The Orthohydrogen Fraction of the NPDGamma Liquid Hydrogen Target

The NPDGamma Experiment at the SNS is measuring the parity-violating correlation $A_\gamma$ between neutron spin and gamma momentum in the reaction $\overrightarrow{n} + p\rightarrow d + \gamma$ with a projected statistically limited precision of a part per $10^{8}$. In the experiment, a polarized cold neutron beam captures on a 16.5~liter cryogenic liquid parahydrogen target. Since orthohydrogen has a large neutron spin flip scattering cross section, with a part per $10^4$ of orthohydrogen expected to produce a relative change in the beam polarization of close to a percent, it is essential for our experiment that the ortho fraction be kept low. An ortho-para conversion loop was incorporated into the target so that the ortho fraction would be held close to its thermodynamic equilibrium level of a few parts per $10^{4}$, and it is important that deviations from thermodynamic equilibrium be well understood. Theoretical and empirical arguments for placing a limit on the orthohydrogen fraction of our liquid hydrogen target will be discussed.