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 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 2.2MeV γ-rays from the capture of polarized cold neutrons on protons (Aγ). The estimated size of Aγ 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.

A 16l 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.

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.

We have built a CsI(Tl) 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 a high cold neutron flux, 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 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 gamma 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.

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.