Search

We report the first observation of the parity-violating gamma-ray asymmetry A_{γ}^{np} in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. A_{γ}^{np} isolates the ΔI=1, ^{3}S_{1}→^{3}P_{1} component of the weak nucleon-nucleon interaction, which is dominated by pion exchange and can be directly related to a single coupling constant in either the DDH meson exchange model or pionless effective field theory. We measured A_{γ}^{np}=[-3.0±1.4(stat)±0.2(syst)]×10^{-8}, which implies a DDH weak πNN coupling of h_{π}^{1}=[2.6±1.2(stat)±0.2(syst)]×10^{-7} and a pionless EFT constant of C^{^{3}S_{1}→^{3}P_{1}}/C_{0}=[-7.4±3.5(stat)±0.5(syst)]×10^{-11} MeV^{-1}. We describe the experiment, data analysis, systematic uncertainties, and implications of the result.

We report the first observation of the parity-violating gamma-ray asymmetry A_{γ}^{np} in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. A_{γ}^{np} isolates the ΔI=1, ^{3}S_{1}→^{3}P_{1} component of the weak nucleon-nucleon interaction, which is dominated by pion exchange and can be directly related to a single coupling constant in either the DDH meson exchange model or pionless effective field theory. We measured A_{γ}^{np}=[-3.0±1.4(stat)±0.2(syst)]×10^{-8}, which implies a DDH weak πNN coupling of h_{π}^{1}=[2.6±1.2(stat)±0.2(syst)]×10^{-7} and a pionless EFT constant of C^{^{3}S_{1}→^{3}P_{1}}/C_{0}=[-7.4±3.5(stat)±0.5(syst)]×10^{-11} MeV^{-1}. We describe the experiment, data analysis, systematic uncertainties, and implications of the result.

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

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/

The “Nab” and “UCNB” collaborations have proposed to measure the correlation parameters in neutron β-decay at Oak Ridge and Los Alamos National Laboratory, using a novel detector design. Two large area, thick, hexagonal-segmented silicon detectors containing 127 pixels per detector will be used to detect the proton and electron from neutron decay. Both silicon detectors are connected by magnetic field lines of a few Tesla field strength, and set on an electrostatic potential, such that protons can be accelerated up to 30 keV in order to be detected. Characteristics of the detector response to low energy conversion electrons and protons from 15 keV to 35 keV, including the evaluation of the dead layer thickness and other contributions to the pulse height defect for proton detection are presented for Si detectors of 0.5 mm and 1 mm of thickness.

The NPDGamma experiment measures the parity-violating (PV) gamma asymmetry from polarized cold neutrons captured on protons at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL). The (PV) neutron spin asymmetry A γ of photons from polarized cold neutron capture on protons is proportional to the ΔI=1 long range weak meson coupling $h_{\pi }^{1}$ between nucleons in the hadronic weak interaction (HWI). Liquid para-hydrogen production data taking concluded in April 2014 and once the background aluminum asymmetry measurements are complete, the PV asymmetry A γ can be extracted. Preliminary results of the analysis of A γ are presented.

The NPDGamma collaboration is performing a measurement of the very small parity-violating asymmetry in the angular distribution of the 2.2 MeV γ - rays from the capture of polarized cold neutrons on protons ( A γ ) . The estimated size of A γ is 5 × 10 − 8 , and the measured asymmetry is proportional to the neutron polarization upon capture. Since the interaction of polarized neutrons with one of the two hydrogen molecular states (orthohydrogen) can lead to neutron spin-flip scattering, it is essential that the hydrogen in the target is mostly in the molecular state that will not depolarize the neutrons ( ≥ 99.8 % parahydrogen). For that purpose, in the first stage of the NPDGamma experiment at the Los Alamos Neutron Science Center (LANSCE), we operated a 16-l liquid hydrogen target, which was filled in two different occasions. The parahydrogen fraction in the target was accurately determined in situ by relative neutron transmission measurements. The result of these measurements indicate that the fraction of parahydrogen in equilibrium was 0.9998 ± 0.0002 in the first data taking run and 0.9956 ± 0.0002 in the second. We describe the parahydrogen monitor system, relevant aspects of the hydrogen target, and the procedure to determine the fraction of parahydrogen in the target. Also assuming thermal equilibrium of the target, we extract the scattering cross-section for neutrons on parahydrogen.

