The NPDGamma collaboration reports results from the first phase of a measurement of the parity violating up-down asymmetry Aγ with respect to the neutron spin direction of γ-rays emitted in the reaction n + p → d + γ 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 (HWI) between nucleons to be induced by the weak interaction between quarks. Aγ in n + p → d + γ is dominated by a ∆I = 1, 3 S1 − 3 P1 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 Aγ = (−1.2 ± 2.1 stat. ± 0.2 sys.) × 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 Aγ,LR = (−1.8 ± 1.9 stat. ± 0.2 sys.) × 10 −7 . In this paper we give a detailed report on the theoretical background, experimental setup, measurements, extraction of the parity-odd and parity-allowed asymmetries, analysis of potential systematic effects, and the LANSCE results. The asymmetry has an estimated size of 5 × 10 −8 and the aim of the NPDGamma collaboration is to measure it to 1 × 10 −8 . The second phase of the measurement will be performed at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory.

For the production of high-density ultracold neutrons (UCNs), we placed 0.8 K superfluid helium in a cold neutron moderator. We resolved previous heat-load problems in the spallation neutron source that were particularly serious below 1 K. With a proton-beam power of 400 MeV×1 μA, a UCN production rate of 4 UCN cm(-3) s(-1) at the maximum UCN energy of E(c)=210 neV and a storage lifetime of 81 s were obtained. A cryogenic test showed that the production rate can be increased by a factor of 10 with the same storage lifetime by increasing the proton-beam power as well as (3)He pumping speed.

We report the first observation of the parity-violating gamma-ray asymmetry A np γ in neutronproton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. A np γ isolates the ∆I = 1, 3 S1 → 3 P1 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 np γ = (−3.0 ± 1.4(stat.) ± 0.2(sys.)) × 10 −8 , which implies a DDH weak πN N coupling of h 1 π = (2.6 ± 1.2(stat.) ± 0.2(sys.)) × 10 −7 and a pionless EFT constant of C 3 S 1 → 3 P 1 /C0 = (−7.4 ± 3.5(stat.) ± 0.5(sys.)) × 10 −11 MeV −1 . We describe the experiment, data analysis, systematic uncertainties, and implications of the result.

The first ultracold-neutron (UCN) production in superfluid helium placed in a spallation neutron source is carried out. A UCN density of 0.7 UCN/cm(3), which can be used in experiments, is achieved for a proton-beam power of 78 W and a He-II temperature of 1.2 K. The present new UCN source is not limited by Liouville's theorem and extraction losses, which were serious problems in the previous sources. The present source has the possibility of extremely high-density UCN production compared with previous UCN sources.

Using velocity-analyzed cold neutrons we observed the production of ultracold neutrons in superfluid 4 He at 0.45 to 1.5 K. Ultracold neutrons are produced for an incident neutron wavelength of 8.78 ±0.06 A which agrees with the single-phonon emission theory. The temperature variation of the ultracold-neutron storage lifetime is also discussed.

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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 a high cold neutron flux, 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 average detector photoelectron yield was determined to be 1300 photoelectrons per MeV. The RMS width seen in the measurement is therefore dominated by the fluctuations in the number of γ rays absorbed in the detector (counting statistics) rather than the intrinsic detector noise. 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 report on the design, operating criteria, and the results of measurements performed to test the detector array.