This work is made available by Hamilton College for educational and research purposes under a Creative Commons BY-NC-ND 4.0 license. For more information, visit http://digitalcommons.hamilton.edu/about.html or contact digitalcommons@hamilton.edu. This document is the publisher's version of an article published in: Physical Review C, vol. 83, no. 1 (2011): 015505 (2011). doi: 10.1103/PhysRevC.83.015505.

The NPDGamma collaboration reports results from the first phase of a measurement of the parity violating up-down asymmetry A(gamma) with respect to the neutron spin direction of gamma rays emitted in the reaction (n) over right arrow + p -> d + 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(gamma) in (n) over right arrow + p -> d + gamma is dominated by a Delta I = 1, S-3(1)-P-3(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(gamma) = [-1.2 +/- 2.1 (stat.) +/- 0.2 (sys.)] x 10(-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(gamma,LR) = [-1.8 +/- 1.9 (stat.) +/- 0.2 (sys.)] x 10(-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 x 10(-8) and the aim of the NPDGamma collaboration is to measure it to 1 x 10(-8). The second phase of the measurement will be performed at the Spallation Neutron Source at Oak Ridge National Laboratory.

The NPDGamma Experiment measures the parity-violating correlation Aγ between neutron spin and photon momentum in the reaction + p → d + γ. Knowledge of Aγ and other parity-violating observables in few-body nuclear systems will provide constraints for a parameterized description of ΔS = 0 parity-violating phenomena free from complications of nuclear structure. The NPDGamma experiment uses a polarized cold pulsed neutron beam, a liquid parahydrogen target, and a cylindrical array of 48 CsI(Tl) scintillation detectors operated in current mode to search for the asymmetry. NPDGamma recently completed the first phase of the program to measure Aγ at the Los Alamos Neutron Science Center with the preliminary result Aγ = (−1.2 ± 2.1(stat.) ± 0.1(sys.)) × 10−7, reproducing the previous upper limit from a measurement at a reactor facility. We discuss the theoretical background and experimental method and report on preliminary analysis of the LANSCE data. The second phase of the program to measure Aγ is in progress at the Spallation Neutron Source at Oak Ridge National Laboratory.

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 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 Cl, Cd, and La targets. Upper limits on the parity-allowed asymmetry sn · (kγ × kn) were set at the level of 7× 10 for all three targets. Parity-violating asymmetries sn · kγ were observed in Cl, Aγ = (−29.1 ± 6.7) × 10, and La, Aγ = (−15.5 ± 7.1) × 10, values consistent with previous measurements.

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 NPDGamma experiment, a search for the small parity-violating gamma-ray asymmetry A(gamma) in polarized cold neutron capture on parahydrogen, is one example. For the NPDGamma 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 He-3 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 phase space of the beam. Systematic effects that the rf spin rotator introduces to the NPDGamma experiment are considered.

structure functioncontains contributions from both the longitudinal and transverse polarizationdistributions within the nucleon. It is sensitive to higher twist effects suchas quark-gluon correlations and quark mass contributions, and is not easilyinterpreted in the QPM where such effects are not included. However, by in-terpreting g

A large area polarized 3He neutron spin filter has been developed and tested in long-term operation. The polarizer cells have area 50cm2 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 5Å. Cell construction, polarizer design, performance, and polarimetry are described in this paper.

The NPD gamma experiment(1) at the Los Alamos Neutron Science Center (LANSCE) is dedicated to measure with high precision the parity violating asymmetry in the emission after capture of spin polarized cold neutrons in para-hydrogen. The measurement will determine unambiguously the weak pion-nucleon-nucleon (pi NN) coupling constant f(pi)(1).

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. Using a statistical approach, we estimate the root-mean-square of the distribution of expected parity-odd correlations (s) over right arrow (n)center dot(k) over right arrow (gamma), where (s) over right arrow (n) is the neutron spin and (k) over right arrow (gamma) is the momentum of the gamma, in the integrated gamma spectrum from the capture of cold polarized neutrons on Al, Cu, and In. 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.

