sensitivity increases of at least another order of magnitude.

The preparation of intercalated graphite compounds is now well established at the Institut Laue Langevin (ILL). We are able to manufacture several types of intercalated crystals, such as KC 8, RbC 8 , KC 24 and RbC 24 compounds, in large quantities and in a reproducible manner. Even though the mosaic distribution is large, these compounds exhibit a high neutron peak reflectivity of about 70% at a neutron wavelength of 9.8 A. The production of such intercalated graphite crystals allows us to build efficient neutron monochromators for wavelengths in the range 6–15 A.

Research and development is being completed for an ultracold neutron (UCN) source to be installed at the PULSTAR reactor on the campus of North Carolina State University (NCSU). The objective is to establish a university-based UCN facility with sufficient UCN intensity to allow world-class fundamental and applied research with UCN. To maximize the UCN yield, a solid ortho-D2 converter will be implemented coupled to two moderators, D2O at room temperature, to thermalize reactor neutrons, and solid CH4, to moderate the thermal neutrons to cold-neutron energies. The source assembly will be located in a tank of D2O in the space previously occupied by the thermal column of the PULSTAR reactor. Neutrons leaving a bare face of the reactor core enter the D2O tank through a 45×45 cm cross-sectional area void between the reactor core and the D2O tank. Liquid He will cool the disk-shaped UCN converter to below 5 K. Independently, He gas will cool the cup-shaped CH4 cold-neutron moderator to an optimum temperature between 20 and 40 K. The UCN will be transported from the converter to experiments by a guide with an inside diameter of 16 cm. Research areas being considered for the PULSTAR UCN source include time-reversal violation in neutron beta decay, neutron lifetime determination, support measurements for a neutron electric-dipole-moment search, and nanoscience applications.

Effective management of liquid water produced in the cathodic reaction of a polymer electrolyte membrane (PEM) fuel cell is essential to achieve high cell efficiency. Few experimental methods are available for in situ measurements of water transport within an operating cell. Neutron radiography is a useful tool to visualize water within a cell constructed of many common materials, including metals. The application of neutron radiography to measurements of water content within the flow field channels of an operating 50 cm2 PEM fuel cell is described. Details of the experimental apparatus, image processing procedure and quantitative analysis are provided. It is demonstrated that water tends to accumulate in the 180° bends of the serpentine anode and cathode flow fields used in this study. Moreover, the effects of both the current density and cathode stoichiometric ratio on the quantity of accumulated water are discussed.

We have performed high precision measurements of the zero-energy neutron scattering amplitudes of gas phase molecular hydrogen, deuterium, and $^{3}$He using neutron interferometry. We find $b_{\mathit{np}}=(-3.7384 \pm 0.0020)$ fm\cite{Schoen03}, $b_{\mathit{nd}}=(6.6649 \pm 0.0040)$ fm\cite{Black03,Schoen03}, and $b_{n^{3}\textrm{He}} = (5.8572 \pm 0.0072)$ fm\cite{Huffman04}. When combined with the previous world data, properly corrected for small multiple scattering, radiative corrections, and local field effects from the theory of neutron optics and combined by the prescriptions of the Particle Data Group, the zero-energy scattering amplitudes are: $b_{\mathit{np}}=(-3.7389 \pm 0.0010)$ fm, $b_{\mathit{nd}}=(6.6683 \pm 0.0030)$ fm, and $b_{n^{3}\textrm{He}} = (5.853 \pm .007)$ fm. The precision of these measurements is now high enough to severely constrain NN few-body models. The n-d and n-$^{3}$He coherent neutron scattering amplitudes are both now in disagreement with the best current theories. The new values can be used as input for precision calculations of few body processes. This precision data is sensitive to small effects such as nuclear three-body forces, charge-symmetry breaking in the strong interaction, and residual electromagnetic effects not yet fully included in current models., Comment: 6 pages, 4 figures, submitted to Physica B as part of the Festschrift honouring Samuel A. Werner at the International Conference on Neutron Scattering 2005

The possible existence of a nonzero electric dipole moment (EDM) of the neutron is of great fundamental interest in itself and directly impacts our understanding of the nature of electro-weak and strong interactions. The experimental search for this moment has the potential to reveal new sources of T and CP violation and to challenge calculations that propose extensions to the Standard Model. The goal of the current experiment is to significantly improve the measurement sensitivity to the neutron EDM over what is reported in the literature. The experiment has the potential to either measure the magnitude of the neutron EDM or to lower the current experimental limit by two orders of magnitude. Achieving these objectives will have a major impact on our understanding of the physics of both weak and strong interactions.