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

A 16 l liquid parahydrogen target has been developed for a measurement of the parity-violating γ -asymmetry in the capture of polarized cold neutrons on protons in the n ⇒ + p → d + γ reaction by the NPDGamma collaboration. The target system was carefully designed to meet the stringent requirements on systematic effects for the experiment and also to satisfy hydrogen safety requirements. The target was designed to preserve the neutron polarization during neutron scattering on liquid hydrogen (LH2), optimize the statistical sensitivity to the n ⇒ + p → d + γ reaction, minimize backgrounds coming from neutron interaction with the beam windows of the target cryostat, minimize LH2 density fluctuations which can introduce extra noise in the gamma asymmetry signal, and control systematic effects. The target incorporates two mechanical refrigerators, two ortho–para convertors, an aluminum cryostat, an aluminum target vessel shielded with 6Li-rich plastic, a hydrogen fill/vent line with a passive recirculation loop to establish and maintain the equilibrium ortho–para ratio, a hydrogen relief system coupled to a vent stack, a gas handling system, and an alarm and interlock system. Low Z, nonmagnetic materials were used for the target vessel and cryostat. Pressure and temperature sensors monitored the thermodynamic state of the target. Relative neutron transmission measurements were used to monitor the parahydrogen fraction of the target. The target was thoroughly tested and successfully operated during the first phase of the NPDGamma experiment conducted at the FP12 beam line at Los Alamos Neutron Science Center (LANSCE). An upgraded version of the target system will be used in the next stage of the experiment, which will be performed at the Fundamental Neutron Physics Beam (FnPB) line of the Spallation Neutron Source at Oak Ridge National Laboratory.

The NPDGamma collaboration has recently completed the first phase of a measurement to determine the size of the weak nucleon–nucleon interaction from cold neutron capture on a liquid hydrogen target. In the framework of the nearly 30 year old DDH model [B. Desplanques, J.F. Donoghue, B.R. Holstein, Annals of Physics 124 (1980) 449], the measured process is explained in terms of the weak pion–nucleon coupling, while the framework of modern effective field theory parameterizes the measured process in terms of the S 1 3 - P 1 3 , long range transition (essentially the Danilov parameter ρ t ) [S.L. Zhu et al., Nuclear Physics A 748 (2005) 435; C.-P. Liu, Phys. Rev. C 75 (2007) 065501]. The couplings in terms of either model are directly proportional to the parity violating up-down asymmetry in the angular distribution of gamma rays with respect to the neutron spin direction in the reaction n ⇒ + p → d + γ . The asymmetry has a predicted size of 5 × 10 - 8 and the aim of the NPDGamma collaboration is to measure it to 20%. The first phase of the measurement was completed at the Los Alamos National Laboratory Neutron Science Center Spallation Source with a preliminary result of ( - 1.1 ± 2.1 stat . ± 0.2 sys . ) × 10 - 7 . Here, we report on the measurements and the results obtained so far. The experiment is currently being installed at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory, for the remainder of its run time.

A large area polarized 3He neutron spin filter has been developed and tested in long-term operation. The polarizer cells have area 50 cm2 and larger allowing access to a large area neutron beam. Polarization of 3He up to 57 ± 1 % was measured by monitoring the neutron transmission through the 3He. The polarization was maintained over the course of several weeks, but the maximum attainable polarization declined over the course of a year-long run as the cell degraded, possibly due to long-term exposure to the neutron beam at high temperatures. The resulting neutron polarization and transmission are velocity dependent and are optimized for 5 A. Cell construction, polarizer design, performance, and polarimetry are described in this paper.

Liquid hydrogen is a dense Bose fluid whose equilibrium properties are both calculable from first principles using various theoretical approaches and of interest for the understanding of a wide range of questions in many body physics. Unfortunately, the pair correlation function $g(r)$ inferred from neutron scattering measurements of the differential cross section $d\sigma \over d\Omega$ from different measurements reported in the literature are inconsistent. We have measured the energy dependence of the total cross section and the scattering cross section for slow neutrons with energies between 0.43~meV and 16.1~meV on liquid hydrogen at 15.6~K (which is dominated by the parahydrogen component) using neutron transmission measurements on the hydrogen target of the NPDGamma collaboration at the Spallation Neutron Source at Oak Ridge National Laboratory. The relationship between the neutron transmission measurement we perform and the total cross section is unambiguous, and the energy range accesses length scales where the pair correlation function is rapidly varying. At 1~meV our measurement is a factor of 3 below the data from previous work. We present evidence that these previous measurements of the hydrogen cross section, which assumed that the equilibrium value for the ratio of orthohydrogen and parahydrogen has been reached in the target liquid, were in fact contaminated with an extra non-equilibrium component of orthohydrogen. Liquid parahydrogen is also a widely-used neutron moderator medium, and an accurate knowledge of its slow neutron cross section is essential for the design and optimization of intense slow neutron sources. We describe our measurements and compare them with previous work., Comment: Edited for submission to Physical Review B