The NPDGamma collaboration has constructed and commissioned an apparatus on flight path 12 at LANSCE to measure with a high precision, 5x10(-9), the small parity-violating gamma-ray asymmetry, A(gamma), 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 NPDGamma experiment will measure the parity-violating directional gamma ray asymmetry A γ in the reaction [Formula: see text]. Ultimately, this will constitute the first measurement in the neutron-proton system that is sensitive enough to challenge modern theories of nuclear parity violation, providing a theoretically clean determination of the weak pion-nucleon coupling. A new beam-line at the Los Alamos Neutron Science Center (LANSCE) delivers pulsed cold neutrons to the apparatus, where they are polarized by transmission through a large volume polarized (3)He spin filter and captured in a liquid para-hydrogen target. The 2.2 MeV gamma rays from the capture reaction are detected in an array of CsI(Tl) scintillators read out by vacuum photodiodes operated in current mode. We will complete commissioning of the apparatus and carry out a first measurement at LANSCE in 2004-05, which would provide a statistics-limited result for A γ accurate to a standard uncertainty of ±5 × 10(-8) level or better, improving on existing measurements in the neutron-proton system by a factor of 4. Plans to move the experiment to a reactor facility, where the greater flux would enable us to make a measurement with a standard uncertainty of ±1 × 10(-8), are actively being pursued for the longer term.

We have built a CsI(TI) gamma-ray detector array for the NPDGamma experiment to search for a small parity-violating directional asymmetry in the angular distribution of 2.2 MeV gamma-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 gamma-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, Al, Cu, and In. B4C, Al, Cu, and In are used throughout the experimental apparatus.

The NPDGamma experiment will measure with a high precision, 5x10(-9), the small parity-violating gammaray asymmetry, A. in polarized cold neutron capture in a para-hydrogen target to determine unambiguously the weak pion-nucleon coupling constant H-pi(1). 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 A(gamma) on Al, B, Cl, Cu, and In. The CI 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 A(gamma)=0 for these nuclei except for Cl. We report the status of the experiment and preliminary results of the 2004 commissioning run.

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.

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 3He 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 35Cl, 113Cd, and 139La targets. Upper limits on the parity-allowed asymmetry sn·(kγ×kn) were set at the level of 7×10−6 for all three targets. Parity-violating asymmetries sn·kγ were observed in 35Cl, Aγ=(−29.1±6.7)×10−6, and 139La, Aγ=(−15.5±7.1)×10−6, values consistent with previous measurements.

We have produced 70%–75% 3He polarization by spin-exchange optical pumping in cells ≈100 cm3 in volume. The polarization achieved is consistent with known spin-exchange and spin-relaxation rates, but only when the recently discovered temperature dependence of 3He relaxation is included. Absolute 3He polarization measurements were performed using two different methods in two different laboratories. The results were obtained with either a spectrally narrowed laser or one type of broadband laser. Based on tests of several larger cells at pressures near 1 bar, we find that the power required to reach the same polarization is typically three times lower for the spectrally narrowed laser. This last result indicates that spectrally narrowed lasers will be important for obtaining the highest polarization in large volume neutron spin filters. Polarization in excess of 55% as obtained in cells up to 640 cm3 in volume and 70% polarization is anticipated with available increases in spectrally narrowed laser power.

An experiment to measure γ-ray asymmetry Aγ with a high precision in neutron-proton radiative capture is under construction at LANSCE. The experiment will determine the weak pion-nucleon coupling constant Hπ1,, 30% of its predicted value.

Motivated by applications to neutron spin filters and recent advances in spectrally narrowed laser diode arrays (LDAs), we are exploring spin exchange optical pumping of He3 at pressures near 1 bar. Among our more interesting results has been the production of glass cells with extremely long relaxation times. The best of these has a lifetime of T1=840 h [where the polarization decays versus time, t, as exp(−t/T1)], dominated by the dipole–dipole contribution of 950 h at a He3 partial pressure of 0.85 bar. Using a broadband LDA, we have obtained 55% He3 nuclear polarization in this cell. These results are particularly relevant to the application of He3-based neutron spin filters to neutron scattering and weak interaction experiments. Applications to magnetometry and polarized gas magnetic resonance imaging are also possible.