The Spallation Neutron Source (SNS), currently under construction at Oak Ridge National Laboratory with an anticipated start-up in early 2006, will provide the most intense pulsed beams of cold neutrons in the world. At a projected power of 1.4 MW, the time averaged fluxes and fluences of the SNS will approach those of high flux reactors. One of the flight paths on the cold, coupled moderator will be devoted to fundamental neutron physics. The fundamental neutron physics beamline is anticipated to include two beam-lines; a broad band cold beam, and a monochromatic beam of 0.89 nm neutrons for ultracold neutron (UCN) experiments. The fundamental neutron physics beamline will be operated as a user facility with experiment selection based on a peer reviewed proposal process. An initial program of five experiments in neutron decay, hadronic weak interaction and time reversal symmetry violation have been proposed.

The design objective of the thermal neutron radiography facility at the National Institute of Standards and Technology (NIST) Center for Neutron Research was to provide a large beam diameter and a high fluence rate in order to produce images of dynamic systems. A thermal neutron beam with a 14 cm diameter thimble was chosen. The beam was initially filtered by a 10 cm thick single crystal bismuth filter cooled with liquid nitrogen. The beam exiting the port is shaped using either a 1 cm or 2 cm diameter pinhole to form a uniform high fluence rate beam at the sample. The resulting neutron beam at the sample has an L / D ratio of 280 with a fluence rate of 1.84×10 7 cm −2 s −1 and 560 with a fluence rate of 4.75×10 6 cm −2 s −1 uniformly spread over a 26 cm diameter beam. To capture the neutron beam image a scintillator and CCD camera is used. The current neutron camera system is limited to a 2.5 s frame rate; however, a high frame rate detector system based on amorphous silicon will allow frame rates to meet the design goal. Samples can be rotated and translated in situ for radiography and tomography applications. This facility became operational in early 2003. Since then the facility has been translated backwards by ≈2.13 m and 5 cm of bismuth was added to the filter. The design of this facility and the impact of the later changes are discussed.

The neutron-induced low temperature (below 5 K) luminescence of neutron shielding and scintillation detection materials is studied. Strong luminescence is observed for the neutron absorbing materials boron nitride (BN) and lithium fluoride (LiF). A measurable, but substantially smaller luminescence is observed from boron oxide (B2O3). An upper bound of 10−3 was determined for the fraction of the luminescence due to time-correlated multiphoton events in the BN. Other materials tested – boron carbide (B4C), polymethyl methacrylate (PMMA or acrylic), expanded polytetrafluoroethylene (PTFE) with an evaporated coating of the downconverting fluor tetraphenyl butadiene (TPB) and a boron/lithium loaded glass – displayed no detectable luminescence. The boron/lithium loaded glass was determined to activate, by the secondary reaction 16O(T,n)18F, with the triton produced in the neutron capture reaction 6Li(n,T)4He.

The 10 B(α,n) 13 N reaction is studied as an activation process in a variety of solid boron-containing neutron shielding materials. The source of α-particles is the neutron capture reaction 10 B(n,α) 7 Li. Samples of boron carbide, boron oxide, and boron nitride are irradiated with thermal neutrons and the rate of 13 N production is determined. 13 N promptly decays, emitting a positron. This positron efficiently annihilates with electrons in the material and the resultant 511 keV gamma ray is detected. For each of the above-mentioned materials, the rate of 13 N production is (1–2) × 10 −10 per captured neutron.

Production of ultracold neutrons (UCN) by single-phonon downscattering of cold neutrons from superfluid helium (the “superthermal process”) utilizes input neutrons only in a narrow wavelength band around 0.89 nm . Delivering a monochromatic 0.89 nm neutron beam to a superfluid helium target reduces backgrounds in the UCN production region with minimal loss in the UCN production rate. The design, construction, and testing of a 0.89 nm neutron monochromator is reported herein. This monochromator is constructed from nine tiled pieces of stage 2 potassium-intercalated graphite with mosaics between 1.1° and 2.1° and reflectivities of (73–91)% at 0.89 nm . In addition to stage 2 potassium-intercalated graphite, fluorophlogopite and stage 1 potassium-intercalated graphite are also characterized.