The NPDGamma γ-ray detector has been built to measure, with high accuracy, the size of the small parity-violating asymmetry in the angular distribution of gamma rays from the capture of polarized cold neutrons by protons. The high cold neutron flux at the Los Alamos Neutron Scattering Center (LANSCE) spallation neutron source and control of systematic errors require the use of current mode detection with vacuum photodiodes and low-noise solid-state preamplifiers. We show that the detector array operates at counting statistics and that the asymmetries due to B4C and (27)Al are zero to with- in 2 × 10(-6) and 7 × 10(-7), respectively. Boron and aluminum are used throughout the experiment. The results presented here are preliminary.

The NPDGamma experiment will measure with high accuracy a very small parity-violating directional asymmetry A γ in the emission of gamma rays from the capture of polarized cold neutrons by protons. High event rates and systematic error considerations require the use of current mode detection with vacuum photo-diodes and low-noise solid-state preamplifiers. The preamplifier requirements, design, and measured properties are described.

An apparatus for measuring parity-violating asymmetries in gamma-ray emission following polarized cold neutron capture was constructed as a 1/10th scale test of the design for the forthcoming n → + p → d + γ experiment at LANSCE. The elements of the polarized neutron beam, including a polarized 3 He neutron spin filter and a radio frequency neutron spin rotator, are described. Using CsI(Tl) detectors and photodiode current mode readout, measurements were made of asymmetries in gamma-ray emission following neutron capture on 35 Cl , 113 Cd , and 139 La targets. Upper limits on the parity-allowed asymmetry s n ·( k γ × k n ) were set at the level of 7×10 −6 for all three targets. Parity-violating asymmetries s n · k γ were observed in 35 Cl , A γ =(−29.1±6.7)×10 −6 , and 139 La , A γ =(−15.5±7.1)×10 −6 , values consistent with previous measurements.

We have measured the brightness of the Flight Path 12 upstream/back-scattering partially coupled cold hydrogen moderator at LANSCE. This measurement was performed in the neutron energy range of 0.8–76.9 meV by using a time-of-flight technique in conjunction with a two-pinhole collimator system. Cold neutrons were observed with a redundant 6Li-loaded glass scintillation detector having an efficiency of close to unity. The detector viewed an area of 0.93 cm2 on the center of the 12×12 cm2 moderator surface. A maximum brightness of 1.25×108 neutrons s−1 sr−1 cm−2 meV−1 μA−1 was measured at 3.3-meV neutron energy. Statistical and systematic errors of the measurement are 3% and 7%, respectively. The measured brightness is compared to the result of a model calculation and there is a significant disagreement, by a factor of 1.5 at the peak. The performance of the m=3 supermirror neutron guide system on Flight Path 12 was also studied with the two pinhole-detector system.

We outline the motivation and conceptual design for a new experiment aimed at a 10-fold improvement in the accuracy of the parity-violating asymmetry A_gamma in the angular distribution of 2.2 MeV gamma rays from the n+p-->d+gamma reaction. This observable is primarily sensitive to the weak pion-nucleon coupling H_pi^1. A proof-of-principle experiment using unpolarized low-energy neutron capture on polyethylene and an array of 12 CsI detectors operated in current mode has been performed. Results of this test experiment including the current mode signal, electronic noise and detector sensitivity to magnetic fields are reported., Comment: 19 pages, 9 eps figures, uses RevTeX and epsfig, submitted to NIM

Measurements of discrete γ-ray spectra have been carried out as a function of incident-neutron energy for nuclei populated in 238U(n, xnγ (x ≤ 4) reactions. The GEANIE spectrometer, comprised of 26 Ge detectors (11 planar and 15 coaxial), was used to detect γ-rays. Neutrons were provided by the “white” neutron source of the Los Alamos Neutron Science Center’s WNR facility. The energy of the incident neutrons was determined using the time-of-flight technique. Absolute cross sections were determined for emission of several γ-rays from low-lying states of 235-238U isotopes (spin up to 10ħ and excitation energy up to ~1 MeV) as a function of incident-neutron energy (1 MeV < En 100 MeV). Uncertainties associated with the spectroscopic analysis of the data are discussed. Predictions of partial γ-ray cross sections from GNASH calculations up to neutron energy En = 30 MeV are compared to the experimental results and are generally in good agreement. Combining the experimental results with the predictions of the nu...

