Investigation of phase transfer properties of light and heavy water by means of energy selective neutron imaging

We report an investigation of the phase transitions between water and ice using neutron imaging as a research tool. Because of the high scattering cross section of hydrogen for slow neutrons, very small amounts of water can easily be quantified by this technique. However, it is not easy to determine the aggregate state (liquid or solid) if the sample does not have a well defined size, as might happen in the case of water produced in PEM fuel cells. It is important to be able to judge the state of the water (or ice), particularly when it is not easily accessible or hidden by the surrounding structure.The neutron interaction properties of the two aggregates (water and ice) were investigated in dynamic series, in which light water and heavy water were frozen under controlled conditions; transmission images were taken continuously during the phase transition. In addition to the expected density reduction of the ice, some other features were also observed in the images, causing the derived neutron transmission data to differ from the tabulated nuclear data.Energy-selective experiments with thermally stabilized water (+10 °C) and ice (−10 °C) showed different interaction behaviors for the two aggregates in the neutron wavelength range of 2–6 Å. We found clear differences in the absolute values of the attenuation cross-sections and the slopes of their energy dependencies. This result might be used in the future as a criterion to distinguish between the aggregate states in arbitrary water or ice assemblies.

► Slopes for energy dependent neutron cross-sections are different for water or ice. ► A non-invasive technique to distinguish liquid water from frozen ice can be developed. ► This method has high impact for effects in the membrane region of a PEM fuel cell. ► The cross-section data needs to be improved in the temperature region around 0 °C.