Detectors for counting low energy (less than 1 MeV ) ionizing events in liquid helium are developed and characterized. These devices employ wavelength shifting fluors to convert extreme ultraviolet (EUV) helium scintillation light to the visible, allowing transport of signal light to room temperature. Three technological approaches are developed and tested: wavelength shifting fiber, composite acrylic tube, and diffuse reflecting tube of expanded teflon. The tube-based detectors have been used to detect magnetically trapped neutrons. All of the technological approaches have utility in other experiments, such as a more sensitive measurement of the neutron electric dipole moment and the monitoring of the low-energy solar neutrino flux.

We investigate the performance of a large-area ð13 mm � 13 mmÞ avalanche photodiode at temperatures ranging from 4.2 to 77 K: We find that the gain, at a given bias voltage, increases with decreasing temperature down to 40 K; below which a premature breakdown phenomenon occurs. The quantum efficiency of the device decreases with decreasingtemperature until approximately 40 K ; at which point it drops abruptly to o15% of its room temperature value. The sensitivity of the device above 40 K makes it a good candidate for detection of scintillation light in lowtemperature systems. r 2003 Elsevier B.V. All rights reserved. PACS: 07.60.Dq; 29.40.Mc; 29.40.Wk

Magnetically stabilized luminescence is observed in hexagonal boron nitride. The luminescence is induced by absorption of cold neutrons and is in the visible region. In the absence of a magnetic field, the photon emission level is observed to decay over several hundred seconds. A fraction of this luminescence can be suppressed if the temperature is T ~ 1.0 T. Subsequent to irradiation and suppression, luminescence can be induced by an increase in T or lowering of B. Possible explanations include stabilization of triplet states or the localization and stabilization of excitons., Comment: 11 pages, 7 figures, to appear in the Journal of Luminescence

We report progress towards magnetic trapping of ultra-cold neutrons (UCN) in preparation for a neutron lifetime measurement. UCN will be produced by inelastic scattering of cold (0.89 nm) neutrons in a reservoir of super#uid 4He and con"ned in a three-dimensional magnetic trap. As the trapped neutrons decay, recoil electrons will generate scintillations in the liquid He, which should be detectable with nearly 100% e$ciency. This direct measure of the number of UCN decays vs. time can be used to determine the neutron beta-decay lifetime. ( 2000 Elsevier Science B.V. All rights reserved.

Long-lived phosphorescence has been observed emanating from superfluid helium following excitation by ionizing radiation. This extreme ultraviolet light is predominantly from the radiative decay of metastable ${\mathrm{He}}_{2}(a{}^{3}{\ensuremath{\Sigma}}_{u}^{+})$ molecules and is detected through frequency down-conversion and photon counting techniques. This allows measurement of the phosphorescence lifetime in a low-molecular-density regime that is not dominated by two-body reactions. A phosphorescence lifetime of $13\ifmmode\pm\else\textpm\fi{}2 \mathrm{s}$ is observed.

We describe how the deformation effect in nuclear spin aligned can be used to identify small d-wave admixtures in predominantly s-wave resonances. Using standard angular momentum penetrabilities, we estimate deformation effect cross sections for resonances below 500 eV. If d-wave admixtures are identified in a number of s-wave resonances, then a test for a parity-conserving, time-reversal violating term in the neutron-nucleus forward scattering amplitude is possible.

We have constructed a 0.4 mole solid 3 He target, cryogenically polarized at 12 mK in a field of 7 T. The 0.04 atoms/b target reached a polarization of 38% in 35 h. Such a target may be applied to any experiment which is tolerant of the large ambient magnetic field and which produces target heating of less than a microwatt. High energy neutron and photon scattering experiments meet these requirements. The target's figure of merit for neutron transmission measurement exceeds that of polarized gas targets by greater than 35. At the Triangle Universities Nuclear Laboratory we have used the target to measure the total cross section differences Δσ T and Δσ L for incident polarized neutrons of energies 2–8 MeV. The cross section difference is sensitive to the excited state structure of the n- 3 He system. The results have been compared to a recent R-matrix analysis of R = 4 scattering and reaction data, and provide support for the 4 He level scheme derived from that analysis.