Absolute partial γ-ray cross sections for discrete γ-ray production in the 239Pu(n,2nγi)238Pu reaction have been measured. The experiments were performed at LANSCE/WNR on the 60R neutron flight path. Reaction γ-rays were measured using the large-scale Compton-suppressed array of Ge detectors, GEANIE. The motivation for this experiment, an overview of the partial γ-ray cross-section measurement, and an introduction to the main experimental issues will be presented. The energy resolution of the Ge detectors allowed identification of reaction γ rays above the background of sample radioactivity and fission γ rays. The use of planar Ge detectors with their reduced sensitivity to neutron interactions and improved line shape was also important to the success of this experiment. Absolute partial γ-ray cross sections are presented for the 61+ → 41+ member of the ground state rotational band in 238Pu, together with miscellaneous other γ-ray partial cross sections. The n,2n reaction cross section shape and magnitude...

The Nab experiment will measure the correlation a between the momenta of the beta particle and antineutrino in neutron decay as well as the Fierz term b which distorts the beta spectrum.

The weak interaction between neutrons and protons has never been resolved experimentally. In analogy with the strong NN interaction, the weak NN interaction at low energy can be parametrized in terms of a meson exchange model with parity violating meson–nucleon couplings. Unlike the measured proton–proton weak interaction, the neutron–proton weak interaction is sensitive to the weak pion–nucleon coupling constant Hπ1. This coupling, which is responsible for the longest-ranged part of the weak NN interaction and is therefore an essential part of any description of weak interactions in nuclei, remains undetermined despite many years of effort. A measurement of the gamma ray directional asymmetry Aγ in the capture of polarized neutrons by parahydrogen has been proposed at Los Alamos National Laboratory. The goal of this experiment is to determine Aγ with a relative standard uncertainty of

We report on polarized beam--polarized target measurements of the spin-dependent neutron-proton total cross-section differences in longitudinal and transverse geometries ( $\ensuremath{\Delta}{\ensuremath{\sigma}}_{L}$ and $\ensuremath{\Delta}{\ensuremath{\sigma}}_{T}$, respectively) between ${E}_{n}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5$ and 20 MeV. Single-parameter phase-shift analyses were performed to extract the phase-shift mixing parameter ${\ensuremath{\varepsilon}}_{1}$, which characterizes the strength of the nucleon-nucleon tensor interaction at low energies. Consistent with the trend of previous determinations at ${E}_{n}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}25$ and 50 MeV, our values for ${\ensuremath{\varepsilon}}_{1}$ imply a stronger tensor force than predicted by meson-exchange nucleon-nucleon potential models and nucleon-nucleon phase-shift analyses.

Discrete {gamma}-ray spectra have been measured as a function of incident neutron energy for nuclei produced in the {sup 196}Pt(n,xn{gamma}) reactions. Spectroscopy was done using the large-scale Compton suppressed Ge {gamma}-ray spectrometer GEANIE. The {open_quotes}white{close_quotes} source neutron beam was produced at the Los Alamos Neutron Science WNR facility. Reaction neutron energy was determined using the time-of-flight technique. Reaction-channel yields were inferred from the measured intensity sum of the 2{sub 1}{sup +}{r_arrow}0{sub 1}{sup +} and the 2{sub 2}{sup +}{r_arrow}0{sub 1}{sup +} transitions for the {sup 196}Pt(n,xn) reactions for x{le}15. Weisskopf-Ewing calculations (including precompound) done with the HMS-ALICE code correctly predict the bulk of the (n,xn) reaction products for low multiplicity. However, they do not accurately predict yield ratios of the different (n,xn) reactions for x{ge}9. In addition, there is no consistent experimental indication of charged-particle reaction channels (n,pxn) for incident neutron energies above 60 MeV where they are predicted to account for approximately 1/3 of the total reaction cross section. Several possible causes are discussed for these discrepancies. Finally, the region of E-J phase space populated in this reaction is probed for several of the strongest reaction channels through the observation of relative yields for different yrast and off-yrast states. {copyright}more » {ital 1998} {ital The American Physical Society}« less

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.

