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

L. Barrón-Palos, Ricardo Alarcon, S. Balascuta, C. Blessinger, J. D. Bowman, T. E. Chupp, S. Covrig, C. B. Crawford, M. Dabaghyan, J. Dadras, M. Dawkins, W. Fox, M. T. Gericke, R. C. Gillis, B. Lauss, M. B. Leuschner, B. Lozowski, R. Mahurin, M. Mason, J. MeiH. Nann, S. I. Penttilä, W. D. Ramsay, A. Salas-Bacci, S. Santra, P. N. Seo, M. Sharma, T. Smith, W. M. Snow, W. S. Wilburn, V. Yuan

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


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 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.

Original languageEnglish (US)
Pages (from-to)579-586
Number of pages8
JournalNuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Issue number1
StatePublished - Dec 11 2011


  • Hadronic weak interaction
  • Liquid hydrogen
  • Neutron capture
  • Neutron polarization
  • Neutron scattering
  • Orthohydrogen
  • Parahydrogen
  • Parity violation

ASJC Scopus subject areas

  • Nuclear and High Energy Physics
  • Instrumentation


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