We report the results of a high-resolution study of neutron resonances below 500 eV in {sup 166}Ho. We find nine previously unreported weak resonances at energies 24.79, 75.08, 119.98, 187.94, 210.86, 227.87, 235.94, 248.60, and 264.98 eV. Bayesian probability analysis of the widths indicates they are formed primarily by {ital s}-wave capture and are therefore negative-parity resonances. If they have measurable {ital d}-wave neutron widths, they are candidates for the fivefold correlation test of time reversal. {copyright} {ital 1996 The American Physical Society.}

Measurements of polarized-neutron--polarized${\mathrm{\ensuremath{-}}}^{3}$He scattering are reported. The target consisted of cryogenically polarized solid $^{3}\mathrm{He}$, with thickness 0.04 atom/b and polarization \ensuremath{\sim}0.4. Polarized neutrons were produced via the $^{3}\mathrm{H}$(p\ensuremath{\rightarrow},n\ensuremath{\rightarrow}${)}^{3}$He or $^{2}\mathrm{H}$(d\ensuremath{\rightarrow},n\ensuremath{\rightarrow}${)}^{3}$He polarization-transfer reactions. The longitudinal and transverse total cross-section differences \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{L}}$ and \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{T}}$ were measured for incident neutron energies 2--8 MeV. The results are compared to phase-shift predictions based on four different analyses of n${\mathrm{\ensuremath{-}}}^{3}$He scattering. The best agreement is obtained with a recent R-matrix analysis of A=4 scattering and reaction data, lending strong support to the $^{4}\mathrm{He}$ level scheme obtained in that analysis. Discrepancies with other phase-shift parametrizations of n${\mathrm{\ensuremath{-}}}^{3}$He scattering exist, attributable in most instances to one or two particular partial waves. \textcopyright{} 1996 The American Physical Society.

The spin-dependent total cross-section difference {Delta}{sigma}{sub {ital T}} has been measured at energies of 7.43, 9.57, and 11.60 MeV for neutrons, polarized transverse to the beam momentum, transmitted through a transversely polarized proton target. In addition, the energy at which {Delta}{sigma}{sub {ital T}} crosses through zero has been determined in the energy range 3.65--6.25 MeV. The measurements were carried out with a polarized TiH{sub 2} target and polarized neutrons from the {sup 3}H(p,n){sup 3}He and {sup 2}H(d,n){sup 3}He reactions. The cross-section differences {Delta}{sigma}{sub {ital T}} are sensitive to the strength of the {sup 3}{ital S}{sub 1}--{sup 3}{ital D}{sub 1} mixing parameter {epsilon}{sub 1}. Phase-shift analyses are used to determine {epsilon}{sub 1} values from the data. The values are in good in agreement with {ital NN} potential models and do not support the low {epsilon}{sub 1} values found in previous studies around 15--20 MeV.

We describe the construction and operation of a solid 3 He polarized nuclear target which we have used for measurements of the spin dependence of the n- 3 He interaction at MeV energies. The target, which contains 0.4 mole of 3 He was polarized to 38% at 12 mK in a field of 7 T. The target is suitable for nuclear physics measurements which are insensitive to the large magnetic field and produce beam heating of tenths of microwatts. We discuss refinements and paths to improved solid 3 He targets at higher polarizations and lower fields.

A summary of the polarized and aligned nuclear targets which have been constructed and used at the Triangle Universities Nuclear Laboratory is given. Statically polarized targets, typically operating at a temperature of 12 mK and a magnetic field of 7 T, have provided significant nuclear polarization in 1 H, 3 He, 27 Al, 93 Nb and 165 Ho. A rotating, aligned 165 Ho target is also in use. A 3 He melting curve thermometer has been developed for use in statically polarized targets. A dynamically polarized proton target is under construction.

Hadronic parity violation uses quark-quark weak interactions to probe nonperturbative strong interaction dynamics through two nonperturbative QCD scales: [Formula: see text] and the fine-tuned MeV scales of NN bound states in low energy nuclear physics. The current and projected availability of high-intensity neutron and photon sources coupled with ongoing experiments and continuing developments in theoretical methods provide the opportunity to greatly expand our understanding of hadronic parity violation in few-nucleon systems. The current status of these efforts and future plans are discussed.

Measurements have been made of \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{T}}$ for polarized neutrons incident on a polarized-proton target from 3.65 to 11.60 MeV. In the energy range near 10 MeV, \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{T}}$ is very sensitive to the nucleon-nucleon tensor interaction. Comparison of the data to potential-model predictions indicate that the tensor interaction is weak, resulting in values of the $^{3}$${\mathit{S}}_{1}$${\mathrm{\ensuremath{-}}}^{3}$${\mathit{D}}_{1}$ mixing parameter ${\mathrm{\ensuremath{\epsilon}}}_{1}$ which are smaller than predicted by any nucleon-nucleon potential model. A smaller tensor force will bring the predictions of local potential models for the triton binding energy into closer agreement with the experimental value.