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.

A newly constructed source of polarized H ± and D ± ions is described. Atomic H or D beams from a dissociator with a cooled nozzle enter a system of two sextupole magnets and several radio-frequency transitions where they are focused and polarized. They enter a downstream electron-cyclotron-resonance-heated plasma ionizer from which positive ions are extracted. When negative ions are desired, they may be produced from the positive beam by charge-exchange in cesium vapor. Emerging beams are intense, have good polarization, low energy spread, and good optical quality. Descriptions are included for all major systems and for diagnostic procedures used to optimize both the intensity and the polarization of the output H ± or D ± beams obtained. Typical operating experience, performance figures, and a description of routine maintenance procedures are given.

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.

A simple and reliable method for determining the concentration of SF 6 in an accelerator gas mixture with N 2 and CO 2 is described. The technique makes use of the low velocity of sound in SF 6 (approximately 1 3 that of air). The sound velocity of the mixture is determined by measuring the spacing of acoustic resonances in a tube filled with the gas. Data from standard gas mixtures containing 0–10% SF 6 are presented, showing that the technique is accurate to approximately 0.5% absolute.

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., Submitted to Proceedings of the Conference CIPANP12, St.Petersburg, Florida, May 2012

JC Penney is a partner with the DOE's Commercial Building Partnerships (CBP) program, working with PNNL to explore energy design measures (EDMs) that may be applied to their building portfolio. A site in Colonial Heights, VA was chosen for a retrofit project; computer modeling predicts 45% improved energy performance compared to baseline operations. This case study reviews EDMs that were selected and their performance as of June 2011.

The NPDGamma collaboration reports results from the first phase of a measurement of the parity violating up-down asymmetry ${A}_{\ensuremath{\gamma}}$ with respect to the neutron spin direction of $\ensuremath{\gamma}$ rays emitted in the reaction $\mathrm{nP\vec}+p\ensuremath{\rightarrow}d+\ensuremath{\gamma}$ using the capture of polarized cold neutrons on the protons in a liquid parahydrogen target. One expects parity-odd effects in the hadronic weak interaction between nucleons to be induced by the weak interaction between quarks. ${A}_{\ensuremath{\gamma}}$ in $\mathrm{nP\vec}+p\ensuremath{\rightarrow}d+\ensuremath{\gamma}$ is dominated by a $\ensuremath{\Delta}I=1$, ${}^{3}{S}_{1}$-${}^{3}{P}_{1}$ parity-odd transition amplitude in the n-p system. The first phase of the measurement was completed at the Los Alamos Neutron Science Center spallation source (LANSCE), with the result ${A}_{\ensuremath{\gamma}}=[\ensuremath{-}1.2\ifmmode\pm\else\textpm\fi{}2.1 (\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.2 (\mathrm{sys}.)]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$. We also report the first measurement of an upper limit for the parity-allowed left-right asymmetry in this reaction, with the result ${A}_{\ensuremath{\gamma},\mathrm{LR}}=[\ensuremath{-}1.8\ifmmode\pm\else\textpm\fi{}1.9 (\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.2 (\mathrm{sys}.)]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$. In this paper we give a detailed report on the theoretical background, experimental setup, measurements, extraction of parity-odd and parity-allowed asymmetries, analysis of potential systematic effects, and LANSCE results. The asymmetry has an estimated size of $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ and the aim of the NPDGamma collaboration is to measure it to $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$. The second phase of the measurement will be performed at the Spallation Neutron Source at Oak Ridge National Laboratory.

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.

Temperature measurement and control are important in brute force polarization experiments. We discuss the installation and use of3He melting curve thermometers in a crystat used to polarize a TiH2 target. Comparison is made between the melting curve thermometers and the60CoCo nuclear orientation thermometer, which is often used in such experiments. The melting curve thermometers provide increased temperature resolution and sensitivity, and were used in a feedback heating system to control temperature to ±5.5 μK at 16.5 mK. The3He melting curve and the60CoCo temperature scales are found to agree within 2% at 15 mK. The present status of the melting curve scale and the effect of a magnetic field on melting curve thermometry are also discussed.

