Determination of the parahydrogen fraction in a liquid hydrogen target using energy-dependent slow neutron transmission

Journal Article · Sat Jan 01 00:00:00 EST 2011 · Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment

DOI:https://doi.org/10.1016/j.nima.2011.07.051· OSTI ID:1265347

Barro´ n-Palos, L. ^[1]

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1. Arizona State Univ., Tempe, AZ (United States).et al

In this study, 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. In addition, assuming thermal equilibrium of the target, we extract the scattering cross-section for neutrons on parahydrogen.

• Research Organization: Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Spallation Neutron Source (SNS)
• Sponsoring Organization: USDOE Office of Science (SC)
• DOE Contract Number: AC05-00OR22725
• OSTI ID: 1265347
• Journal Information: Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment, Vol. 659, Issue 1; ISSN 0168-9002
• Country of Publication: United States
• Language: English

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