Measurement of the scattering cross section of slow neutrons on liquid parahydrogen from neutron transmission

Abstract

Liquid hydrogen is a dense Bose fluid whose equilibrium properties are both calculable from first principles using various theoretical approaches and of interest for the understanding of a wide range of questions in many body physics. Unfortunately, the pair correlation function g(r) inferred from neutron scattering measurements of the differential cross section dσ dΩ from different measurements reported in the literature are inconsistent. We have measured the energy dependence of the total cross section and the scattering cross section for slow neutrons with energies between 0.43 meV and 16.1 meV on liquid hydrogen at 15.6 K (which is dominated by the parahydrogen component) using neutron transmission measurements on the hydrogen target of the NPDGamma collaboration at the Spallation Neutron Source at Oak Ridge National Laboratory. The relationship between the neutron transmission measurement we perform and the total cross section is unambiguous, and the energy range accesses length scales where the pair correlation function is rapidly varying. At 1 meV our measurement is a factor of 3 below the data from previous work. We present evidence that these previous measurements of the hydrogen cross section, which assumed that the equilibrium value for the ratio of orthohydrogen and parahydrogen has been reached in the target liquid, were in fact contaminated with an extra non-equilibrium component of orthohydrogen. Liquid parahydrogen is also a widely-used neutron moderator medium, and an accurate knowledge of its slow neutron cross section is essential for the design and optimization of intense slow neutron sources. We describe our measurements and compare them with previous work.

First Observation of P -odd γ Asymmetry in Polarized Neutron Capture on Hydrogen

“…Two choppers select neutron wavelengths between 3.1-6.6Å from each 60 Hz time-offlight (TOF) pulse and reject neutrons outside this range to prevent lower energy neutrons mixing into the next pulse. The neutron beam intensity was sampled by two 3 He ionization chambers, one upstream (M1) and one downstream (M4) from the hydrogen target [44,46], see After M1, neutrons passed through a SM polarizer and emerged with an average polarization of 94% [47]. The neutron spin was transported to the target by a uniform magnetic field B 0 = 9.5 G aligned within 3 mrad to the +ŷ axis.…”

“…A np γ was determined from interactions of the polarized neutron beam on a 16 l liquid hydrogen (LH 2 ) target in the parahydrogen (p-H 2 ) molecular state [46,51]. Scattering from the S = 0 p-H 2 molecular ground state pre-serves neutron polarization for incident neutron energies which fall below the 14.7 meV threshold for spin-flip scattering into the S = 1 orthohydrogen (o-H 2 ) molecular ground state.…”

ANNI – A pulsed cold neutron beam facility for particle physics at the ESS

“…Since 43% of the neutrons incident on the target are captured by hydrogen [19], the particle flux is used as optimisation criterion. The gain in table 3 corresponds to the ratio of the integral particle flux at the guide exit without frame overlap for ANNI (FOCs set to 2.5-8.5 Å in order to remain below the 14.7 meV threshold for spinflip scattering, see [40]) and FnPB [41] (3.1-6.6 Å, flux from [19] scaled to 1.4 MW nominal power of the Spallation Neutron Source SNS). For the FnPB, the geometrical and transmission loss by the subsequent polarising bender was taken as 50% and taken into account; this loss is included in the simulations for ANNI with polarising bender 2 implemented in the guide (the factor 1/2 for spin selection is Table 4.…”

Monte Carlo calculation of the average neutron depolarization for the NPDGamma experiment

“…Similar to the aluminum calculation, the ∆ r dep correction for hydrogen has a detector ring dependence due to being a distributed volume. ∆ r dep is sensitive to the orthohydrogen concentration (x o ) and calculations were performed at the upper and lower bound orthohydrogen concentrations that were determined along with the measurement of the parahydrogen scattering cross-section [17], which are 0.15% and 0.015%, respectively. Since there was no in situ method of determining the x o for the NPDGamma experiment, the x o distribution is assumed to be flat between the upper and lower bounds, with an uncertainty given by…”

“…The methods to monitor the orthohydrogen concentration available at the time of the construction of the apparatus required feeding electrical signals out of the cryogenic region and hydrogen safety requirements limited the number of wires in the vessel. However, conversion towards a steady state condition with a time constant on the order of days is observed via measurement of the relative neutron transmission and produced a measurement of the parahydrogen scattering cross section [17]. The cross section measurement also yielded a determination of an upper bound (0.15%) on the orthohydrogen concentration in the bulk liquid while the lower limit (0.015%) is given by thermodynamic equilibrium at 15.6 K. The upper and lower bounds on the orthohydrogen concentration are used to as inputs for the calculation of ∆ r dep .…”