The parity-even time-reversal-odd five-fold correlation has been studied in 2 MeV polarized neutron transmission through a rotating, cryogenically-aligned 165Ho target. The analyzing power was found to be (1 ± 6) × 10−4, consistent with time reversal invariance. We obtain a bound of 5 × 10−3 for αT, the ratio of T-odd to T-even couplings in the effective N−N interaction.

We have observed depolarization effects when high intensity cold neutron beams are incident on alkali-metal-spin-exchange polarized He-3 cells used as neutron spin filters. This was first observed as a reduction of the maximum attainable He-3 polarization and was attributed to a decrease of alkali-metal polarization, which led us to directly measure alkali-metal polarization and spin relaxation over a range of neutron fluxes at LANSCE and ILL. The data reveal a new alkali-metal spin-relaxation mechanism that approximately scales as the square root of the neutron capture-flux density incident on the cell. This is consistent with an effect proportional to the recombination-limited ion concentration, but is much larger than expected from earlier work., submitted to Physical Review Letters

The Nab collaboration will perform a precise measurement of 'a', the electron-neutrino correlation parameter, and 'b', the Fierz interference term in neutron beta decay, in the Fundamental Neutron Physics Beamline at the SNS, using a novel electric/magnetic field spectrometer and detector design. The experiment is aiming at the 10^{-3} accuracy level in (Delta a)/a, and will provide an independent measurement of lambda = G_A/G_V, the ratio of axial-vector to vector coupling constants of the nucleon. Nab also plans to perform the first ever measurement of 'b' in neutron decay, which will provide an independent limit on the tensor weak coupling., Comment: 12 pages, 6 figures, 1 table, talk presented at the International Workshop on Particle Physics with Slow Neutrons, Grenoble, 29-31 May 2008; to appear in Nucl. Instrum. Meth. in Physics Research A

High precision fundamental neutron physics experiments have been proposed for the intense pulsed spallation neutron beams at JSNS, LANSCE, and SNS to test the standard model and search for new physics. Certain systematic effects in some of these experiments have to be controlled at the few ppb level. The NPD Gamma experiment, a search for the small parity-violating {gamma}-ray asymmetry A{sub Y} in polarized cold neutron capture on parahydrogen, is one example. For the NPD Gamma experiment we developed a radio-frequency resonant spin rotator to reverse the neutron polarization in a 9.5 cm x 9.5 cm pulsed cold neutron beam with high efficiency over a broad cold neutron energy range. The effect of the spin reversal by the rotator on the neutron beam phase space is compared qualitatively to rf neutron spin flippers based on adiabatic fast passage. We discuss the design of the spin rotator and describe two types of transmission-based neutron spin-flip efficiency measurements where the neutron beam was both polarized and analyzed by optically polarized {sup 3}He neutron spin filters. The efficiency of the spin rotator was measured at LANSCE to be 98.8 {+-} 0.5% for neutron energies from 3 to 20 meV over the full phasemore » space of the beam. Systematic effects that the rf spin rotator introduces to the NPD Gamma experiment are considered.« less

Parity-odd asymmetries in the electromagnetic decays of compound nuclei can sometimes be amplified above values expected from simple dimensional estimates by the complexity of compound nuclear states. In this work we use a statistical approach to estimate the root mean square (RMS) of the distribution of expected parity-odd correlations $\vec{s_{n}} \cdot \vec{k_{\gamma}}$, where $\vec {s_{n}}$ is the neutron spin and $\vec{k_{\gamma}}$ is the momentum of the gamma, in the integrated gamma spectrum from the capture of cold polarized neutrons on Al, Cu, and In and we present measurements of the asymmetries in these and other nuclei. Based on our calculations, large enhancements of asymmetries were not predicted for the studied nuclei and the statistical estimates are consistent with our measured upper bounds on the asymmetries., Comment: 22 pages, 6 figures

Experiments that place strict limits on the electric dipole moment of the neutron (n‐EDM) represent one of the most promising opportunities to search for physics beyond the standard model. Over the last several years, a collaboration based at Los Alamos National Laboratory has been working toward the development of an apparatus that is expected to improve the current n‐EDM limit by two orders of magnitude. A key feature of the proposed experiment involves the detection of ultraviolet scintillation light produced as a result of neutron capture on 3He in superfluid liquid 4He (l‐4He). Typically such events give rise to a large prompt flash of light followed by a train of delayed photons (‘after‐pulses’). We recently performed an experiment to examine correlations between the amplitude of the initial light pulse and the number of after‐pulses that are produced when a dilute mixture of 3He in l‐4He is irradiated with a neutron beam. Data from this experiment suggest that events associated with reaction products from neutron capture on 3He can be distinguished from background events associated with other forms of ionizing radiation such as gamma rays. A sample of data from our experiment is presented.