We report the measurements of depolarization probabilities of polarized $^3$He in a rectangular acrylic cell with a deuterated tetraphenyl butadiene-doped deuterated polystyrene coating filled with superfluid $^4$He at $\sim$330 mk with a magnetic holding field of $\sim$7.3 G. We achieve a wall depolarization probability of $\sim1.0\times10^{-7}$. Such a surface will find application in a new experiment searching for the neutron electric dipole moment and other applications.

The neutron beta-decay lifetime plays an important role both in understanding weak interactions within the framework of the Standard Model and in theoretical predictions of the primordial abundance of 4He in Big Bang Nucleosynthesis. In previous work, we successfully demonstrated the trapping of ultracold neutrons (UCN) in a conservative potential magnetic trap. A major upgrade of the apparatus is nearing completion at the National Institute of Standards and Technology Center for Neutron Research (NCNR). In our approach, a beam of 0.89 nm neutrons is incident on a superfluid 4He target within the minimum field region of an Ioffe-type magnetic trap. A fraction of the neutrons is downscattered in the helium to energies, 5 pages, 5 figures

As part of an on‐going program utilizing magnetically trapped ultracold neutrons (UCNs), we are developing a technique that offers the possibility of improving the precision of the neutron lifetime by more than an order of magnitude. The experiment works by loading an Ioffe‐type superconducting magnetic trap with UCNs through inelastic scattering of 0.89 nm neutrons with phonons in superfluid 4He. Trapped neutrons are detected when they decay; charged decay electrons ionize helium atoms in the superfluid resulting in scintillation light that is recorded in real time using photomultiplier tubes. At present, we are installing a larger and deeper superconducting magnetic trap into our apparatus, implementing techniques to reduce background events, and working to increase the neutron decay detection efficiency. We report the status of the construction of the improved apparatus.

In the meson exchange model of weak nucleon-nucleon (NN) interactions, the exchange of virtual mesons between the nucleons is parameterized by a set of weak meson exchange amplitudes. The strengths of these amplitudes from theoretical calculations are not well known, and experimental measurements of parity-violating (PV) observables in different nuclear systems have not constrained their values. Transversely polarized cold neutrons traveling through liquid helium experience a PV spin rotation due to the weak interaction with an angle proportional to a linear combination of these weak meson exchange amplitudes. A measurement of the PV neutron spin rotation in helium (φ PV ( n ,α)) would provide information about the relative strengths of the weak meson exchange amplitudes, and with the longitudinal analyzing power measurement in the p + α system, allow the first comparison between isospin mirror systems in weak NN interaction. An earlier experiment performed at NIST obtained a result consistent with zero: φ PV ( n ,α) = (8.0 ±14(stat) ±2.2(syst)) ×10(-7) rad / m[1]. We describe a modified apparatus using a superfluid helium target to increase statistics and reduce systematic effects in an effort to reach a sensitivity goal of 10(-7) rad/m.

Monte Carlo simulations are being performed to design and characterize the neutron optics components for the two fundamental neutron physics beamlines at the Spallation Neutron Source. Optimization of the cold beamline includes characterization of the guides and benders, the neutron transmission through the 0.89 nm monochromator, and the expected performance of the four time-of-flight choppers. The locations and opening angles of the choppers have been studied using a simple spreadsheet-based analysis that was developed for other SNS chopper instruments. The spreadsheet parameters are then optimized using Monte Carlo techniques to obtain the results presented in this paper. Optimization of the 0.89 nm beamline includes characterizing the double crystal monochromator and the downstream guides. The simulations continue to be refined as components are ordered and their exact size and performance specifications are determined.

We compute classical trajectories of Ultracold neutrons (UCNs) in a superconducting Ioffe-type magnetic trap using a symplectic integration method. We find that the computed escape time for a particular set of initial conditions (momentum and position) does not generally stabilize as the time step parameter is reduced unless the escape time is short (less than approximately 10 s). For energy intervals where more than half of the escape times computed for UCN realizations are numerically well determined, we predict the median escape time as a function of the midpoint of the interval.

We report progress on an experiment to measure the neutron lifetime using magnetically trapped neutrons. Neutrons are loaded into a 1.1 T deep superconducting Ioffe-type trap by scattering 0.89 nm neutrons in isotopically pure superfluid (4)He. Neutron decays are detected in real time using the scintillation light produced in the helium by the beta-decay electrons. The measured trap lifetime at a helium temperature of 300 mK and with no ameliorative magnetic ramping is substantially shorter than the free neutron lifetime. This is attributed to the presence of neutrons with energies higher than the magnetic potential of the trap. Magnetic field ramping is implemented to eliminate these neutrons, resulting in an [Formula: see text] trap lifetime, consistent with the currently accepted value of the free neutron lifetime.

A measurement of the $n\text{\ensuremath{-}}^{3}\mathrm{He}$ coherent scattering length using neutron interferometry is reported. The result, ${b}_{c}=(5.8572\ifmmode\pm\else\textpm\fi{}0.0072)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$, improves the measured precision of any single measurement of ${b}_{c}$ by a factor of eight; the previous world average, ${b}_{c}=(5.74\ifmmode\pm\else\textpm\fi{}0.04)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$, now becomes ${b}_{c}=(5.853\ifmmode\pm\else\textpm\fi{}0.007)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$. Measurements of the $n\text{\ensuremath{-}}p$, $n\text{\ensuremath{-}}d$, and $n\text{\ensuremath{-}}^{3}\mathrm{He}$ coherent scattering lengths have now been performed using the same technique, thus allowing one to extract the scattering length ratios: parameters that minimize systematic errors. We obtain values of ${b}_{n^{3}\mathrm{He}}∕{b}_{np}=(\ensuremath{-}1.5668\ifmmode\pm\else\textpm\fi{}0.0021)$ and ${b}_{nd}∕{b}_{np}=(\ensuremath{-}1.7828\ifmmode\pm\else\textpm\fi{}0.0014)$. Using the new world average value of ${b}_{c}$ and recent high-precision spin-dependent scattering length data also determined by neutron optical techniques, we extract new values for the bound singlet and triple scattering lengths of ${b}_{0}=(9.949\ifmmode\pm\else\textpm\fi{}0.027)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$ and ${b}_{1}=(4.488\ifmmode\pm\else\textpm\fi{}0.017)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$ for the $n\text{\ensuremath{-}}^{3}\mathrm{He}$ system. The free nuclear singlet and triplet scattering lengths are ${a}_{0}=(7.456\ifmmode\pm\else\textpm\fi{}0.020)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$ and ${a}_{1}=(3.363\ifmmode\pm\else\textpm\fi{}0.013)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$. The coherent scattering cross section is ${\ensuremath{\sigma}}_{c}=(4.305\ifmmode\pm\else\textpm\fi{}0.007)\phantom{\rule{0.3em}{0ex}}\mathrm{b}$ and the total scattering cross section is ${\ensuremath{\sigma}}_{s}=(5.837\ifmmode\pm\else\textpm\fi{}0.014)\phantom{\rule{0.3em}{0ex}}\mathrm{b}$. Comparisons of ${a}_{0}$ and ${a}_{1}$ to the only existing high-precision theoretical predictions for the $n\text{\ensuremath{-}}^{3}\mathrm{He}$ system, calculated using a resonating group technique with nucleon-nucleon potentials incorporating three-nucleon forces, have been performed. Neutron scattering length measurements in few-body systems are now sensitive enough to probe small effects not yet adequately treated in present theoretical models.

The time dependence of extreme ultraviolet (EUV) fluorescence following an ionizing radiation event in liquid helium is observed and studied in the temperature range from 250 mK to 1.8 K. The fluorescence exhibits significant structure including a short $(\ensuremath{\sim}10\mathrm{ns})$ strong initial pulse followed by single photons whose emission rate decays exponentially with a $1.6\ensuremath{-}\ensuremath{\mu}\mathrm{s}$ time constant. At an even longer time scale, the emission rate varies as ``1/time'' (inversely proportional to the time after the initial pulse). The intensity of the ``1/time'' component from $\ensuremath{\beta}$ particles is significantly weaker than those from $\ensuremath{\alpha}$ particles or neutron capture on ${}^{3}\mathrm{He}.$ It is also found that for $\ensuremath{\alpha}$ particles, the intensity of this component depends on the temperature of the superfluid helium. Proposed models describing the observed fluorescence are discussed.