The NPDGamma collaboration has constructed and commissioned an apparatus on flight path 12 at LANSCE to measure with a high precision, 5×10−9, the small parity‐violating gamma‐ray asymmetry, Aγ, in polarized neutron capture on protons. This asymmetry can be determined unambiguously the weak pion‐nucleon coupling constant. To study the hadronic weak interaction at low energy, the collaboration has used the NPDGamma apparatus to measure parity‐violating gamma‐ray asymmetries in compound nuclei with cold neutrons. Using the statistical model of compound nuclei and spectroscopic information of the target nuclei, we can set upper limit on the spreading width of the hadronic weak interaction for intermediate‐mass nuclei. We describe the experiment and the preliminary results of measured gamma‐ray asymmetries of Al, Sc, Ti, Mn, and Co.

The abBA collaboration is developing a new type of field‐expansion spectrometer to measure neutron beta decay angular parameters, a, b, B, and A, to the 0.1% precision level. This precision will be achieved by combining three new technical approaches; a pulsed cold neutron beam, a 3He neutron spin filter, and segmented large‐area thin‐dead layer silicon detectors. Both the electron and proton resulting from the decay will be guided by electric and magnetic fields and detected in coincidence by two 2π solid‐angle silicon detectors. For the neutron polarization‐dependent observables A and B, a novel precision neutron polarimetry technique has been developed. The parameters a and b will be obtained from the proton time‐of‐flight and the measured electron energy spectrum. Measurement of the four parameters in the same apparatus provides a redundant determination of parameter λ=gA/gV, providing a test of the standard electroweak interaction.

The NPDGamma experiment will measure with a high precision, 5×10−9, the small parity‐violating gamma‐ray asymmetry, Aγ, in polarized cold neutron capture in a para‐hydrogen target to determine unambiguously the weak pion‐nucleon coupling constant Hπ1. For the experiment the collaboration has built a new high‐flux pulsed cold neutron beam line at LANSCE. In 2004, we first commissioned the beam line and then the apparatus with exception of the hydrogen target. The sensitivity of the apparatus was tested by measuring Aγ on Al, B, Cl, Cu, and In. The Cl has a well‐known large parity‐violating gamma‐ray asymmetry that was used to verify the performance of the apparatus. The other nuclei that were studied during the commissioning run are present in materials used for construction of the experiment and are, therefore, possible sources of the false asymmetries since backgrounds are expected to be about 10% of the signal from the neutron capture on hydrogen. We measured Aγ≈0 for these nuclei except for Cl. We repo...

We have built a CsI(Tl) γ‐ray detector array for the NPDGamma experiment to search for a small parity‐violating directional asymmetry in the angular distribution of 2.2 MeV γ‐rays from the capture of polarized cold neutrons by protons with a sensitivity of several ppb. The weak pion‐nucleon coupling constant can be determined from this asymmetry. The small size of the asymmetry requires control of systematic errors at the ppb level, and the use of current‐mode γ‐ray detection with vacuum photo diodes and low‐noise solid‐state preamplifiers. The detectors were tested for noise performance, sensitivity to magnetic fields, pedestal stability, and cosmic background. False asymmetries due to gain changes and electronic pickup in the detector system were measured to be consistent with zero to an accuracy of 10−9 in a few hours. We show that the detector array operates at counting statistics and present asymmetry results for B4C , CCl4 , 27Al, Cu, and In. B4C , 27Al, Cu, and In are used throughout the experiment...

A new pulsed cold neutron beam line, flight path 12, has been commissioned at LANSCE by the NPDGamma collaboration. The beam line was designed for fundamental nuclear physics experiments. We present the measured brightness of the unique backscattering moderator viewed by the flight path 12 neutron guide and report results for guide performance measurements. The peak neutron flux out of the guide is dN/dE= 2.4×105 neutrons/meV/cm2/s/μA at 2 meV neutron energy.